/*
 * $Id: xhb-diff.txt 16709 2011-05-02 22:39:29Z vszakats $
 */

This text describes most important differences between Harbour and xHarbour
with some references to Clipper and other compatible compilers like xBase++,
CLIP, FlagShip.
Many thanks to Pritpal and Viktor for updating this text.
I hope that it will be updated in the future also by xHarbour developers,
It describes status of both compiler at the end of October 2009:
   Harbour 2.0.0 beta3 (revision 12788)
   xHarbour 1.2.1 (revision 6629)

Przemek,

(Przemyslaw Czerpak, druzus/at/acn.waw.pl)



###    COMPILE TIME SUPPORT FOR MERGING MULTIPLE .PRG MODULES    ###
====================================================================
Clipper allows to compile many .prg modules included by @<name>.clp
and/or SET PROCEDURE TO ... / DO ... [ WITH ... ] into single output
object. In such compilation it supports, separated for each .prg file,
file wide declarations when -n switch is used and allows to use more
then one static function with the same name if each of them is declared
in different .prg modules. This code illustrates such situation:

      /***** t1. prg *****/
      static s  := "t01:s"
      static s1 := "t01:s1"
      proc main()
         ? "===="
         ? s, s1
         p1();p2();p3()
         ? "===="
         do t2
         ? "===="
      return
      proc p1        ; ? "t01:p1"
      static proc p2 ; ? "t01:p2"
      static proc p3 ; ? "t01:p3"
      init proc pi ; ? "init t01:pi"
      exit proc pe ; ? "exit t01:pe"

      /***** t2. prg *****/
      static s  := "t02:s"
      static s2 := "t02:s2"
      proc t2()
         ? s, s2
         p1();p2();p3()
      return
      static proc p1 ; ? "t02:p1"
      proc p2        ; ? "t02:p2"
      static proc p3 ; ? "t02:p3"
      init proc pi ; ? "init t02:pi"
      exit proc pe ; ? "exit t02:pe"

It needs -n switch for file wide declarations and uses static/init/exit
functions with the same names but declared in different modules.
It can be compiled and linked by Clipper and Harbour, i.e.:
      cl t1.prg /n/w/es2
or:
      hbmk2 t1.prg -n -w -es2
and then executed.

xHarbour does not have such functionality and above code has to be adopted
to work with this compiler. Additionally it does not work well with case
sensitive file systems what can be seen in above example where it converts
"t1" to "T1" and then tries to include "T1.prg".

For users which have old Clipper code written for DOS file systems with
mixed upper and lower letters in file names used directly or indirectly
by procedure name, Harbour provides compile time switches which enable
automatic filename conversions for all files opened by compiler:
      -fn[:[l|u]|-]    set filename casing (l=lower u=upper)
      -fd[:[l|u]|-]    set directory casing (l=lower u=upper)
      -fp[:<char>]     set path separator
      -fs[-]           turn filename space trimming on or off (default)
This functionality is also local to Harbour so cannot be used with xHarbour
as workaround for above problem though it should be easy to add it to this
compiler in the future.

Both compilers support runtime switches for file name conversions.
      SET FILECASE LOWER | UPPER | MIXED
      SET DIRCASE LOWER | UPPER | MIXED
      SET DIRSEPARATOR <cDirSep>
      set( _SET_TRIMFILENAME, <lOnOff> )
which can be used in programs not intended to work with different
file system(s) and with different OS(s).



###    NEW LANGUAGE STATEMENTS    ###
=====================================
1. FOR EACH
   Harbour support all xHarbour functionality and it offers also additional
   features which are not available in xHarbour.
   a) it allows to iterate more then one variable
         FOR EACH a, b, c IN aVal, cVal, hVal
            ? a, b, c
         NEXT
   b) it allows to set descending order by DESCEND flag, f.e.:
         FOR EACH a, v IN aVal, cVal DESCEND
            ? a, b
         NEXT
   c) it has native support for hashes:
         FOR EACH x IN { "ABC" => 123, "ASD" => 456, "ZXC" => 789 }
            ? x, "@", x:__enumKey()
         NEXT
   d) it allows to assign string items, f.e.:
         s := "abcdefghijk"
         FOR EACH c IN @s
            IF c $ "aei"
               c := UPPER( c )
            ENDIF
         NEXT
         ? s      // AbcdEfghIjk
   e) it gives OOP interface to control enumerator variables what
      is very important when more then one variable is iterated or
      when FOR EACH is called recursively, f.e.:
         hVal := { "ABC" => 123, "ASD" => 456, "ZXC" => 789 }
         FOR EACH x IN hVal
            ? x:__enumIndex(), ":", x:__enumKey(), "=>", x:__enumValue(), ;
              "=>", x:__enumBase()[ x:__enumKey() ]
         NEXT
   f) it gives very flexible OOP mechanism to overload FOR EACH behavior
      for user define objects adding to above enumerator methods also
      __enumStart(), __enumStop(), __enumSkip() methods what allows to
      implement many different enumeration algorithms depending on used
      data
   g) it does not have any hardcoded limitations for recursive calls
      (it's limited only by available memory and HVM stack size), f.e.:
         proc main()
            p( 0 )
         return
         proc p( n )
            local s := "a", x
            ? n
            if n < 1000
               for each x in s
                  p( n + 1 )
               next
            endif
         return
   In xHarbour there is function HB_ENUMINDEX() which is supported by
   Harbour in XHB library.

2. WITH OBJECT / END[WITH]
   In Harbour it does not have any hardcoded limitations for recursive
   calls (it's limited only by available memory and HVM stack size), f.e.:
      proc main()
         p( 0 )
      return
      proc p( n )
         ? n
         if n < 1000
            with object n
               p( n + 1 )
            end
         endif
      return
   It also uses OOP interface just like FOR EACH, so it's possible to
   use :__withObject() to access / assign current WITH OBJECT value.
   In xHarbour there are functions HB_QWITH(), HB_WITHOBJECTCOUNTER()
   and HB_RESETWITH() which are supported by Harbour in XHB library.

3. SWITCH / [ CASE / [EXIT] / ... ] OTHERWISE / END[SWITCH]
   In Harbour it uses jump table with predefined values what gives
   significant speed improvement in comparison to sequential PCODE
   evaluation just like in DO CASE statements.
   In xHarbour SWITCH does not use such jump table and generated
   PCODE is similar to the one used for DO CASE or IF / ELSEIF
   and only the main switch value calculation is optimized and
   reused for all statements so speed improvement is relatively
   small.
   In xHarbour instead of OTHERWISE the DEFAULT clause is used.
   As SWITCH values Harbour supports integer numbers and strings, f.e.:
      switch x
         case 1         ; [...]
         case 10002     ; [...]
         case "data"    ; [...]
         otherwise      ; [...]
      endswitch
   xHarbour supports only integer numbers and one character length strings
   like "A", "!", "x", " ", ...

4. BEGIN SEQUENCE [ WITH <errBlock> ]
   [ RECOVER [ USING <oErr> ] ]
   [ ALWAYS ]
   END SEQUENCE

   It's unique to Harbour. In xHarbour limited version of above statement
   exists as:
      TRY
      [ CATCH [<oErr>] ]
      [ FINALLY ]
      END
   TRY gives exactly the same functionality as:
      BEGIN SEQUENCE WITH { |e| break(e) }

With the exception to SWITCH implementation, in all other statements
described above, xHarbour causes performance reduction in PCODE evaluation
even if user does not use them at all. In Harbour they are implemented in
different way which does not cause any overhead and slowness for other code.



###    EXTENDED CODEBLOCKS    ###
=================================
Both compilers support compile time extended codeblocks which allow
to use statements but with a little bit different syntax. Harbour uses
standard Clipper codeblock delimiters {}, f.e.:
      ? eval( { | p1, p2, p3 |
                ? p1, p2, p3
                return p1 + p2 + p3
              }, 1, 2, 3 )
and xHarbour <>, f.e.:
      ? eval( < | p1, p2, p3 |
                ? p1, p2, p3
                return p1 + p2 + p3
              >, 1, 2, 3 )

In Harbour extended codeblocks works like nested functions and support
all functions attributes, f.e. they can have own static variables or
other declarations which are local to extended codeblocks only and
do not effect upper function body.
In xHarbour the compiler was not fully updated for such functionality
and extended codeblocks were added to existing compiler structures what
causes that not all language constructs work in extended codeblocks
and creates a set of very serious compiler bugs, f.e., like in this code
with syntax errors but which is compiled by xHarbour without even single
warning giving unexpected results at runtime:
      #ifndef __XHARBOUR__
         #xtranslate \<|[<x,...>]| => {|<x>|
         #xcommand > [<*x*>]       => } <x>
      #endif
      proc main()
         local cb, i
         for i:=1 to 5
            cb := <| p |
                     ? p
                     exit
                     return p * 10
                  >
            ?? eval( cb, i )
         next
      return
It's possible to create many other similar examples which are mostly
caused by missing in the compiler infrastructure for nested functions
support.
This can be fixed if someone invest some time to clean xHarbour compiler.



###    HASH ARRAYS    ###
=========================
Both compilers have support for hash arrays. They are similar to
normal arrays but also allow to use non integer values as indexes
like string, date, non integer number or pointer (in Harbour) items.
They can be created using => for list of keys and values enclosed
inside {}, f.e.:
      hVal := { "ABC"    => 123.45, ;
                100.1    => date(), ;
                100.2    => 10,     ;
                100      => 5,      ;
                date()-1 => .t. }
and then items can be accessed using [] operator, f.e.:
      ? hVal[ "ABC" ]      // 123.45
      ? hVal[ 100 ]        // 5
      ? hVal[ date()-1 ]   // .t.
      ? hVal[ 100.2 ]      // 10
      ? hVal[ 100.1 ]      // date()

By default hash items in both compiler support automatic adding new elements
on assign operation. It can be disabled using one of hash item functions.
Harbour has additional extension which allows to enable autoadd with default
values also for access operation and reference operator. It also supports
well passing hash array items by reference and has some other minor
extensions.

xHarbour does not support autoadd on access or reference operations and
passing hash array items by reference does not work (see passing array and
hash item by reference).

Both compilers have set of functions to make different operations on hash
arrays which give similar functionality. In Harbour they use HB_H prefix
(f.e. HB_HSCAN()) in xHarbour H prefix (f.e. HSCAN())

xHarbour has additional functionality which can be enabled for each hash
array using HSetAACompatibility() function. It's an index where is stored
information about the order in which items were added to hash array and
set of functions with HAA prefix to operate on hash array items using this
index instead of real position in hash array, i.e. HAAGETVALUEAT() which
works like HGETVALUEAT().
In Harbour such functionality also exists and it's enabled by HB_HKEEPORDER()
function but the internal implementation is completely different. Harbour
does not emulate associative arrays by special index which keeps assign order
but it uses real natural order in hash array. It means that associative array
indexes are equal to regular hash indexes so it does not need any translation
between them and Harbour users can use regular hash array functions for
associative array indexes. So separated functions for accessing by other
index are not necessary. The Harbour implementation is also more efficient
because it does not introduce linear index updated each time new key is
added to hash array.
Harbour emulates HAA*() xHarbour functions in XHB library but only for
compatibility with existing xHarbour code.



###    REFERENCES TO VARIABLES STORED IN ARRAYS    ###
======================================================
In xHarbour the behavior of references stored in array is reverted in
comparison to Clipper or Harbour.
In Clipper and Harbour VM executing code like:
         aVal[ 1 ] := 100
clears unconditionally 1-st item in aVal array and assigns to it value 100.
xHarbour checks if aVal[ 1 ] is an reference and in such case it resolves
the reference and then assign 100 to the destination item.
On access Clipper and Harbour VM executing code like:
         x := aVal[ 1 ]
copy to x the exact value stored in aVal[ 1 ]. xHarbour checks is aVal[ 1 ]
is an reference and in such case it resolves the reference and then copy to x
the value of reference destination item.
It can be seen in code like:
      proc main
         local p1 := "A", p2 := "B", p3 := "C"
         ? p1, p2, p3
         p( { @p1, p2, @p3 } )
         ? p1, p2, p3

      proc p( aParams )
         local x1, x2, x3

         x1 := aParams[ 1 ]
         x2 := aParams[ 2 ]
         x3 := aParams[ 3 ]

         x1 := lower( x1 ) + "1"
         x2 := lower( x2 ) + "2"
         x3 := lower( x3 ) + "3"

Harbour and Clipper shows:
      A B C
      a1 B c3
but xHarbour:
      A B C
      A B C

It's not Clipper compatible so in some cases it may cause portability
problems. F.e. code like above was used in Clipper as workaround for
limited number of parameters (32 in Clipper). But it allows to directly
assign items of arrays returned by hb_aParams() and updating corresponding
variables passed by references (see functions with variable number of
parameters below).
Anyhow the fact that xHarbour does not have '...' operator which can
respect existing references in passed parameters and does not support
named parameters in functions with variable number of parameters causes
that reverted references introduce limitation, f.e. it's not possible
to make code like:
   func f( ... )
      local aParams := hb_aParams()
      if len( aParams ) == 1
         return f1( aParams[ 1 ] )
      elseif len( aParams ) == 2
         return f2( aParams[ 1 ], aParams[ 2 ] )
      elseif len( aParams ) >= 3
         return f3( aParams[ 1 ], aParams[ 2 ], aParams[ 3 ] )
      endif
   return 0

which will respect references in parameters passed to f() function.



###    PASSING ARRAY AND HASH ITEMS BY REFERENCE    ###
=======================================================
Harbour supports passing array and hash items by reference, f.e.:
      proc main()
         local aVal := { "abc", "klm", "xyz" }, ;
               hVal := { "qwe"=>"123", "asd"=>"456", "zxc"=>"789" }
         ? aVal[1], aVal[2], aVal[3], hVal["qwe"], hVal["asd"], hVal["zxc"]
         p( @aVal[2], @hVal["asd"] )
         ? aVal[1], aVal[2], aVal[3], hVal["qwe"], hVal["asd"], hVal["zxc"]

      proc p( p1, p2 )
         p1 := '[1]'
         p2 := '[2]'

Compiled by Harbour above code shows:
      abc klm xyz 123 456 789
      abc [1] xyz 123 [2] 789

In xHarbour only passing array items by reference works but does not work
passing hash items by reference though it does not generate either
compile time or run time errors so the above code can be compiled and
executed but it shows:
      abc klm xyz 123 456 789
      abc [1] xyz 123 456 789



###    PASSING OBJECT VARIABLES BY REFERENCE    ###
===================================================
Both compilers support passing object variables by reference though this
functionality in xHarbour is limited to pure instance or class variables
only and does not work for SETGET methods. In Harbour it works correctly.
This code illustrates the problem:

      proc main()
         local oBrw := tbrowseNew()
         ? oBrw:autoLite
         oBrw:autoLite := !oBrw:autoLite
         ?? "=>", oBrw:autoLite
         p( @oBrw:autoLite )
         ?? "=>", oBrw:autoLite

      proc p( x )
         x := !x

Harbour prints:
      .T.=> .F.=> .T.
but xHarbour prints:
      .T.=> .F.=> .F.
without generating any compile or run time errors.



###    DETACHED LOCALS AND REFERENCES    ###
============================================
When local variables are used in codeblocks then it's possible that
the codeblocks will exist after leaving the function when they were
created. It's potentially very serious problem which have to be resolved
to avoid internal VM structure corruption. In Clipper, Harbour and
xHarbour special mechanism is used in such situation. Local variables
are "detached" from VM stack so they are still accessible after leaving
the function, f.e.:
      proc make_cb()
         local n := 123
         return {|| ++n  }
We call such variables "detached locals".
Here there are two important differences between Clipper and [x]Harbour.
In Clipper variables are detached when function exits (returns) and it
does not know which variables were used but simply detach whole local
variable frame from VM stack. It's very important to know that because
it can be source of serious memory problems in OS like DOS.
This simple code illustrates it:

      // link using RTLINK and run with //e:0 //swapk:0
      // repeat test second time with additional non empty parameter <x>
      #define N_LOOPS 15
      #xcommand FREE MEMORY => ? 'free memory: ' + ;
                                 AllTrim( Str( Memory( 104 ) ) )
      proc main( x )
         local n, a
         a := array( N_LOOPS )
         FREE MEMORY
         for n := 1 to N_LOOPS
            a[n] := f( x )
            FREE MEMORY
         next
      return
      func f(x)
         local cb, tmp, ref
         tmp := space( 60000 )
         if empty( x )
            cb := {|| .t. }
         else
            cb := {|| ref }
         endif
      return cb

If you execute above program with non empty parameter then 'tmp' variable
is detached with codeblock which uses 'ref' variable and not released as
long as codeblock is still accessible. It means that in few iterations
all memory are allocated and program crashes. Clipper's programmers should
know that and be careful when use detached local and if necessary clear
explicitly other local variables before returning from the function by
setting NIL to them.
In Harbour and xHarbour only variables explicitly used in codeblocks
are detached and detaching is done when codeblock is created and original
local variables are replaced by references. It is possible because Harbour
and xHarbour support multilevel references chains so it works correctly
also for local parameters passed be reference from parent functions.
In Clipper only one level references are supported what creates second
important differences. When Clipper detaches frame with local parameters
then it has to unreference all existing references breaking them. This code
illustrates it:

      proc main()
         local cb, n := 100
         mk_block( @cb, @n )
         ? "after detaching:"
         ? eval( cb ), n
      return
      proc mk_block( cb, n )
         n := 100
         cb := {|| ++n }
         ? "before detaching:"
         ? eval( cb ), n
      return

Above code compiled by Clipper shows:

      before detaching:
             101        101
      after detaching:
             102        101

so after detaching the references to 'n' variable is broken and codeblocks
access his own copy of this variables.
In Harbour it works correctly so the results are correct and it shows:

      before detaching:
             101        101
      after detaching:
             102        102

In xHarbour ( for unknown to me reasons ) Clipper bug is explicitly emulated
though it was possible to fix it because xHarbour inherited from Harbour
the same early detaching mechanism with multilevel references so just like
in Clipper xHarbour programmers have to carefully watch for possibly broken
references by detached locals and add workarounds for it if necessary.



###    FUNCTIONS WITH VARIABLE NUMBER OF PARAMETERS    ###
==========================================================
Both compilers supports them though xHarbour is limited to all parameters
and does not support unnamed parameters. In Harbour you can declare some
named parameters and then unnamed just like in many other languages, f.e.:
      func f( p1, p2, p3, ... )
The unnamed parameters can be used in different statements passing them
by '...' operator, f.e. as array items:
      proc main()
         AEval( F( "1", "2", "A", "B", "C" ), {|x, i| qout( i, x ) } )
      func f( p1, p2, ... )
         ? "P1:", p1
         ? "P2:", p2
         ? "other parameters:", ...
         return { "X", ... , "Y", ... "Z" }
or as array indexes:
      proc main()
         local a := { { 1, 2 }, { 3, 4 }, 5 }
         ? aget( a, 1, 2 ), aget( a, 2, 1 ), aget( a, 3 )
      func aget( aVal, ... )
      return aVal[ ... ]

or as function parameters:
      proc main()
         info( "test1" )
         info( "test2", 10, date(), .t. )
      proc info( msg, ... )
         qout( "[" + msg +"]: ", ... )

The '...' operator saves references when push parameters and it can be
used also in codeblocks, f.e.:
      bCode := { | a, b, c, ... | qout( a, b, c ), qout( "[", ..., "]" ) }

All parameters can be accessed also using hb_aParams() function but
in xHarbour it works correctly only for functions which does not use
any local parameters or declared with variable number of parameters
or when number of declared parameters is not smaller then number of
passed parameters. This code illustrates it:
      proc main()
         p1("A","B","C")
         p2("A","B","C")
         p3("A","B","C")
         p4("A","B","C")
         p5("A","B","C")
      proc p1
         ? procname()+"(), parameters:", pcount()
         aeval( hb_aParams(), {|x,i| qout(i,"=>",x) } )
      proc p2
         local l
         ? procname()+"(), parameters:", pcount()
         aeval( hb_aParams(), {|x,i| qout(i,"=>",x) } )
      proc p3(x)
         ? procname()+"(), parameters:", pcount()
         aeval( hb_aParams(), {|x,i| qout(i,"=>",x) } )
      proc p4(...)
         ? procname()+"(), parameters:", pcount()
         aeval( hb_aParams(), {|x,i| qout(i,"=>",x) } )
      proc p5(a,b,c,d,e)
         ? procname()+"(), parameters:", pcount()
         aeval( hb_aParams(), {|x,i| qout(i,"=>",x) } )

In xHarbour it's only possible to declare all parameters as unnamed, f.e.:
      func f( ... )
and then access them using hb_aParams() or PVALUE() (in Harbour it's called
HB_PVALUE()) function. There is no support for named parameters and ...
operator.

In xHarbour due to reverted behavior of references stored in array items
assign operation to items in array returned by hb_aParams() changes
corresponding parameters passed by reference. It does not happen in
Harbour where item references stored in arrays work like in Clipper.



###    HB_ARRAYTOPARAMS() FUNCTION    ###
=========================================
Harbour has special function which allows to convert array into
list of items which can be used as function parameters, array
values or array indexes in the same way as '...' operator:
   hb_arrayToParams( <aValue> ) -> <aValue>[ 1 ] [, <aValue>[ N ] ]
i.e.:
      proc main( ... )
         local aParams := hb_aParams(), n
         /* remove parameters starting with "--" */
         n := 1
         while n < len( aParams )
            if left( aParams[ n ], 2 ) == "--"
               hb_adel( aParams, n, .t. )
            else
               ++n
            endif
         enddo
         ? "Public parameters:", hb_arrayToParams( aParams )
      return

Note for Clipper users: 'hb_adel( aParams, n, .t. )' in above example
works like 'adel( aParams, n ); asize( aParams, len( aParams ) - 1 )'.

This functionality is unique to Harbour and xHarbour does not support
hb_arrayToParams() or similar function.



###    MACRO MESSAGES    ###
============================
Both compilers Harbour and xHarbour supports macros as messages.
Clipper does not. This example shows such macro messages usage:

      proc main()
         memvar var
         local o := errorNew(), msg := "cargo"
         private var := "CAR"

         o:&msg := "<cargo>"
         o:&( upper( msg ) ) += "<value>"
         ? o:&var.go

Users who want to test it in xHarbour should change:
      o:&( upper( msg ) ) += "<value>"
to:
      o:&( upper( msg ) ) := o:&( upper( msg ) ) + "<value>"
because using macro messages with <op>= operators or pre/post
incrementation/decrementation causes that xHarbour compiler GPFs during
compilation.



###    MULTIVALUE MACROS    ###
===============================
In the early Harbour days was added basic support for multivalue macros
which can be evaluated to list of values, f.e.:
      ? &("1,2,3")
should show:
      1,  2,  3
The implementation of this extension was not accepted by many of Harbour
developers and it was one of the main reasons of the xHarbour fork.
In Harbour it was later fully removed and implemented from scratch using
different internal algorithms and structures. Now Harbour supports multivalue
macros in code like:
      proc main()
         local s1 := "'a', 'b', 'c'", s2 := "1,3", a
         ? &s1
         a := { { "|", &s1, 'x', &s2, 'y' }, 'x', &s2 }
         ? "a[1] items:"
         aeval( a[1], { |x,i| qout( i, x ) } )
         ? "a["+s2+"] =>", a[ &s2 ]
      return

xHarbour which inherited the original implementation after over 6 years
still cannot execute correctly above code.



###    USING [] OPERATOR FOR STRING ITEMS    ###
================================================
xHarbour supports using [] operator to access single characters in
string items. Harbour doesn't by default but it has strong enough
OOP API to allow adding such extension without touching core code
even by user at .prg level. It was implemented in Harbour in XHB.LIB.
This code can be compiled and executed by both compilers:
      #ifndef __XHARBOUR__
         #include "xhb.ch" // add xHarbour emulation to Harbour
      #endif
      proc main()
         local s := "ABCDEFG"
         ? s, "=>", s[2], s[4], s[6]
         s[2] := lower( s[2] )
         s[4] := lower( s[4] )
         s[6] := lower( s[6] )
         ?? " =>", s
      return

Warning!. There is one difference in above implementation introduced
intentionally to Harbour. xHarbour never generates errors for wrong
indexes in [] operator used for string items but simply returns "",
f.e. add to above code:
      ? ">" + s[0] + "<", ">" + s[1000] + "<"
If [] operator is used for other type of items RTE is generated.
Harbour will generate RTE in all cases. If someone needs strict XHB
compatibility here then he should adopt code overloading [] operator
for strings in XHB.LIB for his own preferences removing the RTE.



###    NEGATIVE INDEXES IN [] OPERATOR USED TO ACCESS ITEMS FROM TAIL    ###
============================================================================
xHarbour supports negative indexes in [] operator. They are used
to access items from tail, f.e. aVal[ -1 ] is the same as
aVal[ len( aVal ) - 1 ].
By default Harbour core code does not give such functionality but
it has strong enough OOP API to allow adding such extension without
touching core code even by user at .prg level. It was implemented
in Harbour in XHB.LIB.
This code can be compiled and executed by both compilers:

      #ifndef __XHARBOUR__
         #include "xhb.ch" // add support for negative indexes in Harbour
      #endif
      proc main()
         local s := "ABCDEF", a := {"1", "2", "3", "4", "5", "6"}
         ? s, "=>", s[1], s[2], s[3], s[4], s[5], s[6], "=>", ;
           s[-1], s[-2], s[-3], s[-4], s[-5], s[-6]
         ? a[1], a[2], a[3], a[4], a[5], a[6], "=>", ;
           a[-1], a[-2], a[-3], a[-4], a[-5], a[-6]
      return

Warning! see above note about indexes out of bound used with [] operator
and string items.



###    USING ONE CHARACTER LENGTH STRING AS NUMERIC VALUE    ###
================================================================
xHarbour uses one byte strings as numeric values corresponding to ASCII
value of the byte in a string, f.e.:
      ? "A" * 10 // 650
By default Harbour core code does not give such functionality but
it has strong enough OOP API to allow adding such extension without
touching core code even by user at .prg level. It was implemented
in Harbour in XHB.LIB.
This code can be compiled and executed by both compilers:

      #ifndef __XHARBOUR__
         #include "xhb.ch"
      #endif
      proc main()
         local c := "A"
         ? c * 10, c - 10, c + 10, c * " ", chr( 2 ) ^ "!"
      return

and gives the same results.
Anyhow the emulation is not full here. It works only for .prg code.
In xHarbour standard C API functions/macros were modified to use
one byte string items as numbers. It's potential source of very serious
problems, f.e. ordSetFocus("1") should chose index called "1" or maybe
index 49 or what should return ascan( {49,"1"}, "1" ) (1 or 2)? so
Harbour developers decided to not add anything like that to core code
so in Harbour functions written in C refuse to accept one byte string
as number and code like
      ? str( "0" )
generates runtime error instead of printing '        48'.



###    OOP INTERFACE TO HASH ARRAYS    ###
==========================================
xHarbour allows to access items in hash array using OOP interface.
hVal[ "ABC" ] := 100 can be alternatively written as hVal:ABC := 100.
Using OOP interface is slower then [] operator but it works for all
indexes which are valid upper case [x]Harbour identifiers.
By default Harbour core code does not give such functionality but
it has strong enough OOP API to allow adding such extension without
touching core code even by user at .prg level. It was implemented
in Harbour in XHB.LIB.
This code can be compiled and executed by both compilers:

      #ifndef __XHARBOUR__
         #include "xhb.ch"
      #endif
      proc main()
         local hVal := {=>}
         hVal["ABC"] := 100
         hVal:QWE := 200
         hVal:ZXC := 300
         hVal:QWE += 50
         ? hVal:ABC, hVal:QWE, hVal:ZXC
         ? hVal["ABC"], hVal["QWE"], hVal["ZXC"]
      return

Some Harbour users used to compile Harbour core code with HB_HASH_MSG_ITEMS
macro which enables such functionality directly in core code but it's not
necessary with current code and it exists for historical reasons.
It's possible that in the future support for above macro will be removed
or it will be replaced by runtime switch which can be enabled/disabled for
each hash array separately.



###    $ OPERATOR EXTENSIONS (arrays and hashes)    ###
=======================================================
In Harbour and xHarbour $ operator can be used to check if some key
or hash pair belongs to hash, f.e.:
      ? "abc" $ { "qwe"=>100, "abc"=>200, "zxc"=>300 }
In xHarbour $ operator can be also used to check if value belongs
to array. It works like ASCAN() but with exact comparison for strings,
f.e.:
      ? "abc" $ { "qwe", "abc", "zxc" } // result .T.
      ? "a" $ { "qwe", "abc", "zxc", { "a" } } // result .F.
By default Harbour core code does not allow to use $ operator for arrays
and generate the same as Clipper RT error but it has strong enough OOP API
to allow adding such extension without touching core code even by user at
.prg level. It was implemented in Harbour in XHB.LIB so both compilers
can compile and execute this code:
      #ifndef __XHARBOUR__
         #include "xhb.ch"
      #endif
      proc main()
         ? "abc" $ { "qwe", "abc", "zxc" } // result .T.
         ? "a" $ { "qwe", "abc", "zxc", { "a" } } // result .F.
      return

Warning! XBase++ also support $ operator for arrays but it makes non
         exact comparison so ` "a" $ { "abc" } ' gives .T. in XBase++
         and .F. in xHarbour or in Harbour when xHarbour compatibility
         library is used. Harbour users who need strict XBase++ compatibility
         should create own code to overload $ operators used for arrays
         which will follow exact XBase++ rules.
         CLIP also has some code for such extension but it has two bugs.
         1-st is a semi bug: it uses non exact comparison but reverts
         arguments so ` "abc" $ { "a" } ' gives .T.
         2-nd which is critical: it has wrong stop condition so does
         not stop scanning when locates 1-st matching item. It should
         be fixed and I do not know what will be the final CLIP behavior.



###    NEW BIT OPERATORS    ###
===============================
xHarbour support five new bit operators: &, |, ^^, <<, >>
      <x> & <y>      - bit and
      <x> | <y>      - bit or
      <x> ^^ <y>     - bit xor
      <x> << <y>     - bit shift left
      <x> >> <y>     - bit shift right
Harbour does not have such operators but it has set of bit functions
(HB_BIT*()) which are fully optimized at compile time giving such
functionality without extending language syntax and introducing new
operators:
      <x> & <y>      => HB_BITAND( <x>, <y> )
      <x> | <y>      => HB_BITOR( <x>, <y> )
      <x> ^^ <y>     => HB_BITXOR( <x>, <y> )
      <x> << <y>     => HB_BITSHIFT( <x>, <y> )
      <x> >> <y>     => HB_BITSHIFT( <x>, -<y> )



###    IN, HAS, LIKE OPERATORS    ###
=====================================
xHarbour has three operators defined as identifier: IN, HAS, LIKE
IN is synonym of $ operator and it's translated by lexer to $.
It does not give any new functionality. For portable code use $.
HAS and LIKE are regular expressions operators. In Harbour they have
the same functionality as HB_REGEXHAS() and HB_REGEXLIKE() functions:
      <x> HAS <y>    => HB_REGEXHAS( <y>, <x> )
      <x> LIKE <y>   => HB_REGEXLIKE( <y>, <x> )

Using identifiers as operators is not compatible with Clipper preprocessor
precedence rules and introduces bugs to language syntax so Harbour will
never support it directly as operators. Using operators for regular
expression introduce yet another unpleasant thing. It forces linking
regular expression library with final application even if it does not
use RegEx at all. In Harbour RegEx library is optional and linked only
when user use one of HB_REGX*() functions or explicitly use REQUEST
command.



###    PRE/POST INCREMENTATION/DECREMENTATION AND <op>= OPERATORS    ###
========================================================================
Clipper compiling PRG code with expressions containing operators like
pre and post ++ and -- and also <op>= internally translates them to
normal operators and then generated PCODE for such translated depressions,
i.e.:
   a += 10
   b /= c
   ++d
   e--
   ? --f
   ? g++
is translated to:
   a := a + 10
   b := b / c
   d := d + 1
   e := e - 1
   ? ( f := f - 1 )
   ? g ; g := g + 1

As you can see it causes that modified expression is used more then once.
Usually twice but in some cases (i.e. '? g++' in example above even 3 times.
For simple expression like variables such internal compile time modification
is not visible for users and does not create any difference.
Anyhow if modified expression is complex one which is evaluated and changes
some states (expression with side effects) then such compiler behavior creates
very serious problems. In practice in Clipper it's not possible to use code
like:
   aValue[ ++i ] += field->QUANTITY       // (*)
because this line is translated to:
   aValue[ ++i ] := aValue[ ++i ] + field->QUANTITY
then to:
   aValue[ i := i + 1 ] := aValue[ i := i + 1 ] + field->QUANTITY
for := operator the left side is calculated first so effectively Clipper
generates code which works like:
   aValue[ i + 2 ] := aValue[ i + 1 ] + field->QUANTITY
   i := i + 2
what is very far from the initial (*) expression.
In Harbour we decided to fix it and all expressions modified by pre and
post ++/-- and <op>= operators are evaluated exactly once. It means that
now Harbour users can safely write code with (*) like expressions.
Anyhow if someone needs strict Clipper behavior then -kc Harbour compiler
switch restore it.
Here is simple code which illustrates it:

   proc main()
      local a, i, n
      a := afill( array( 10 ), 100 )
      i := n := 0
      while i < len( a )
         a[ ++i ] -= n++
      enddo
      aeval( a, { |x| qout( x ) } )
   return

This problem is also fixed in xBase++ so it's possible to exchange code
using such expressions between Harbour and xBase++.
In xHarbour the behavior of such expressions in in practice undefined.
In most of cases it behaves like Clipper (i.e. it gives the same wrong
results in above example) but not always because some not cleanly
implemented extensions changed above behavior for chosen operators
executed in some context (i.e. modifying -= to += in above example
causes that xHarbour generates correct code which give the same results
as Harbour and xBase++). Additionally so far no one has tried to control
such things in xHarbour compiler so the behavior of different operators
and/or context where they behaves differently has been changing few times
as side effect of some other modifications.

The correct behavior of pre and post ++/-- and <op>= operators is also
very important for OOP programmers because it guaranties that messages
are send to exactly the same object. This example illustrates it:

   proc main()
      local o
      o := errorNew()
      ? "assign..."
      ?? f( o ):cargo := 0
      ? "predec..."
      ?? --f( o ):cargo
      ? "postinc..."
      ?? f( o ):cargo++
   return
   func f( o )
      ?? " F()"
   return o

In Harbour and xBase++ function F() is executed only once for each
expression but in Clipper and xHarbour once for 'assign', then twice for
'predec' and finaly three times for 'postinc'. In this code function f()
returns the same object 'o' on each call so it's not a problem but code
where the expression can return different objects will not work correctly
so OOP programmers working with Clipper or xHarbour should remember about
it and not create such code.



###    GLOBAL / GLOBAL EXTERNAL (GLOBAL_EXTERN)    ###
======================================================
xHarbour support application wide static variables called GLOBALs.
To declare GLOBAL variable you have to put
      GLOBAL <varname> [ := <initValue> ]
before the 1-st function in compiled .prg module and use -n compiler
switch. In xHarbour GLOBAL variables cannot be declared inside function
body. If user wants to use GLOBAL variable in different .prg module
then he has to add declaration
      GLOBAL EXTERNAL <varname>
to his code before the 1-st function in compiled .prg module and use
-n compiler switch. Also, GLOBAL EXTERNAL declaration cannot be used
in function body. Unlike other variables GLOBALs declared in xHarbour
reserve used names so they cannot be used in the same module in local
function declarations, f.e. xHarbour cannot compile code like:
      GLOBAL var
      GLOBAL EXTERNAL var2
      func F1( var )
         return var * 2
      func F2()
         FIELD var2
         return var2
due to name conflicts. In general users should be careful using the same
names for different type of variables in xHarbour.
GLOBAL variables in xHarbour need static link time bindings so they
do not work with dynamically loaded PCODE functions from .hrb files
or shared libraries (.dll, .so, .sl, .dyn, ...). Just like STATICs
they cannot be accessed by macro compiler.
Harbour does not support GLOBALs.



###    DATETIME/TIMESTAMP VALUES    ###
=======================================
Both compilers support DATETIME/TIMESTAMP values though they use different
implementation.

In Harbour it's new type TIMESTAMP for which VALTYPE() function returns
"T". It has its own HVM arithmetic similar to the one used by DATE type
but not exactly the same. The difference (-) between two TIMESTAMP values
is represented as number where integer part is number of days and fractional
part is time in given day. Non-exact comparison (=, >, <, >=, <=) for
TIMESTAMP and DATA value assumes that both values are equal if the date
part is the same. Such semantic is also respected by native RDDs when
mixed DATE and TIMESTAMP values are used in indexes, seeks, scopes, etc.
When number is added to DATE type then like in Clipper only integer part
increase (decrease) DATE value but when it's added to TIMESTAMP value then
fractional part is also significant. When TIMESTAMP value is added to DATE
value then as result new TIMESTAMP value is calculated. Here is detail
information about relational and arithmetic operators in Harbour.
Timestamp values in relational operators <, <=, >, >=, =, ==
      - When two timestamp values are compared then VM compares date and
        time parts in both values.
      - When date and timestamp values are used in <, <=, >, >=, =
        operations then VM compares only date part in both values.
      - When date and timestamp values are used in == operation then VM
        compares date part in both values and then check if time part
        of timestamp value is 0.
Timestamp values in + and - math operations:
      <t> + <t> => <t>
      <t> - <t> => <n>
      <t> + <n> => <t>
      <n> + <t> => <t>
      <t> - <n> => <t>
      <d> + <t> => <t>
      <t> + <d> => <t>
      <d> - <t> => <n>
      <t> - <d> => <n>
   when number is result or argument of timestamp operation then its integer
   part is a number of days and fractional part represents time in day.

In xHarbour DATE type was extended to hold information about time. Clipper
compatible DATE arithmetic in HVM was modified to respect fractional part
in numbers which was used for time part.
The xHarbour DATETIME implementation introduces incompatibilities to Clipper
(f.e. compare Clipper and xHarbour results in code like: '? date() + 1.125'
so in some cases existing Clipper code has to be carefully adopted to work
correctly with xHarbour but it's fully functional solution though it needs
some minor fixes in conversions between datetime values and numbers.

The difference between Harbour and xHarbour can be seen in this code:
      proc main()
         local dVal, tVal
         dVal := date()
         tVal := dVal + {^ 02:00 }  // {^ 02:00 } timestamp
                                    // constant, see below
         ? valtype(dVal), valtype(tVal)
         ? dVal; ? tVal
         dVal += 1.125  // In Clipper and Harbour it increases
                        // date value by 1
         tVal += 1.25   // it it increases timestamp value by 1 day
                        // and 6 hours
         ? dVal; ? tVal
         ? dVal = tVal  // In Harbour .T. because date part is the same
         ? date() + 0.25, date() + 0.001 == date()
      return
Harbour shows:
      D T
      05/20/09
      05/20/09 02:00:00.000
      05/21/09
      05/21/09 08:00:00.000
      .T.
      05/20/09 .T.

and xHarbour shows:
      D D
      05/20/09
      06/25/21 02:00:00.00
      05/21/09 03:00:00.00
      06/26/21 08:00:00.00
      .F.
      05/20/09 06:00:00.00 .F.

Recently to xHarbour "T" type was introduced with some of Harbour TIMESTAMP
code in RDD but VM was not modified to follow Harbour/RDD modifications.
So now some parts of xHarbour are not synced and I cannot say what is the
goal of DATETIME values and their arithmetic in xHarbour VM for the future.



###    LITERAL DATE AND TIMESTAMP VALUES    ###
===============================================
Both compilers tries to support VFP like datetime constant values
in the following format:
      { ^ [ YYYY-MM-DD [,] ] [ HH[:MM[:SS][.FFF]] [AM|PM] ] }
In Harbour and VFP the following characters can be used as date delimiters:
"-", "/" and "."
xHarbour supports only "/" as date delimiter.
There is no limit on number of digits in YYYY, MM, DD, HH, MM, SS, FFF
parts. Important is only their value. This is the format in semi PP notation:
      { ^ <YEAR> <sep:/-> <MONTH> <sep:/-> <DAY> [[<sep2:,>]
        [ <HOUR> [ : <MIN> [ : <SEC> [ . <FRAQ> ] ] ] [AM|PP] ] }
NOTE: there is one important difference between Harbour and VFP/xHarbour in
decoding above format. In VFP and xHarbour when date part is missed then
it's set by default to: 1899-12-30 so this code:
      { ^ 12:00 }
gives the same results as:
      { ^ 1899/12/30 12:00 }
Harbour does not set any default date value when timestamp constant contains
only time part.
Harbour supports VFP syntax only in Compiler. It's not supported in
macrocompiler and it can be disabled in the future so it's not suggested
to use with Harbour programs.

Only Harbour supports date constant (in compiler and macrocompiler) in the
form d"YYYY-MM-DD" f.e.:
      d"2009-05-20"
Also delimiter "/" or "." can be used instead of "-".

Only Harbour supports timestamp constant (in compiler and macrocompiler)
in the form t"YYYY-MM-DD HH:MM:SS.fff"
The exact accepted timestamp pattern is is:
      YYYY-MM-DD [H[H][:M[M][:S[S][.f[f[f[f]]]]]]] [PM|AM]
f.e.:
      tValue := t"2009-03-21 5:31:45.437 PM"
or:
      YYYY-MM-DDT[H[H][:M[M][:S[S][.f[f[f[f]]]]]]] [PM|AM]
with literal "T" as date and time part delimiters (XML timestamp format),
f.e.:
      tValue := t"2009-03-21T17:31:45.437"
The following characters can be used as date delimiters: "-", "/", "."
if PM or AM is used HH is in range < 1 : 12 > otherwise in range < 0 : 23 >

Harbour compiler and macrocompiler support also date constants in the
form 0dYYYYMMDD f.e.:
      0d20090520.



###    EXTENDED LITERAL STRING IN COMPILER AND MACROCOMPILER    ###
===================================================================
Harbour and xHarbour support extended strings with C like escaped values
as e"...", f.e.:
      ? e"Helow\r\nWorld \x21\041\x21\000abcdefgh"
They works in compiler and macrocompiler but xHarbour macrocompiler does
not support strings with embedded 0 so they are not fully functional here.



###    SYMBOL ITEMS AND FUNCTION REFERENCES    ###
==================================================
Harbour supports SYMBOL items ( VALTYPE(funcSym) == "S" ) which can be used
as function or message references. They have similar functionality to
SYMBOL objects in Class(y) and understands NAME, EXEC and EVAL messages.
They can be created literally by compiler using @<funcName>(), f.e:
      funcSym := @str()
and in such case they also create explicit reference (link time binding)
to given functions or by macro compiler, f.e.:
      funcSym := &("@upper()")
what does not create any explicit bindings. This is simple code which
uses symbol items:
      proc main()
         local funcSym
         funcSym := @str()
         ? funcSym:name, "=>", funcSym:exec( 123.456, 10, 5 )
         funcSym := &("@upper()")
         ? funcSym:name, "=>", funcSym:exec( "Harbour" )
      return

xHarbour does not have such functionality though it can create at
compile time function references using a little bit different syntax:
      ( @<funcName>([<anyExpList,...>]) )
the different syntax is side effect of bugs in grammar rules only and
probably will be fixed somewhere (macrocompiler does not support such
syntax at all).
In xHarbour above code creates pointer item ( VALTYPE(funcSym) == "P" )
which can be used in some cases like in Harbour but because xHarbour VM
does not know if given pointer item is function reference or not then in
such context xHarbour has to accept any pointer items as function
references so any user mistake can cause GPF or some HVM structure
corruptions.



###    OOP SCOPES    ###
========================
Harbour and xHarbour supports scopes in class declaration.
It's possible to declare exported, protected and hidden messages and then
at runtime scopes are checked. Anyhow there are important differences in
both implementation. In xHarbour for internal implementation .prg module
symbol table was used so all classes declared in the same .prg file (or
#included from the same .prg file) and also all other functions in this
file have exactly the same scope and works like internal methods which
can access hidden data. Harbour is Class(y) compatible so each class
and external functions are separated. If programmer needs to give some
rights to other classes or external functions then he can explicitly
declare friend classes and functions by simple:
      FRIEND CLASS <ClassName,...>
and:
      FRIEND FUNCTION <FuncName,...>
It's also possible to give unprotected access to all other functions
declared in the same .prg module just like in xHarbour by using
'MODULE FRIENDLY' clause in class declaration, i.e.:
      CLASS myCls FROM parent MODULE FRIENDLY
         [...]
      ENDCLASS
So in Harbour all scopes aspects are explicitly controlled by programmer
and AFAIK there are no side effects forced by internal implementation.
Important is also the fact in Harbour each codeblock block created explicitly
or by macrocompiler inherits rights from functions when it was created
and then has the same scope. It means that it's not possible to break scope
protection using codeblocks and it's very important functionality for real
private (hidden) data because it makes it accessible for codeblocks created
by object methods.

AFAIK xHarbour does not have such functionality and codeblock scopes depends
on module symbol table.



###    OOP AND MULTIINHERITANCE    ###
======================================
Harbour and xHarbour support multiple inheritance just like Class(y).
Anyhow only Harbour correctly resolves possible name conflicts
by casting. xHarbour and Class(y) in Clipper does not work correctly
if few ancestors have instance variables with the same name even if
explicit casting is used. This code illustrate the problem:

      #ifdef __HARBOUR__
         #include "hbclass.ch"
      #else
         #include "class(y).ch"
      #endif
      proc main()
         local o
         o := mycls():new()
         o:var := "VAR"
         o:cls1:var := "VAR1"
         o:cls2:var := "VAR2"
         o:cls3:var := "VAR3"
         ? o:var, o:cls1:var, o:cls2:var, o:cls3:var
         o:var := "[var]"
         o:cls1:var := "[var1]"
         o:cls2:var := "[var2]"
         o:cls3:var := "[var3]"
         ? o:var, o:cls1:var, o:cls2:var, o:cls3:var
      return

      CREATE CLASS MYCLS FROM CLS1, CLS2, CLS3
      EXPORTED:
         VAR      VAR
      ENDCLASS

      CREATE CLASS CLS1
      EXPORTED:
         VAR      VAR
      ENDCLASS

      CREATE CLASS CLS2
      EXPORTED:
         VAR      VAR
      ENDCLASS

      CREATE CLASS CLS3
      EXPORTED:
         VAR      VAR
      ENDCLASS

Expected results are:
      VAR VAR1 VAR2 VAR3
      [var] [var1] [var2] [var3]
and Harbour shows such results.
xHarbour and Class(y) gives wrong results:
      VAR VAR3 VAR3 VAR3
      [var] [var3] [var3] [var3]

Please note that Harbour does not make any tricks with compile time static
bindings. It uses only dynamic bindings and resolves all problems with
instance area super casting.

Correct instance area super casting is very important functionality when
program is created by few programmers and it's necessary to create classes
which inherits from many other ones written be different programmers to
resolve possible name conflicts. So for bigger projects is one of the most
important thing.



###    OOP AND PRIVATE/HIDDEN DATAs    ###
==========================================
In bigger projects where different programmers work on different classes
which are later used as ancestors of some new classes it's extremely important
to give support for real private (hidden) data for each class programmer
which will not cause problems if name conflicts appears. Otherwise
each method and instance variable name used by each programmer has to be
agreed with project manager to eliminate possible name conflicts. In really
big projects such name conflicts can be nightmare a which eliminates some
languages.

Harbour resolves this problem automatically. All hidden variables and
messages are automatically casted to the class which is the owner of
calling code. To make it working Harbour needs fully functional scope
protection and multiinheritance (see OOP SCOPES and OOP AND MULTIINHERITANCE).
Everything is done automatically without any explicit casting or other
then declaring variables or methods as HIDDEN source code modification.
This code illustrates it. We have two classes and each of them has its own
_private_ 'temp' instance variable and 'set' method. 3-rd class inherits
from both of them and executes it's public methods which internally
access hidden ones but without any explicit casting. In Harbour all is done
automatically by HVM:

      #include "hbclass.ch"
      PROC MAIN()
         LOCAL o
         o := mycls():new()
         o:action( "X" )
         o:action( "Y" )
         WAIT
      RETURN

      CREATE CLASS MYCLS FROM CLS1, CLS2
      EXPORTED:
         METHOD   ACTION
      ENDCLASS

      METHOD ACTION( x )
         ? ::plus( x )
         ? ::minus( x )
      RETURN Self


      CREATE CLASS CLS1
      HIDDEN:
         VAR      TEMP
         METHOD   SET
      EXPORTED:
         METHOD   PLUS
      ENDCLASS

      METHOD SET( x )
         IF ::TEMP == NIL
            ::TEMP := "C1"
         ENDIF
         ::TEMP += ":" + UPPER( x )
      RETURN Self

      METHOD PLUS( x )
      RETURN ::SET( x ):TEMP


      CREATE CLASS CLS2
      HIDDEN:
         VAR      TEMP
         METHOD   SET
      EXPORTED:
         METHOD   MINUS
      ENDCLASS

      METHOD SET( x )
         IF ::TEMP == NIL
            ::TEMP := "c2"
         ENDIF
         ::TEMP -= ">" + LOWER( x )
      RETURN Self

      METHOD MINUS( x )
      RETURN ::SET( x ):TEMP

Expected results from this code are:
         C1:X
         c2>x
         C1:X:Y
         c2>x>y

and Harbour shows exactly such results.
Neither xHarbour nor Class(y) has such functionality what seems to be very
serious limitation for big projects.
In xHarbour above code shows:
      c2>x
      c2>x>x
      c2>x>x>y
      c2>x>x>y>y

To compile it with classy it's necessary to make some minor modification
and change in CLS1:
      METHOD SET
to:
      MESSAGE SET METHOD SET1
and in CLS2
      METHOD SET
to:
      MESSAGE SET METHOD SET2
updating method names in the code.
Then it can be compiled but like in xHarbour only one SET method is visible
and only one TEMP instance variable so it shows only:
      C1:X
and then generates scope violation error because unlike xHarbour Class(y)
does not define all classes as "MODULE FRIENDLY".



###    OOP AND CLASS OBJECT/CLASS MESSAGES    ###
=================================================
Neither Harbour nor xHarbour supports class objects, class messages
and class instance variables like Class(y) does.

But both compilers support shared variables which can be shared with
all object of the same class or shared variables which can be
shared with all object of given class and its descendants.
Class methods are not supported at all by both compilers (they cannot be
without class object) though xHarbour wrongly use class messages as normal
messages.

In the future we plan to introduce real class objects like in Class(y) and
some other languages (i.e. XBASE++) so it's important for portability to write
code which is Class(y) friendly and never use class function directly as
constructor, i.e. instead of writing code like:
      o := mycls( p1, p2, p3 )
programmer should use:
      o := mycls():new( p1, p2, p3 )
otherwise the code will not work with future Harbour versions supporting
real class objects.



###    TYPED OBJECT VARIABLES    ###
====================================
Harbour support strong typed object variables, f.e.:
      CREATE CLASS MyClass
         VAR   var1 AS INTVAR
         VAR   var2 AS NUMERIC
         VAR   var3 AS DATE
         VAR   var3 AS CHARACTER
      ENDCLASS
And validates assigned values at runtime just like in Class(y).
xHarbour can compile above code but AS <type> is only source code
decoration for this language.



###    OBJECT DESTRUCTORS    ###
================================
Both Harbour and xHarbour support object destructors and both
compilers uses reference counters and garbage collector to
execute destructors. Anyhow the low level implementation is
different in the two compilers. In Harbour HVM is reentrant safe
so the implementation of destructors is much simpler. It also
keeps full control about reference counters and detects user
code errors bound with complex items in .prg and C code what
greatly helps to detect problems in user code or 3-rd party
libraries and gives protection against internal HVM memory
corruption. In the last years it also helped to located few
very hard to exploit bugs in core code.

In xHarbour there is some basic protection but it was never
fully functional. Sometimes it may cause that internal error
message is generated but in most of cases internal HVM memory
is silently corrupted. A good example which illustrates what
may happen is in Harbour SVN in harbour/tests/destruct.prg
This code can be compiled and executed by Harbour and xHarbour.
But only Harbour version detects user errors generating RTE
and is necessary in all cases to keep internal memory cleaned
protecting against corruption. The xHarbour version behavior
is random because this .prg code corrupts internal HVM memory.
It's possible that it will be executed without any visible
errors or it will generate GPF or internal error. But in all
cases it can be well seen using tools like valgrind or CodeGuard
that it corrupts internal memory so the results are unpredictable.
I.e. this is part of valgrind log generated during execution
of above destruct.prg compiled by xHarbour:

   ==22709== Invalid read of size 2
   ==22709==    at 0x426E235: hb_gcItemRef (garbage.c:646)
   ==22709==    by 0x425EDE5: hb_memvarsIsMemvarRef (memvars.c:2396)
   ==22709==    by 0x426E674: hb_gcCollectAll (garbage.c:847)
   ==22709==    by 0x426E899: HB_FUN_HB_GCALL (garbage.c:1179)
   ==22709==  Address 0x485b096 is 22 bytes inside a block of size 56 free'd
   ==22709==    at 0x4023B7A: free ()
   ==22709==    by 0x42837A9: hb_arrayReleaseBase (arrays.c:1588)
   ==22709==    by 0x428381A: hb_arrayRelease (arrays.c:1602)
   ==22709==    by 0x4256B94: hb_itemClear (fastitem.c:248)
   [...]
   ==22709== Invalid write of size 2
   ==22709==    at 0x426E4C6: hb_gcItemRef (garbage.c:652)
   ==22709==    by 0x425EDE5: hb_memvarsIsMemvarRef (memvars.c:2396)
   ==22709==    by 0x426E674: hb_gcCollectAll (garbage.c:847)
   ==22709==    by 0x426E899: HB_FUN_HB_GCALL (garbage.c:1179)
   ==22709==  Address 0x485b096 is 22 bytes inside a block of size 56 free'd
   ==22709==    at 0x4023B7A: free ()
   ==22709==    by 0x42837A9: hb_arrayReleaseBase (arrays.c:1588)
   ==22709==    by 0x428381A: hb_arrayRelease (arrays.c:1602)
   ==22709==    by 0x4256B94: hb_itemClear (fastitem.c:248)
   [...]
   ==22709== Invalid read of size 4
   ==22709==    at 0x426E223: hb_gcItemRef (garbage.c:603)
   ==22709==    by 0x425EDE5: hb_memvarsIsMemvarRef (memvars.c:2396)
   ==22709==    by 0x426E674: hb_gcCollectAll (garbage.c:847)
   ==22709==    by 0x426E899: HB_FUN_HB_GCALL (garbage.c:1179)
   ==22709==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
   ==22709==  Process terminating with default action of signal 11 (SIGSEGV)
   ==22709==  Access not within mapped region at address 0x0

Harbour executes above code cleanly without any errors:

   ==28376== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 3 from 1)
   ==28376== malloc/free: in use at exit: 0 bytes in 0 blocks.
   ==28376== malloc/free: 6,164 allocs, 6,164 frees, 10,323,064 bytes allocated.
   ==28376== All heap blocks were freed -- no leaks are possible.

Missing protection and error detection is not only problem for programmers.
It also causes that in xHarbour core code few bad bugs have not been fixed
so far though they exist for years. It also extremely time consuming problem
for core developers when users or 3-rd party developers reports problems with
some memory corruption and it's necessary to locate the reason.
I remember how much time I lost for such things working on xHarbour
in the past so later when I moved to Harbour it was one of the main goal
for me to resolve the problem.

In xHarbour there is disabled code which was designed to detect such
problems and it can be enabled by setting HB_ARRAY_USE_COUNTER_OFF macro
but it causes horrible runtime overhead and was never finished to be ready
as production extension. Such solution seems to be dummy way so I do not
believe that it will be ever finished.



###    SCALAR CLASSES    ###
============================
Both compilers support scalar classes and allows to add OOP functionality
to native types like numeric, character, array, hash, codeblock, date, ...
It's possible to overload default scalar classes provided by Harbour and
xHarbour or use ASSOCIATE CLASS command to bound any class with some native
type. It's also possible to overload the behavior of some operators if it's
not already defined for given types. Anyhow it's not possible to change
operator precedence which is the same for all types and defined at compile
time.



###    RUNTIME CLASS MODIFICATION    ###
========================================
Harbour and xHarbour use dynamic bindings so it's possible to modify class
definitions at runtime. To avoid possible very serious problems which can
be caused by such class modification Harbour provides LOCKED clause in class
definition. xHarbour does not have such functionality and even documented
command like OVERRIDE CLASS or EXTEND CLASS as official features though
they break inheritance scheme and never worked correctly in the whole
xHarbour history. They are available in Harbour only in xHarbour compatible
library after including xhbcls.ch for porting existing xHarbour code to
Harbour and work exactly like in xHarbour having the same problems so they
are not planned to be part of Harbour core code.



###    ARRAY AND STRING PREALLOCATION    ###
============================================
Both compilers uses array and string preallocation to optimize resize
operation, f.e. in code like:
      s := ""
      for i := 0 to 255
         s += chr( i )
      next
or:
      a := {}
      for i := 0 to 255
         aadd( a, repl( chr( i ), 5 ) )
      next
the string inside 's' variable and array inside 'a' variable are not
resized 256 times but much seldom. It gives noticeable speed improvement
with the cost of additional memory used by application. Anyhow in Harbour
internal HVM and item structures are smaller then in xHarbour so usually
the total memory usage in Harbour, even with extensive preallocation, is
smaller then in the same application compiled by xHarbour with disabled
string and array preallocation.

The speed difference can be really huge in some application. It's possible
to create test code which will be 100 times faster due to this feature
so it's really important functionality.

The internal implementation of these optimization in both compilers is
different. In xHarbour only chosen left side expression are optimized
(in practice only 'var += ...' and 'arr[ <n> ] += ...') but in Harbour
all with the exception to 'o:<msg> += ...' optimization which can be
enabled only optionally at Harbour compile time using HB_USE_OBJMSG_REF
macro (there is no -k? option to control it by user yet), it can be well
seen in code like:
      proc main()
         memvar v
         local t, i, s
         private v := ""
         t := secondsCPU()
         s := "v"
         for i := 1 to 100000
            &s += l2bin( i )
         next
         t := secondsCPU() - t
         ? "time:", t, "sec."
      return
it's enough to compare time difference:
      Harbour  0.13 sec.
      xHarbour 15.91 sec.



###    DIVERT STATEMENT    ###
==============================
xHarbour supports

      DIVERT TO <pFuncPointer> | <pMessagePointer> ;
             [ OF <oNewSelfOrNil> ] [ FLAGS <nDivertFlags> ]

statement which allow to switch current function/method execution context
to the new one. The detail description of the above extension can be found
in xHarbour ChangeLog.027 at:
      2007-12-07 22:30 UTC-0500 Ron Pinkas <ron/at/xharbour.com>

It's very dangerous extension which needs really deep knowledge about
compiler and HVM internals to use it safely, because, for called function
HVM does not create new stack frame. Instead it simply adopts current
one. It means that programmer using above statement have to exactly know
HVM registers used by different language constructions and which ones are
correctly saved and then restored or cleaned by DIVERT statement.
Any mistake can cause unpredictable behavior like GPF or internal HVM
structure corruption.

The worst thing in above extension is the fact that it effectively blocks
using HVM stack by compiler for optimization (i.e. to store some values in
temporary stack variables) and also adding some new language constructions
which may try to use HVM stack as temporary holder for some values so in
the future such new language constructions will have to use explicitly
defined new HVM registers just like the ones defined in current xHarbour
VM for WITH OBJECT or FOR EACH statements increasing unnecessary overhead
also in code which will not use either DIVERT TO or new language
constructions, so it introduces very serious problem for further compiler
improvement and syntax extensions.

In theory it should be faster method to switch the execution context
than normal call but in practice it introduces new overheads directly
or indirectly (by blocking some possible improvements) so in summary
it causes slowness.

Probably only some examples which uses DIVERT TO with bigger number
of parameters in a loop can show some speed improvement but in real
life application in most of cases it's necessary to call functions/
procedures/methods in standard way which will be slower so in fact
it reduces overall performance.

I tried to make some tests with above statement but quite fast I exploited
few bugs in the pure DIVERT TO implementation in HVM so I was not able
to make serious tests for interactions with different language constructions.
xHarbour simply GPFs or makes some other strange things before and I do
not have time to check what exactly is broken in this case or analyze
possible bad side effects looking at xHarbour core code.
I only scanned xHarbour core code for real life DIVERT TO usage and
the only one practical usage of above extension in xHarbour core code
is workaround for missing '...' operator working like in Harbour
(see: FUNCTIONS WITH VARIABLE NUMBER OF PARAMETERS) so I do not think
that Harbour users will ever ask about it.

For all of the above reasons I think that Harbour will never have
anything like that at least as documented language extension.



###    NAMESPACEs    ###
========================
xHarbour has compile time support for namespaces which uses by default
static bindings with optional support for dynamic bindings in HVM which
is necessary for .hrb and macrocompiler. To exchange information about
defined by user namespaces xHarbour generates <namespace>.xns files
during compilation of .prg ones which define public functions in given
namespace. These files are automatically included by xHarbour compiler
if namespace is requested in PRG code. They are also necessary for C
compiler to resolve static bindings so they are explicitly included in
.c files generated by xHarbour.

xHarbour compiler seems to always overload existing .xns files and
there is no description for format of .xns files. It means that without
some manual modification namespace can be defined only in single .prg
file.

In xharbour CVS in doc/namespace.txt is basic description for used syntax
but I cannot find any deeper information about expected functionality so
it's hard for me to comment on it. I simply do not know the final goals
for this feature and why it was introduced. I do not even know if it has
been finished or not and some modifications/extensions are planned.
After few tests I can only say that it does not work correctly on case
sensitive file systems so probably it's not widely used functionality
if no one reported the problem so it has not been fixed.
If xHarbour developers can make some short description for this feature
then I'll add it to this text.

Harbour does not have such functionality and AFAIK none of developers
has planed to add similar to xHarbour namespace functionality.
Also none of Harbour users requested about it so seems that it will
not be added to Harbour.

Anyhow Harbour users and developers requested about functionality
which allow to control public symbol scopes at runtime.
It's necessary to create limited in functionality (reduced list of
public functions which can be accessed) environment to execute untrusted
code, i.e. loaded from .hrb files or dynamically compiled and registered
.prg code.

In the future I plan to work on such functionality. If possible I'll
try to resolve also some other problems which can appear in programs
loading and executing external code like support for thread local public
symbols. It should greatly help to improve some programs like HTTP
servers. Probably this code should also help in implementing dynamic
namespace support so maybe we introduce it but seems that it will be
completely different thing then in current xHarbour.



###    MULTI WINDOW GTs AND RUNTIME GT SWITCHING    ###
=======================================================
Harbour allows to initialize more then one GT drivers at runtime
and switch between active GTs in single application. It can be
done using the following functions:
      HB_GTCREATE( <cGtName>, [<nStdIn>], [<nStdOut>], [<nStdErr>] ) -> <pGT>
      HB_GTSELECT( <pGT> ) -> <pPrevGT>
if all references to allocated GT drivers are cleaned in visible items
then GT windows is automatically closed.

Number of allocated GTs depends on existing resources. I.e. some GTs like
GTWVT or GTXWC for each hb_gtCreate() call creates new console window so
application can create many different windows and switch between them.
It's very nice feature for MT mode because current GT driver in Harbour
is thread attribute so each thread in Harbour can allocate it's own
independent console window if platform support GT driver with such
possibility (graphical GTs like GTWVT, GTXWC, GTWVG).
Some GTs like GTWIN, GTOS2 or GTDOS have to share the same resources
(OS console windows, video screen buffer) so their parallel usage is
limited but allocating them does not block allocating any other GTs,
i.e. application can use one GTWIN console (MS-Windows console window)
and many GTWVT console windows.

This feature can be used also with non graphical GTs which uses stream
input/output like GTTRM, GTCGI or GTSTD, i.e. user can create terminal
server and for each internet connection allocate new GTTRM or GTSTD
driver or can execute CGI procedures in newly allocated GTCGI drivers
with direct output redirection to given handle passed to hb_gtCreate().
In some other cases it can be usable to execute some part of code using
GTNUL driver to pacify all output and disable input.

This functionality is also supported by virtual GT drivers GTCTW used
to emulate Clipper Tools window system. GTCTW has also separate support
for MT mode and keep current window as thread attribute so each thread
can use its own CT window independently.

In Harbour contrib there are some GTs and mixed GT/GUI libraries which
extensively use multi window functionality like GTWVG, GTQTC.

Not all GTs in Harbour has been adopted to work simultaneously in multi
thread mode with more then one instance so user should not allocate more
then one instance of the following GTs GTDOS, GTWIN, GTOS2, GTCRS, GTSLN,
GTPCA, GTALLEG.

These GTs are limited by resources or have alternative GT drivers with
similar functionality so it's hard to say if Harbour developers invest
time in updating them.

The above functionality is local to Harbour and does not exists in
xHarbour. Anyhow xHarbour borrowed from Harbour most of new GT code so
it should be possible to implement (at least partially due to limited
in xHarbour MT mode) some of the above features.



###    MULTI THREAD SUPPORT    ###
==================================
Both compilers support multi thread programming and in both
compilers to use threads it's necessary to use MT version of VM.

In Harbour it's only single HVM library because in both versions
(ST and MT) Harbour VM gives exactly the same public API so it's
not necessary to create separate versions of any other libraries
or create different .prg code for MT and ST modes.
The separate ST version of VM can be linked with applications
which do not create any threads to improve the performance because
it's a little bit faster then MT VM. The speed difference can be
compared using speedtst.prg (results below). For other platforms/C
compilers it may be a little bit bigger or smaller anyhow it's
not very huge and Harbour in MT mode is still much faster then
xHarbour in ST mode.

For .prg programmer the only difference between applications linked
with single thread VM and multi thread VM is in hb_threadStart()
results. This function linked with ST VM always fails and new thread
is never created.

Harbour gives set of functions with hb_thread*() and hb_mutex*()
prefix to manage threads and synchronize them. It also supports
THREAD STATIC variables what allows to easy adopt any existing code
to MT mode. Whole Clipper compatible .prg API using internally static
variables like GET system was adopted for MT mode and work with thread
local statics.

Most of core code had been rewritten to be reentrant safe so it does
not have to be protected by any mutexes what greatly helps in
scalability. Below is a short description which shows which resources
are thread local which ones are shared and how they are initialized:
      Thread attributes inherited from parent thread:
         - code page
         - language
         - SETs
         - default RDD
         - active GT driver/console window
         - I18N translation set
      Thread attributes initialized to default value:
         - public variable GetList := {}
           (of course if public variables are not shared with parent
           thread).
         - error block (initialized by calling ERRORSYS())
         - math error handler and math error block
         - default macro compiler features setting (controlled by
           hb_setMacro())
         - RDDI_* settings in core RDDs (some 3-rd party RDDs may use
           global settings)
         - thread static variables
      Resources which can be shared or copied form parent to child thread
      on user request when thread is created:
         - public variables
         - private variables
      Global resources synced automatically:
         - functions and procedures
         - class definitions
         - RDDs
         - GT drivers
         - lang modules
         - codepage modules
      Global resources:
         - .prg static variables (Harbour core code does not use them)
      Thread local resources:
         - memvar (public and private) variables (except the ones which
           are shared with parent threads)
         - work areas
         - thread static variables
In practice the only one not synced automatically element in Harbour
is write access to the same item with complex variable. This part
have to be synced by user and automatic synchronization here will
strongly reduce scalability.

Harbour also support xBase++ compatible MT API with SYNC and CLASS SYNC
methods, SIGNAL and THREAD classes, workarea zones and thread functions.
It allows to easy port xbase++ code to Harbour. The main difference
between Harbour and xbase++ is write protection to complex items.
Harbour gives only read protection so threads can access the same
complex variable simultaneously without any restrictions as long
as it's not modified (assigned) by some other thread. Assign operations
to variable which can be accessed simultaneously by other threads
need explicit synchronization by user code. In xBase++ it should
be synced automatically so users have to use own synchronization
only to keep synced his application logic.

For sure xBase++ is more user friendly here. Anyhow such solution
strongly reduce scalability so the cost of such automatic protection
is very high. Additionally in most of cases simultaneous access and
assign operations on the same complex variable need additional
user synchronization to eliminate race condition so it does not
help much in real programming. Anyhow it's interesting in some
cases so it's possible that we add such functionality to Harbour
in alternative HVM library to not reduce current performance.

In xHarbour for MT mode it's necessary to use different set of RTL libraries
not only MT version of VM lib. MT mode change the behavior of core features
and many parts of xHarbour core code have been never adopted to MT mode so
it's hard to clearly define which resources are thread local and which ones
are global and what should be synchronized by user. Many things are not
synced at all and race conditions can cause internal HVM structure
corruptions, i.e. class creation code where many threads can try to
register the same class simultaneously so it needs yet a lot of work to
reach real production functionality. It also needs serious cleanup because
there are race conditions even in basic MT API. Missing from the beginning
clear definition of MT mode caused that in last years not all xHarbour
modifications were MT safe and some of them even broke existing functionality.
xHarbour MT API is present only in MT version of HVM and supports only some
subset of Harbour functionality.
The only one xHarbour MT feature not existing in current Harbour code is
support for:
      CRITICAL [STATIC] FUNCTION | PROCEDURE( [<xParams,...> )
but in current xHarbour CVS such functionality is broken. The problem can
be seen at compile time so looks that none of core developers were using
it in last years. This feature in xHarbour was implemented only in .c
code generated by xHarbour compiler (-gc[0-3] output) so it was never
working with .hrb (-gh) files or some interpreters like xBaseScript.
xHarbour has support for SYNC methods which were designed to replicate
xBase++ functionality but their real behavior is not xBase++ and Harbour
compatible at all. It seems to be close to SYNC CLASS method in xBase++
and Harbour though it's not exactly the same. There is no support for
real SYNC methods like in xBase++ and xBase++ MT programming functions
and classes.

In summary MT mode in xHarbour looks like a work in progress, started few
years ago and never finished. Instead some other core code modifications
in last years systematically introduced new non MT safe code and in few
cases even broke existing MT extensions.

I cannot say if xHarbour developers plan to make sth with it.
Now I cannot even describe it well because looking at the xHarbour source
I do not know what is intentional decision, what is a bug in implementation
and what is unfinished code, i.e. such code:
      JoinThread( StartThread( @func() ) )
in xHarbour has race condition and can generate RT error if new thread
finished before join operation begin. For me it's bad design decision
or fatal side effect of wrong implementation but I do not plan to guess.
This is very trivial example and I see in core code much more serious
and more complicated problems I had to resolve in Harbour which are not
touched at all in xHarbour.

I know that some people created MT programs using xHarbour but for me
current xHarbour MT mode is not production ready. At least it is very
far from current Harbour functionality and quality.



###    HARBOUR TASKS AND MT SUPPORT IN DOS    ###
=================================================
Harbour supports threads also in system without native thread support.
It has special code which emulates task switching so Harbour MT programs
can be compiled and executed in systems like DOS.
Harbour tasks are used by default in DOS builds in MT HVM.
They can be enabled in all other builds instead of native thread support
though on some platforms not tested so far which does not support POSIX
<ucontext.h> task code may need some modifications to safe/restore execution
context.

For .prg code Harbour tasks behave like native threads but it's important
to know that Harbour does not add in some magic way MT support to OS-es
like DOS or to CRTL. If one task executes OS or CRTL code then the context
is not switched. It will be switched when it will return from such code.



###    BACKGROUND TASK    ###
=============================
In xHarbour after setting _SET_BACKGROUNDTASKS to .T. main HVM loop
periodically (once per some number of iterations defined by user in
_SET_BACKGROUNDTICK) executes user defined code interrupting current
context. This functionality is called Background Tasks. Because it's
defined in main HVM loop then it does not work for code generated with
-gc3 compiler switch. This functionality is also enabled for MT HVM
version though with different internal semantic. When _SET_BACKGROUNDTASKS
is .T. all threads try to execute user defined code using thread local
HVM loop counter initialized to some fixed value (HB_VM_UNLOCK_PERIOD=5000)
instead of_SET_BACKGROUNDTICK.

Harbour does not have such functionality and probably will not have in
such form for the following reasons:
      1. it works only from HVM loop so it will not work for code which
         does not use it, f.e. generated with -gc3 compiler switch
      2. such implementation causes same speed overhead even if programmer
         does not use background tasks
      3. the frequency of executing background task is very irregular
      4. in MT programs background tasks are executed in different way
         then in ST mode and the frequency depends on number of threads
         and their activity.
      5. in Harbour MT programs such functionality can be very easy
         implemented by user without touching core code.
      6. it's possible that in the future HVM will support asynchronous
         events what is more general mechanism and also allows to easy
         implement background tasks without touching core code.



###    CODEBLOCK SERIALIZATION / DESERIALIZATION    ###
=======================================================
xHarbour has support for codeblock serialization. It's done by simple
storing compiled codeblock body (PCODE) in string variable and then
restoring it from such variable. Harbour does not support it intentionally
and in such form it will never be supported by HVM because it can work
only in some very limited situations and then can be source of very bad
bugs like internal HVM memory corruption or absolutely unexpected runtime
behaviors. Compiled PCODE contains references to symbol tables or even
memory addresses (i.e. macrocompiled codeblocks). If application stores
such codeblock in some external holder, i.e. memo file then after restoring
by different instance of the same application memory addresses can be
different so for macrocompiled codeblocks it's not safe at all. For
codeblocks compiled by compiler it can work until exactly the same
application is used. But any modifications in the code can change
symbol table so after restoring the result can be unpredictable.
Codeblock like:
      {|x| qout( x ) }
can be defined in code where just after QOUT() in symbol table is FERASE().
Small modification in application code can cause that symbols will be
moved and above codeblock after restoring will effectively execute FERASE()
instead of QOUT(). What does it mean for programs which makes sth like:
      eval( cb, "The most import file is:" )
      eval( cb, cDataFile )
      eval( cb, "Please make backup." )
I do not have to explain.
With full respect to xHarbour users IMO such "extension" is giving knife
into monkey hands.
Codeblock serialization can be implemented in safe way but it needs at
least some partial PCODE (de|re)compiler. If Harbour users find codeblock
serialization as important extension then maybe we create such PCODE
recompiler and such functionality will be added. Anyhow it will be necessary
to keep recompiler code synced with compiler and it will be additional job
in any future compiler modifications which can change generated PCODE.



###    NATIVE RDDs    ###
=========================
Both compilers shares most of RDD code. The differences are only in
some recently added features to both compilers like timestamp values
or MT mode support which are in xHarbour not fully implemented yet.
Some of the most recent modifications in xHarbour core code are local
to xHarbour only and I hope will never be part of Harbour core code.
In xHarbour RDDs starting with "RE" prefix, i.e. REDBFCDX are simple
copy of original RDDs where hb_file*() functions are replaced with
hb_fileNet*() so these are the same RDDs but with different file
transfer layer. Instead of standard OS file IO they use dedicate
file server on TCP IP socket.

In Harbour it's possible to dynamically register many alternative
RDD IO APIs and use them simultaneously in one application.
HBNETIO library in Harbour implements this so without touching even
single line in RDD code it works for all core RDDs and also 3-rd
party ones if they use Harbour RDD IO API (hb_file*() functions).
Additionally client and server code in HBNETIO are fully MT safe
so can be used in MT programs without any problems.
Harbour has also similar library called HBMEMIO which allows to
use native RDDs with pseudo files stored in memory instead of real
files.
This feature was recently copied from Harbour to xHarbour but I
haven't tested if it works or not. For sure RE*DBF RDDs still exists.

In both compilers maximal file size for tables, memos and indexes is
limited only by OS and file format structures. Neither Harbour nor
xHarbour introduce own limits here.
The maximal file size for DBFs is limited by number of records
2^32-1 = 4294967295 and maximal record size: 2^16-1 = 65535 what
gives nearly 2^48 = 256TB as maximal .dbf file size.
The maximal memo format size depends on used memo type: DBT, FPT
or SMT and size of memo block. It's limited by maximal number of memo
blocks = 2^32 and size of memo block so it's 2^32*<size_of_memo_block>.
The default memo block size for DBT is 512 bytes, FPT - 64 bytes and
for SMT 32 bytes. So for standard memo block sizes the maximum are:
DBT->2TB, FPT->256GB, SMT->128GB. The maximal memo block size in
Harbour is 2^32 and minimal is 1 byte and it can be any value between
1 and 65536 and then any number of 64KB blocks. The last limitation
is introduced as workaround for some wrongly implemented in other
languages memo drivers which were setting only 16 bits in 32bit field
in memo header. Most of other languages has limit for memo block
size at 2^15 and the block size has to be power of 2. Some of them
also introduce minimal block size limits. If programmers plans to share
data with programs compiled by such languages then he should check
their documentation to not create memo files which cannot be accessed
by them.

Maximal NTX file size for standard NTX files is 4GB and it's limited
by internal NTX structures. Enabling 64bit locking in [x]Harbour change
slightly used NTX format and increase maximum NTX file size to 4TB.
The NTX format in [x]Harbour has also many other extensions like support
for multitag indexes or using record number as hidden part of index key
and many others which are unique to [x]Harbour. In practice all of CDX
extensions are supported by NTX in [x]Harbour.
The NSX format in [x]Harbour is also limited by default to 4GB but like
in NTX enabling 64bit locking extend it to 4TB. It also supports common
to NTX and CDX set of features.

The CDX format is limited to 4GB and so far [x]Harbour does not support
extended mode which can increase the size up to 2TB with standard page
length and it can be bigger in all formats if we introduce support for
bigger index pages. Of course all such extended formats are not binary
compatible with original ones and so far can be used only by [x]Harbour
RDDs though in ADS the .adi format is such extended CDX format so maybe
in the future it will be possible to use .adi indexes in our CDX RDD.

Of course all of the above sizes can be reduced by operating system (OS)
or file system (FS) limitations so it's necessary to check what is
supported by environment where [x]Harbour applications are executed.



###    REGULAR EXPRESSIONS    ###
=================================
Harbour and xHarbour support regular expressions. In xHarbour they are bound
with HVM and always present. In Harbour they are fully optional. Harbour
supports few different regular expression libraries and it's possible to
use platform or CRTL native libs or PCRE which is included in Harbour.
Harbour always uses native for given library API. xHarbour supports only
PCRE modified to use POSIX regex interface which does not support strings
with embedded chr(0) characters.

Both compilers can store and reuse compiled regular expressions.
xHarbour stores them in string items and Harbour in GC pointer items.
Portable code should respect these differences.



###    INET SOCKETS    ###
==========================
Both compilers support IP4 sockets with similar .prg API created by
Giancarlo Niccolai though both use different low level implementations.
In xHarbour IP4 functions have Inet*() prefix.
In Harbour they have hb_inet*() prefix in core code and xHarbour Inet*()
functions are available in XHB library.

Harbour has also public C socket API (hbsocket.h) which is not reduced
to IP4 protocols only and is the base low level code used for .prg level
[hb_]inet*() function implementation. This socket implementation was
rewritten from scratch, it's MT safe and was designed to hide platform
differences in BSD socket implementation. It also works in DOS builds
using WATT-32 library.
This API is available for users also at PRG level by hb_socket*() functions.



###    I18N SUPPORT    ###
==========================
Harbour and xHarbour compilers have build-in support for
internationalization (I18N) and it's enabled during compilation using
the same compiler time switch -j[<file>] but the low level implementation
in compilers and at runtime is completely different.

In xHarbour during compilation with -j switch compiler looks for all
call like I18N( <cConstValue> ) collects them and then stores them using
internal xHarbour binary format in file with .hil. The base name of such
file is taken from compiler .prg module or set by user by optional <file>
argument of -j switch. If file already exists then strings extracted from
compiled code are always appended to existing file without merging or
checking for duplicated strings. Duplicated strings are eliminated by
'hbdict' program which should be used to create translation table.
This programs reads .hil file show all existing strings and allow to
type translation for each existing string and then saves the translation
table in files with .hit extension using another internal xHarbour binary
format. At runtime application may load .hit file and then I18N() function
will look if string passed as 1-st parameter exists in translation table
and if yes then it returns translated string and if not it returns original
string. Only pure 1 to 1 translation exists without any extensions like
context domains, support for plural forms or automatic CP translations.
Detail information about I18N in xHarbour can be found in xHarbour CVS
in doc/hbi18n.txt file.

Harbour compiler is very close to 'gettext' functionality with some
additional extensions. It recognize the following functions at compile
time:
      hb_i18n_gettext( <cText> [, <cDomain> ] )
      hb_i18n_gettext_strict( <cText> [, <cDomain> ] )
      hb_i18n_gettext_noop( <cText> [, <cDomain> ] )
      hb_i18n_ngettext( <nCount>, <cText> [, <cDomain> ] )
      hb_i18n_ngettext_strict( <nCount>, <cText> [, <cDomain> ] )
      hb_i18n_ngettext_noop( <nCount>, <cText> [, <cDomain> ] )
and then generates .pot text files which are gettext compatible so it's
possible to use gettext tools to process them (merge, translate, update,
etc.). Additionally Harbour has own tool called hbi18n which can merge
.pot files, add automatic translations from other .po[t] or .hbl files
and generate compiled Harbour I18N binary modules as .hbl files.
Harbour supports plural forms translations, context domains, automatic
CP translations, etc. The plural form support in Harbour is extended
in comparison to gettext and allows to use non US languages as base.
In MT mode each thread inherits I18N translation module from parent
thread but then can set and use its own one.
There is a set of HB_I18N_*() functions available for programmers at
runtime which allows to make different operations on compiled I18N
modules and also .po[t] files.
Using -w3 switch during compilation enable additional validation of
hb_i18n_[n]gettext*() parameters so compiler generates warnings.
Just like in original gettext it's suggested to use #define or #xtranslate
macros instead of direct calls to hb_i18n_[n]gettext*() functions, i.e.:
      #xtranslate _I( <x,...> ) => hb_i18n_gettext( <x> )
      #xtranslate _IN( <x,...> ) => hb_i18n_ngettext( <x> )
or:
      #xtranslate I18N( <x,...> ) => hb_i18n_gettext( <x> )
It allows to keep source code shorter and if necessary easy switch to
STRICT (for strict parameter validation) or NOOP (disabled at compile
time I18N support) versions.
Additionally Harbour compiler can recognize user I18N functions. They
have the same name as above hb_i18n_*() functions but with additional
user '_*' suffix so they are in the form like:
      hb_i18n_[n]gettext{_strict,_noop,}_*([<params,...>])
Using them users can easy introduce their own I18N runtime modules.
To reduce dependencies on external tools by default Harbour uses own
format for compiled .po[t] files but it's planned to add also native
runtime gettext support as optional user I18N interface.



###    ZLIB    ###
==================
Harbour RTL gives support at .prg level to ZLIB (HB_Z*()) and GZIP
(HB_GZ*()) functions. In xHarbour RTL only ZLIB compression/decompression
is available by: HB_COMPRESS(), HB_UNCOMPRESS(), HB_COMPRESSBUFLEN(),
HB_COMPRESSERROR() and HB_COMPRESSERRORDESC() functions.
In Harbour these functions are available in XHB library.
Original Harbour ZLIB API is different. It does not have any non MT safe
extensions, it's protected against possible overflows and it's more closer
to original ZLIB one.



###    SERIAL PORT SUPPORT    ###
=================================
Harbour gives common multiplatform PRG and C level API for serial port
communication by hb_com*() functions in core libraries. It also implements
few other known in Clipper serial port interfaces in contrib libraries
like CT3 COM_*() functions in hbct library or basic support for Telepathy
interface in hbtpathy library. All such implementations use at low level
code the core hb_com*() interface so they can work on all platforms.
xHarbour CVS contains only telepath library with some very basic Telepathy
interface which is not available for all platforms.
xHarbour.com distribute also library compatible with CT3 COM*() interface
but only for MS-Windows builds.
It's also possible to ask on xHarbour.news group for HBCOM library.
AFAIK binaries are available for MS-Windows and Linux.
In Harbour the interface given by HBCOM library is also available
inside HBCOMM contrib library which internally uses hb_com*() core
API so it's portable between different OS-es.



###    MACRO COMPILER    ###
============================
In Harbour macro compiler supports the same expressions as compiler.
The only one and documented difference is support for VFP like datetime
constant values {^...} which are supported only by compiler.
It's guaranteed that any valid expressions not longer then 8MB will be
cleanly compiled and executed by Harbour. It's possible to compile and
execute longer expressions (in practice there is no maximal size limit)
but in such case it's not guaranteed that all expressions can be compiled,
f.e. macro expressions containing string constant values longer then 16MB
cannot be compiled. It's only guaranteed that if Harbour cannot compile
macro expression then it generates RT error and never generates broken
code.

In xHarbour there are differences between macro compiler and compiler
and not all expressions supported at compile time can be used in macro
compiler. The maximum size for valid expressions which are always correctly
compiled by xHarbour is 32KB. Expressions longer then 32KB can be compiled
without any RT error by xHarbour but it's possible that wrong code will be
generated for them which may cause any unpredictable behavior so user should
not use such expressions. There is no clean way in xHarbour to check if macro
expression longer then 32KB will be correctly compiled.

Just like Clipper neither Harbour nor xHarbour support statements in macro
compiler so extended codeblocks also cannot be compiled.
Now such functionality is supported only by compiler library in Harbour.

In Clipper the documented maximum size of expression which can be compiled
by macro compiler is 256. Sometime Clipper's macro compiler can compile
a little bit longer expressions generating correct PCODE anyhow just like
in xHarbour it may also cause some unpredictable results (i.e. application
crash) though in most of cases it's RT error during macro compilation.



###    COMPILER LIBRARY    ###
==============================
Harbour has compiler library which allows to integrate Harbour compiler
with user applications and use it at runtime to compile new code which
can be also immediately executed or dynamically registered as part of
code. In Harbour compiler code is fully MT safe so the compiler library
can be safely used in MT programs, f.e. for parallel compilation.
Now compiler library is on pure GPL license so any programs which wants
to use it have to respect the GPL license too.



###    PP LIBRARY    ###
========================
Harbour and xHarbour has PP library which allows to use preprocessor
in user .prg code. Current PP in both compilers are fully MT safe without
any internal locks so it can be used concurrently in MT programs.



###    LEXER    ###
===================
xHarbour uses SIMPLEX as lexer for compiler and macro compiler.
Introducing this lexer was next reason of the xHarbour fork.
Internally SIMPLEX uses a lot of static variables and C compile time
macros to speed-up some operations. It's faster than lexer used before
based on code generated by FLEX but it's not MT safe and very hard
to debug due to code hidden by nested C macros. The reduced version of
SIMPLEX is used by xHarbour in macro compiler.

The preprocessor and lexer are not integrated in xHarbour. The preprocessed
code is converted from tokens used by PP to a string and then this string
is divided to tokens again by SIMPLEX.

Harbour compiler accepts tokens used by preprocessor so in fact it does
not have separate compiler lexer at all. The compiler lexer code (complex.c)
is only simple translator of token values between PP and bison terminal
symbols. For macro compiler Harbour by default uses small lexer (macrolex.c)
which is optimized for speed but it can also use PP lexer when Harbour is
compiled with HB_MACRO_PPLEX. All Harbour lexers are fully MT safe without
any internal locks. They are also much faster then xHarbour ones.



###    CONTRIB LIBRARIES    ###
===============================
Some of xHarbour core libraries like CT, TIP or ODBC are supported by
Harbour as contrib library. In practice Harbour covers whole xHarbour
functionality with some fixes and extensions. In contrib tree Harbour
has also many other libraries which are unique to Harbour, like HBCURL,
RDDSQL, HBSSL, GTWVG, HBQT, HBXBP, ... which give many important
extensions. Some of them can be easily ported to xHarbour but some others
not due to not fully functional MT model in xHarbour or missing some
important functionality like thread safe support for many GTs in single
application and multi window GTs with runtime window switching. It's
possible that in the future some important fixes and extensions will
be done in xHarbour core code what allow to port some of Harbour contrib
libraries to xHarbour.

Detail description of contrib libraries needs separate text and will be
much longer then this whole document and maybe will be created in the
future.



###    PORTABILITY    ###
=========================
Both compilers were ported to many different platforms on different hardware.
The list of supported platforms is really long: different *nixes (Linux,
MacOSX, SunOS, HP-UX, *BSD,...) on little and big endian 32 and 64 bit
machines, DOS, OS2, Win32, Win64 and WinCE (Harbour only).
In practice they can be ported quite easy to nearly each OS and hardware
if it passes two important conditions: it's ASCII based machine and has
support for 8bit bytes. Due to cleaner code it's easier to port Harbour
to new platform then xHarbour but the difference seems to not be huge
for someone who has enough knowledge about destination platform.
Now all popular operating systems are supported by both compilers
with the exception to WinCE which is not supported by xHarbour yet.



###    C LEVEL COMPATIBILITY    ###
===================================
The main difference between Harbour and xHarbour in public C API
is in value quantifiers. Harbour fully respect 'const' variable
attribute and does not remove it from returned values. It helps to
keep code clean and locate places where string constant values
are overwritten what is illegal. It also helps C compiler to better
optimize the code because it has additional information that some
variables/memory regions are readonly and cannot be modified during
code execution what can give some additional speed improvement.
If some C code does not respect it then during compilation with
Harbour header files C compilers usually generate warning and
C++ ones errors, i.e. code like:
      char * pszValue = hb_parc( 1 );
is wrong because hb_parc() returns pointer to readonly strings
which cannot be changed by user code. So this code should be
changed to:
      const char * pszValue = hb_parc( 1 );
Now if compiled code tries to make sth like:
      pszValue[ index ] = 'X';
what is illegal in Harbour, xHarbour and also in Clipper for pointers
returned by _parc() function then compiler generate warning or error
about writing to readonly location.
In summary it means that Harbour forces to keep code clean and use
more strict declarations but xHarbour and Clipper don't and wrong
code can be silently compiled without any warnings or errors.
The second important difference is in hb_par*() and hb_stor*() functions.
In Harbour there are hb_parv*() and hb_storv*() functions which accepts
parameters like in Clipper _par*() and _stor*() functions with optional
array indexes, i.e.:
      const char * pszValue = hb_parvc( 1, 1 );
      int          iValue   = hb_parvni( 1, 2 );
and functions without 'v' which do not allow to access array items, i.e.:
      const char * pszValue = hb_parc( 1 );
      int          iValue   = hb_parni( 2 );
These functions (without 'v') are also much more restrictive on accepted
parameters and do not make hidden parameter translations, i.e. hb_parl()
returns TRUE only for logical parameters and does not accept numeric
values like hb_parvl() or _parl().
New functions without array index support were introduced to safely access
parameters without strict type checking. Such code:
      HB_FUNC( NOT )
      {
         int iValue = _parni( 1 );
         hb_retni( ~iValue );
      }
is not fully safe when array parameter is passed from .prg level, i.e.
      ? NOT( { 5, 4, 7 } )
In such case the behavior is unpredictable because _parni() function
tries to access from C function frame unexciting parameter with array
index. Depending on used hardware (CPU) different things may happen.
On some platforms _parni( 1 ) always return 0, on some other random
value from the array and on some others with hardware function frame
protection it generates exception and application crash. To make the
above code safe programmer should change it to:
      HB_FUNC( NOT )
      {
         int iValue = ISNUM( 1 ) ? _parni( 1 ) : 0;
         hb_retni( ~iValue );
      }
or:
      HB_FUNC( NOT )
      {
         int iValue = _parni( 1, 0 );
         hb_retni( ~iValue );
      }
xHarbour does not have hb_par*() and hb_stor*() functions without
optional array index parameters so they cannot be safely used to
access parameters without type checking. It may also create problems
when xHarbour code is moved to Harbour because hb_par*() and
hb_stor*() functions in xHarbour work like hb_parv*() and hb_storv*()
in Harbour. It can be seen in code like:
      int iValue = hb_parni( 1, 1 );
C compiler refuses to compile such code with Harbour header files
and it has to be modified to:
      int iValue = hb_parvni( 1, 1 );
For portable code which access array items using hb_par/hb_stor API
I suggest to use hb_parv*() and hb_storv*() functions and add in
some header file:
      #ifdef __XHARBOUR__
         #define hb_parvc        hb_parc
         #define hb_parvni       hb_parni
         [...]
         #define hb_storvc       hb_storc
         #define hb_storvni      hb_storni
         [...]
      #endif
or use Clipper compatible functions defined in extend.api: _par*()
and _stor*().
I hope that in the future xHarbour will have above #define translation
in core code or maybe even support for hb_parv*() and hb_storv*() functions.

Next important difference between Harbour and xHarbour is access to internal
HVM structures. In Harbour internal HVM structures are hidden by default so
programmers cannot access them, i.e. PHB_ITEM is mapped as 'void *' pointer
so code like:
      iValue = pItem->item.asInteger.value;
cannot be compiled and programmers should change it to use official API:
      iValue = hb_itemGetNI( pItem );
It's very important for core developers because public and internal C API
is separated so we can easy introduce modifications in HVM internals without
worrying about backward compatibility and it allows to keep the same code
for different HVM versions (i.e. for MT and ST modes) so in Harbour only
HBVM library is different for MT mode and all others are the same.
Such separation also greatly increase C code quality eliminating possible
GPF traps, wrong behavior in code created without enough knowledge about
HVM internals or code which blocks adding new extensions.
It's sth what should be done in xHarbour core code where lot of code is
simply wrong due to direct access to HVM internals, i.e.
source/rtl/txtline.c[299]:
      Term[i] = (char *) (&Opt)->item.asString.value;
and we have GPF trap if given array item is not a string.
source/rtl/version[108]:
      PHB_ITEM pQuery = hb_param( 1, HB_IT_INTEGER );
      [...]
      if( pQuery )
      {
         int iQuery = pQuery->item.asInteger.value;
         [...]
so it does not work if passed numeric parameter is not internally stored
as HB_IT_INTEGER value and programmer cannot control it yourself, i.e.
parameters can be read from table or passed from remote client and then
after deserialization stored as HB_IT_LONG or HB_IT_DOUBLE.
source/rtl/direct.c[124]:
      HB_ITEM_NEW( SubDir );
      hb_fsDirectory( &SubDir, szSubdir, szAttributes, FALSE, TRUE );
      hb_fsDirectoryCrawler( &SubDir, pResult, szFName, szAttributes, sRegEx );
This code creates complex item (array) on C stack so garbage collector does
not know anything about it so it cannot be activated otherwise it will cause
GPF. It means that this code blocks adding automatic garbage collector
activation in memory manager.
These are only examples and anyone can find many other similar problems
in xHarbour core code so in my opinion API separation with core code
cleanup is sth what has to be done in xHarbour in the future.
Of course such separation does not mean that we can forbid 3-rd party
programmers to access HVM internals. Programmers creating code for Harbour
can include before any other Harbour header files "hbvmint.h", i.e.:
      #include "hbvmint.h"
      #include "hbapi.h"
and it enable access to HVM internals anyhow in such case we do not
guaranty that such code will work in the future Harbour versions so
we strongly recommend to not use it.
In Harbour core code only HBVM library is compiled with access to
HVM internals. All other code uses official API. In contrib code
"hbvmint.h" is used in few places in XHB library to emulate some core
HVM xHarbour extensions.

In xHarbour there are few functions which allows to create string items
without terminating '\0' characters like: hb_itemPutCRaw(),
hb_itemPutCRawStatic(), hb_retclenAdoptRaw(). Because string items
without terminating '\0' character exploit problems in any C code which
needs ASCIIZ string creating possible GPF traps in all str*() and similar
functions then in Harbour such functionality is illegal and will never
be added to core code. Additionally in Harbour functions corresponding to
xHarbour functions: hb_retclenStatic(). hb_itemPutCLStatic() have additional
protection against creating such strings and generate internal error when
programmer tries to pass wrong strings without trailing '\0' character.
It may exploit problems in wrong code ported to Harbour.

Most of other public C functions in Harbour and xHarbour are compatible
anyhow there are some differences i.e. in hash array or timestamp API
which have to be covered by #ifdef __XHARBOUR__ conditional compilation.
Programmers creating portable code have to now about them.
For people who wants to use only extended API for code used for Clipper
and [x]Harbour I suggest to use small definition:
      #ifndef __HARBOUR__
         #define HB_FUNC( fun )  CLIPPER fun ( void )
      #endif
This definition allows to easy create code which can be compiled with
Clipper and Harbour or xHarbour header files (extend.api).
It can be even added to Clipper's extend.api and then we can compile
this code:
      #include "extend.api"
      HB_FUNC( NOT )
      {
         int iValue = _parni( 1, 0 );
         hb_retni( ~iValue );
      }
for Clipper, Harbour and xHarbour. Please only remember that linkers
used with [x]Harbour are case sensitive and always use upper letters
for function names. In Clipper it was not necessary.



###    HBRUN / XBSCRIPT    ###
==============================
xHarbour has .prg code interpreter mostly written also as .prg code.
It's called xbscript. AFAIK It's also based for commercial product
xBaseScript distributed by xHarbour.com.
In Harbour the same functionality was reached in just few lines in
a program called hbrun because it's possible to use compiler library.
HBRUN can compile any given code just like Harbour Compiler (in fact
it uses the same compiler code from compiler library) and then dynamically
execute it.

Unlike XBSCRIPT the HBRUN does not cause any performance reduction in
comparison to standalone compilation with Harbour compiler to PCODE
(-gc[0-2], -gh) because exactly the same PCODE is generated and later
executed in both cases. Also the compilation phase is many times faster
then in XBSCRIPT.
The commercial version of xBaseScript has some extensions which allows
to integrate it with IE and/or IIS. HBRUN from Harbour does not have such
functionality yet.



###    HBMK2    ###
===================
It's very nice new tool in Harbour which hides differences between
platforms and C compiler used with Harbour. It should help new
users to start working with Harbour (i.e. we can create copies of
hbmk2 executable file called 'clipper' and 'rtlink' or 'blinker'
and then it's possible to use them with original Clipper make file
projects (i.e. in .rmk files created for Clipper and RMAKE).
hbmk2 also nicely helps in creating binaries for different platforms
(cross compilation), i.e. in creating WinCE programs in Linux or
desktop Windows.

There are many features supported by this tool which can greatly
help new and advanced Harbour users. Their detail description is
not this text goal. If someone is interested in list of supported
options then he can use hbmk2 with '--help' parameter. In the nearest
future hbmk2 should replace hb* scripts.

This tool is unique to Harbour and does not exist in xHarbour.



###    PERFORMANCE AND RESOURCE USAGE    ###
============================================
Harbour internal structures are optimized for memory usage and are smaller
than xHarbour ones so total memory usage by Harbour is also smaller.
The difference depends on type of code but usually Harbour needs about
20%-25% less memory then xHarbour. The size of final executable is also
usually smaller due to reduced dependencies which are necessary for pure
HVM code though in current days it's less important issue.
Much bigger difference is in performance.

Both compiler shares a lot of common C code in some subsystems like RDDs
so in many operations which needs a lot of CPU to execute the same C code
the performance is nearly the same. The difference begins to be noticeable
when we compare HVM performance and time necessary to execute .prg code.
Harbour is ~75% faster in ST mode and over 100% faster in MT mode.
In harbour/tests/speedtst.prg we have simple test which can be used to
compare performance of different compilers.
The tests below were done using AMD Phenom(tm) 8450 Triple-Core Processor
2100 MHZ with 32bit Linux kernels.

Here are results for ST mode:

   2009.07.28 20:48:12 Linux 2.6.25.20-0.4-pae i686
   Harbour 2.0.0beta2 (Rev. 11910) GNU C 4.4 (32-bit) x86
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.03
   ====================================================================
   [ T001: x := L_C ]..............................................0.02
   [ T002: x := L_N ]..............................................0.02
   [ T003: x := L_D ]..............................................0.02
   [ T004: x := S_C ]..............................................0.05
   [ T005: x := S_N ]..............................................0.03
   [ T006: x := S_D ]..............................................0.03
   [ T007: x := M->M_C ]...........................................0.06
   [ T008: x := M->M_N ]...........................................0.04
   [ T009: x := M->M_D ]...........................................0.04
   [ T010: x := M->P_C ]...........................................0.06
   [ T011: x := M->P_N ]...........................................0.04
   [ T012: x := M->P_D ]...........................................0.04
   [ T013: x := F_C ]..............................................0.15
   [ T014: x := F_N ]..............................................0.25
   [ T015: x := F_D ]..............................................0.10
   [ T016: x := o:Args ]...........................................0.14
   [ T017: x := o[2] ].............................................0.10
   [ T018: round( i / 1000, 2 ) ]..................................0.21
   [ T019: str( i / 1000 ) ].......................................0.55
   [ T020: val( s ) ]..............................................0.28
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.39
   [ T022: dtos( d - i % 10000 ) ].................................0.33
   [ T023: eval( { || i % 16 } ) ].................................0.36
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.20
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.29
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.20
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.31
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.24
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.23
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................2.95
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................2.97
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.34
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........0.62
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.21
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.23
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.23
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.23
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.23
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.23
   [ T040: ascan( a, i % 16 ) ]....................................0.24
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................2.34
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....0.70
   [ T043: x := a ]................................................0.04
   [ T044: x := {} ]...............................................0.12
   [ T045: f0() ]..................................................0.06
   [ T046: f1( i ) ]...............................................0.10
   [ T047: f2( c[1...8] ) ]........................................0.10
   [ T048: f2( c[1...40000] ) ]....................................0.09
   [ T049: f2( @c[1...40000] ) ]...................................0.09
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.11
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.31
   [ T052: f2( a ) ]...............................................0.11
   [ T053: x := f4() ].............................................0.45
   [ T054: x := f5() ].............................................0.22
   [ T055: x := space(16) ]........................................0.17
   [ T056: f_prv( c ) ]............................................0.31
   ====================================================================
   [ total application time: ]....................................20.27
   [ total real time: ]...........................................20.51

   2009.07.28 20:48:46 Linux 2.6.25.20-0.4-pae i686
   xHarbour build 1.2.1 Intl. (SimpLex) (Rev. 6517) GNU C 4.4 (32 bit) ?
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.09
   ====================================================================
   [ T001: x := L_C ]..............................................0.02
   [ T002: x := L_N ]..............................................0.00
   [ T003: x := L_D ]..............................................0.02
   [ T004: x := S_C ]..............................................0.00
   [ T005: x := S_N ]..............................................0.01
   [ T006: x := S_D ]..............................................0.01
   [ T007: x := M->M_C ]...........................................0.02
   [ T008: x := M->M_N ]...........................................0.02
   [ T009: x := M->M_D ]...........................................0.01
   [ T010: x := M->P_C ]...........................................0.02
   [ T011: x := M->P_N ]...........................................0.06
   [ T012: x := M->P_D ]...........................................0.01
   [ T013: x := F_C ]..............................................0.18
   [ T014: x := F_N ]..............................................0.20
   [ T015: x := F_D ]..............................................0.10
   [ T016: x := o:Args ]...........................................0.27
   [ T017: x := o[2] ].............................................0.05
   [ T018: round( i / 1000, 2 ) ]..................................0.30
   [ T019: str( i / 1000 ) ].......................................0.69
   [ T020: val( s ) ]..............................................0.34
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.51
   [ T022: dtos( d - i % 10000 ) ].................................0.44
   [ T023: eval( { || i % 16 } ) ].................................0.54
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.36
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.43
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.33
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.58
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.48
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.51
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................4.70
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................5.62
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.64
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........0.87
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.28
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.28
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.27
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.28
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.27
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.28
   [ T040: ascan( a, i % 16 ) ]....................................0.47
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................3.95
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....0.89
   [ T043: x := a ]................................................0.01
   [ T044: x := {} ]...............................................0.11
   [ T045: f0() ]..................................................0.14
   [ T046: f1( i ) ]...............................................0.19
   [ T047: f2( c[1...8] ) ]........................................0.19
   [ T048: f2( c[1...40000] ) ]....................................0.18
   [ T049: f2( @c[1...40000] ) ]...................................0.18
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.22
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.39
   [ T052: f2( a ) ]...............................................0.18
   [ T053: x := f4() ].............................................0.81
   [ T054: x := f5() ].............................................0.49
   [ T055: x := space(16) ]........................................0.33
   [ T056: f_prv( c ) ]............................................0.75
   ====================================================================
   [ total application time: ]....................................34.52
   [ total real time: ]...........................................34.96

And here are results for MT mode:

   2009.07.28 20:52:43 Linux 2.6.25.20-0.4-pae i686
   Harbour 2.0.0beta2 (Rev. 11910) (MT) GNU C 4.4 (32-bit) x86
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.04
   ====================================================================
   [ T001: x := L_C ]..............................................0.07
   [ T002: x := L_N ]..............................................0.02
   [ T003: x := L_D ]..............................................0.02
   [ T004: x := S_C ]..............................................0.05
   [ T005: x := S_N ]..............................................0.03
   [ T006: x := S_D ]..............................................0.04
   [ T007: x := M->M_C ]...........................................0.05
   [ T008: x := M->M_N ]...........................................0.05
   [ T009: x := M->M_D ]...........................................0.05
   [ T010: x := M->P_C ]...........................................0.05
   [ T011: x := M->P_N ]...........................................0.05
   [ T012: x := M->P_D ]...........................................0.04
   [ T013: x := F_C ]..............................................0.16
   [ T014: x := F_N ]..............................................0.24
   [ T015: x := F_D ]..............................................0.10
   [ T016: x := o:Args ]...........................................0.15
   [ T017: x := o[2] ].............................................0.09
   [ T018: round( i / 1000, 2 ) ]..................................0.22
   [ T019: str( i / 1000 ) ].......................................0.62
   [ T020: val( s ) ]..............................................0.28
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.41
   [ T022: dtos( d - i % 10000 ) ].................................0.39
   [ T023: eval( { || i % 16 } ) ].................................0.44
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.27
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.38
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.24
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.40
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.29
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.31
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................2.83
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................3.16
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.38
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........0.75
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.28
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.29
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.29
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.29
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.29
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.29
   [ T040: ascan( a, i % 16 ) ]....................................0.30
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................2.87
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....0.81
   [ T043: x := a ]................................................0.03
   [ T044: x := {} ]...............................................0.13
   [ T045: f0() ]..................................................0.07
   [ T046: f1( i ) ]...............................................0.11
   [ T047: f2( c[1...8] ) ]........................................0.11
   [ T048: f2( c[1...40000] ) ]....................................0.11
   [ T049: f2( @c[1...40000] ) ]...................................0.11
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.14
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.33
   [ T052: f2( a ) ]...............................................0.11
   [ T053: x := f4() ].............................................0.45
   [ T054: x := f5() ].............................................0.24
   [ T055: x := space(16) ]........................................0.19
   [ T056: f_prv( c ) ]............................................0.38
   ====================================================================
   [ total application time: ]....................................23.10
   [ total real time: ]...........................................23.78

   2009.07.28 20:53:24 Linux 2.6.25.20-0.4-pae i686
   xHarbour build 1.2.1 Intl. (SimpLex) (Rev. 6517) (MT) GNU C 4.4 (32 bit) ?
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.20
   ====================================================================
   [ T001: x := L_C ]..............................................0.00
   [ T002: x := L_N ]..............................................0.00
   [ T003: x := L_D ]..............................................0.00
   [ T004: x := S_C ]..............................................0.00
   [ T005: x := S_N ]..............................................0.00
   [ T006: x := S_D ]..............................................0.00
   [ T007: x := M->M_C ]...........................................0.19
   [ T008: x := M->M_N ]...........................................0.23
   [ T009: x := M->M_D ]...........................................0.18
   [ T010: x := M->P_C ]...........................................0.24
   [ T011: x := M->P_N ]...........................................0.24
   [ T012: x := M->P_D ]...........................................0.21
   [ T013: x := F_C ]..............................................0.17
   [ T014: x := F_N ]..............................................0.21
   [ T015: x := F_D ]..............................................0.07
   [ T016: x := o:Args ]...........................................0.36
   [ T017: x := o[2] ].............................................0.05
   [ T018: round( i / 1000, 2 ) ]..................................0.43
   [ T019: str( i / 1000 ) ].......................................0.83
   [ T020: val( s ) ]..............................................0.41
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.65
   [ T022: dtos( d - i % 10000 ) ].................................0.56
   [ T023: eval( { || i % 16 } ) ].................................0.78
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.47
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.66
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.49
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.88
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.73
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.72
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................5.69
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................6.18
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.79
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........1.12
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.37
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.36
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.42
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.36
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.37
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.38
   [ T040: ascan( a, i % 16 ) ]....................................0.64
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................6.21
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....1.29
   [ T043: x := a ]................................................0.00
   [ T044: x := {} ]...............................................0.15
   [ T045: f0() ]..................................................0.21
   [ T046: f1( i ) ]...............................................0.26
   [ T047: f2( c[1...8] ) ]........................................0.27
   [ T048: f2( c[1...40000] ) ]....................................0.27
   [ T049: f2( @c[1...40000] ) ]...................................0.25
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.33
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.57
   [ T052: f2( a ) ]...............................................0.27
   [ T053: x := f4() ].............................................0.99
   [ T054: x := f5() ].............................................0.65
   [ T055: x := space(16) ]........................................0.38
   [ T056: f_prv( c ) ]............................................1.24
   ====================================================================
   [ total application time: ]....................................50.90
   [ total real time: ]...........................................51.49

Please note that in xHarbour MT mode many important things resolved in
Harbour have not been implemented so far so it's hard to say how may
look final performance.

Recently I've repeated above tests on the same Hardware but using 64-bit
Linux kernels and 64bit Harbour builds and here the difference is even
bigger. Here are results for ST mode:

   2009.10.27 13:24:41 Linux 2.6.31.3-1-desktop x86_64
   Harbour 2.0.0beta3 (Rev. 12776) GNU C 4.4 (64-bit) x86-64
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.02
   ====================================================================
   [ T001: x := L_C ]..............................................0.03
   [ T002: x := L_N ]..............................................0.02
   [ T003: x := L_D ]..............................................0.02
   [ T004: x := S_C ]..............................................0.03
   [ T005: x := S_N ]..............................................0.03
   [ T006: x := S_D ]..............................................0.03
   [ T007: x := M->M_C ]...........................................0.03
   [ T008: x := M->M_N ]...........................................0.04
   [ T009: x := M->M_D ]...........................................0.03
   [ T010: x := M->P_C ]...........................................0.04
   [ T011: x := M->P_N ]...........................................0.03
   [ T012: x := M->P_D ]...........................................0.03
   [ T013: x := F_C ]..............................................0.10
   [ T014: x := F_N ]..............................................0.17
   [ T015: x := F_D ]..............................................0.08
   [ T016: x := o:Args ]...........................................0.11
   [ T017: x := o[2] ].............................................0.06
   [ T018: round( i / 1000, 2 ) ]..................................0.15
   [ T019: str( i / 1000 ) ].......................................0.32
   [ T020: val( s ) ]..............................................0.18
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.28
   [ T022: dtos( d - i % 10000 ) ].................................0.28
   [ T023: eval( { || i % 16 } ) ].................................0.29
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.18
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.24
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.18
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.27
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.22
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.22
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................2.05
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................2.41
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.27
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........0.54
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.17
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.19
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.18
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.19
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.19
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.18
   [ T040: ascan( a, i % 16 ) ]....................................0.22
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................2.01
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....0.53
   [ T043: x := a ]................................................0.02
   [ T044: x := {} ]...............................................0.08
   [ T045: f0() ]..................................................0.05
   [ T046: f1( i ) ]...............................................0.08
   [ T047: f2( c[1...8] ) ]........................................0.07
   [ T048: f2( c[1...40000] ) ]....................................0.07
   [ T049: f2( @c[1...40000] ) ]...................................0.08
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.09
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.21
   [ T052: f2( a ) ]...............................................0.08
   [ T053: x := f4() ].............................................0.34
   [ T054: x := f5() ].............................................0.16
   [ T055: x := space(16) ]........................................0.12
   [ T056: f_prv( c ) ]............................................0.21
   ====================================================================
   [ total application time: ]....................................15.60
   [ total real time: ]...........................................15.61

   2009.10.27 13:25:10 Linux 2.6.31.3-1-desktop x86_64
   xHarbour build 1.2.1 Intl. (SimpLex) (Rev. 6629) GNU C 4.4 (64 bit) ?
   THREADS: 0
   N_LOOPS: 1000000
   [ T000: empty loop overhead ]...................................0.06
   ====================================================================
   [ T001: x := L_C ]..............................................0.05
   [ T002: x := L_N ]..............................................0.03
   [ T003: x := L_D ]..............................................0.04
   [ T004: x := S_C ]..............................................0.05
   [ T005: x := S_N ]..............................................0.03
   [ T006: x := S_D ]..............................................0.05
   [ T007: x := M->M_C ]...........................................0.05
   [ T008: x := M->M_N ]...........................................0.04
   [ T009: x := M->M_D ]...........................................0.04
   [ T010: x := M->P_C ]...........................................0.06
   [ T011: x := M->P_N ]...........................................0.04
   [ T012: x := M->P_D ]...........................................0.04
   [ T013: x := F_C ]..............................................0.25
   [ T014: x := F_N ]..............................................0.17
   [ T015: x := F_D ]..............................................0.11
   [ T016: x := o:Args ]...........................................0.30
   [ T017: x := o[2] ].............................................0.07
   [ T018: round( i / 1000, 2 ) ]..................................0.32
   [ T019: str( i / 1000 ) ].......................................0.87
   [ T020: val( s ) ]..............................................0.32
   [ T021: val( a [ i % 16 + 1 ] ) ]...............................0.48
   [ T022: dtos( d - i % 10000 ) ].................................0.43
   [ T023: eval( { || i % 16 } ) ].................................0.62
   [ T024: eval( bc := { || i % 16 } ) ]...........................0.40
   [ T025: eval( { |x| x % 16 }, i ) ].............................0.47
   [ T026: eval( bc := { |x| x % 16 }, i ) ].......................0.37
   [ T027: eval( { |x| f1( x ) }, i ) ]............................0.66
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]......................0.56
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ].................0.58
   [ T030: x := &( "f1(" + str(i) + ")" ) ]........................5.95
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]................6.57
   [ T032: x := valtype( x ) +  valtype( i ) ].....................0.76
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]..........0.90
   [ T034: x := a[ i % 16 + 1 ] == s ].............................0.25
   [ T035: x := a[ i % 16 + 1 ] = s ]..............................0.25
   [ T036: x := a[ i % 16 + 1 ] >= s ].............................0.24
   [ T037: x := a[ i % 16 + 1 ] <= s ].............................0.25
   [ T038: x := a[ i % 16 + 1 ] < s ]..............................0.25
   [ T039: x := a[ i % 16 + 1 ] > s ]..............................0.24
   [ T040: ascan( a, i % 16 ) ]....................................0.46
   [ T041: ascan( a, { |x| x == i % 16 } ) ].......................4.10
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s,s2,a2 ]....0.92
   [ T043: x := a ]................................................0.04
   [ T044: x := {} ]...............................................0.16
   [ T045: f0() ]..................................................0.21
   [ T046: f1( i ) ]...............................................0.23
   [ T047: f2( c[1...8] ) ]........................................0.25
   [ T048: f2( c[1...40000] ) ]....................................0.24
   [ T049: f2( @c[1...40000] ) ]...................................0.24
   [ T050: f2( @c[1...40000] ), c2 := c ]..........................0.28
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]....................0.41
   [ T052: f2( a ) ]...............................................0.24
   [ T053: x := f4() ].............................................0.94
   [ T054: x := f5() ].............................................0.63
   [ T055: x := space(16) ]........................................0.41
   [ T056: f_prv( c ) ]............................................0.86
   ====================================================================
   [ total application time: ]....................................37.14
   [ total real time: ]...........................................37.16

Harbour is noticeable faster in 64 bit mode and xHarbour slower but
I haven't analyzed the reasons. Few years ago xHarbour was also faster
in 64bit modes so it's probably problem with some compile time settings.
Harbour build system allows to use different compile time switches
for static and shared libraries and the bigger difference is probably
result of better tuned switches for static libraries which cannot be
used for shared ones so xHarbour cannot use them by default.

The last test we can make is scalability in real multi CPU environment.

   2009.07.28 21:07:56 Linux 2.6.25.20-0.4-pae i686
   Harbour 2.0.0beta2 (Rev. 11910) (MT)+ GNU C 4.4 (32-bit) x86
   THREADS: 2
   N_LOOPS: 1000000
                                                           1 th.  2 th.  factor
   ============================================================================
   [ T001: x := L_C ]____________________________________  0.19   0.07 ->  2.65
   [ T002: x := L_N ]____________________________________  0.12   0.06 ->  1.97
   [ T003: x := L_D ]____________________________________  0.12   0.06 ->  1.97
   [ T004: x := S_C ]____________________________________  0.21   0.26 ->  0.81
   [ T005: x := S_N ]____________________________________  0.15   0.08 ->  1.99
   [ T006: x := S_D ]____________________________________  0.16   0.08 ->  2.00
   [ T007: x := M->M_C ]_________________________________  0.19   0.13 ->  1.42
   [ T008: x := M->M_N ]_________________________________  0.18   0.09 ->  1.98
   [ T009: x := M->M_D ]_________________________________  0.21   0.14 ->  1.51
   [ T010: x := M->P_C ]_________________________________  0.18   0.13 ->  1.45
   [ T011: x := M->P_N ]_________________________________  0.17   0.09 ->  1.89
   [ T012: x := M->P_D ]_________________________________  0.17   0.09 ->  2.00
   [ T013: x := F_C ]____________________________________  0.44   0.30 ->  1.48
   [ T014: x := F_N ]____________________________________  0.57   0.32 ->  1.81
   [ T015: x := F_D ]____________________________________  0.32   0.17 ->  1.94
   [ T016: x := o:Args ]_________________________________  0.44   0.24 ->  1.81
   [ T017: x := o[2] ]___________________________________  0.29   0.16 ->  1.83
   [ T018: round( i / 1000, 2 ) ]________________________  0.59   0.28 ->  2.12
   [ T019: str( i / 1000 ) ]_____________________________  1.21   0.69 ->  1.74
   [ T020: val( s ) ]____________________________________  0.61   0.33 ->  1.86
   [ T021: val( a [ i % 16 + 1 ] ) ]_____________________  0.93   0.50 ->  1.85
   [ T022: dtos( d - i % 10000 ) ]_______________________  0.90   0.56 ->  1.60
   [ T023: eval( { || i % 16 } ) ]_______________________  1.00   1.24 ->  0.81
   [ T024: eval( bc := { || i % 16 } ) ]_________________  0.59   0.32 ->  1.83
   [ T025: eval( { |x| x % 16 }, i ) ]___________________  0.81   1.03 ->  0.79
   [ T026: eval( bc := { |x| x % 16 }, i ) ]_____________  0.59   0.30 ->  1.96
   [ T027: eval( { |x| f1( x ) }, i ) ]__________________  0.93   1.10 ->  0.85
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]____________  0.71   0.38 ->  1.89
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ]_______  0.70   0.40 ->  1.74
   [ T030: x := &( "f1(" + str(i) + ")" ) ]______________  5.71   3.36 ->  1.70
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]______  6.36   4.21 ->  1.51
   [ T032: x := valtype( x ) +  valtype( i ) ]___________  0.86   0.51 ->  1.68
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]  1.52   0.85 ->  1.80
   [ T034: x := a[ i % 16 + 1 ] == s ]___________________  0.62   0.35 ->  1.80
   [ T035: x := a[ i % 16 + 1 ] = s ]____________________  0.67   0.38 ->  1.75
   [ T036: x := a[ i % 16 + 1 ] >= s ]___________________  0.66   0.37 ->  1.76
   [ T037: x := a[ i % 16 + 1 ] <= s ]___________________  0.66   0.38 ->  1.73
   [ T038: x := a[ i % 16 + 1 ] < s ]____________________  0.67   0.38 ->  1.75
   [ T039: x := a[ i % 16 + 1 ] > s ]____________________  0.65   0.33 ->  1.98
   [ T040: ascan( a, i % 16 ) ]__________________________  0.67   0.33 ->  2.01
   [ T041: ascan( a, { |x| x == i % 16 } ) ]_____________  5.87   3.63 ->  1.62
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s ]  1.88   1.45 ->  1.29
   [ T043: x := a ]______________________________________  0.15   0.07 ->  1.96
   [ T044: x := {} ]_____________________________________  0.37   1.00 ->  0.37
   [ T045: f0() ]________________________________________  0.23   0.14 ->  1.58
   [ T046: f1( i ) ]_____________________________________  0.31   0.20 ->  1.57
   [ T047: f2( c[1...8] ) ]______________________________  0.32   0.18 ->  1.80
   [ T048: f2( c[1...40000] ) ]__________________________  0.31   0.17 ->  1.78
   [ T049: f2( @c[1...40000] ) ]_________________________  0.30   0.21 ->  1.40
   [ T050: f2( @c[1...40000] ), c2 := c ]________________  0.36   0.24 ->  1.51
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]__________  0.74   0.42 ->  1.78
   [ T052: f2( a ) ]_____________________________________  0.31   0.16 ->  1.95
   [ T053: x := f4() ]___________________________________  0.96   0.57 ->  1.66
   [ T054: x := f5() ]___________________________________  0.59   0.39 ->  1.53
   [ T055: x := space(16) ]______________________________  0.48   0.32 ->  1.50
   [ T056: f_prv( c ) ]__________________________________  0.86   0.56 ->  1.53
   ============================================================================
   [   TOTAL   ]_________________________________________ 46.76  30.75 ->  1.52
   ============================================================================
   [ total application time: ]...................................106.45
   [ total real time: ]...........................................77.52


   2009.07.28 21:11:44 Linux 2.6.25.20-0.4-pae i686
   xHarbour build 1.2.1 Intl. (SimpLex) (Rev. 6517) (MT)+ GNU C 4.4 (32 bit) ?
   THREADS: 2
   N_LOOPS: 1000000
                                                           1 th.  2 th.  factor
   ============================================================================
   [ T001: x := L_C ]____________________________________  0.46   0.20 ->  2.33
   [ T002: x := L_N ]____________________________________  0.37   0.18 ->  2.04
   [ T003: x := L_D ]____________________________________  0.38   0.22 ->  1.68
   [ T004: x := S_C ]____________________________________  0.42   0.38 ->  1.10
   [ T005: x := S_N ]____________________________________  0.36   0.21 ->  1.69
   [ T006: x := S_D ]____________________________________  0.39   0.19 ->  2.07
   [ T007: x := M->M_C ]_________________________________  1.39   1.22 ->  1.14
   [ T008: x := M->M_N ]_________________________________  1.42   1.12 ->  1.27
   [ T009: x := M->M_D ]_________________________________  1.44   1.16 ->  1.24
   [ T010: x := M->P_C ]_________________________________  1.46   1.27 ->  1.15
   [ T011: x := M->P_N ]_________________________________  1.43   1.10 ->  1.30
   [ T012: x := M->P_D ]_________________________________  1.40   1.10 ->  1.27
   [ T013: x := F_C ]____________________________________  0.76   0.41 ->  1.84
   [ T014: x := F_N ]____________________________________  0.76   0.43 ->  1.76
   [ T015: x := F_D ]____________________________________  0.53   0.28 ->  1.87
   [ T016: x := o:Args ]_________________________________  1.04   0.96 ->  1.09
   [ T017: x := o[2] ]___________________________________  0.51   0.25 ->  2.04
   [ T018: round( i / 1000, 2 ) ]________________________  1.15   0.98 ->  1.17
   [ T019: str( i / 1000 ) ]_____________________________  1.97   1.51 ->  1.30
   [ T020: val( s ) ]____________________________________  1.19   0.86 ->  1.38
   [ T021: val( a [ i % 16 + 1 ] ) ]_____________________  1.65   1.23 ->  1.35
   [ T022: dtos( d - i % 10000 ) ]_______________________  1.54   1.14 ->  1.35
   [ T023: eval( { || i % 16 } ) ]_______________________  1.91   2.78 ->  0.68
   [ T024: eval( bc := { || i % 16 } ) ]_________________  1.46   1.34 ->  1.09
   [ T025: eval( { |x| x % 16 }, i ) ]___________________  1.60   2.77 ->  0.58
   [ T026: eval( bc := { |x| x % 16 }, i ) ]_____________  1.32   1.30 ->  1.02
   [ T027: eval( { |x| f1( x ) }, i ) ]__________________  2.13   3.10 ->  0.69
   [ T028: eval( bc := { |x| f1( x ) }, i ) ]____________  1.75   1.78 ->  0.98
   [ T029: eval( bc := &("{ |x| f1( x ) }"), i ) ]_______  1.77   1.79 ->  0.99
   [ T030: x := &( "f1(" + str(i) + ")" ) ]______________ 11.34  14.11 ->  0.80
   [ T031: bc := &( "{|x|f1(x)}" ), eval( bc, i ) ]______ 13.19  17.26 ->  0.76
   [ T032: x := valtype( x ) +  valtype( i ) ]___________  2.11   1.65 ->  1.28
   [ T033: x := strzero( i % 100, 2 ) $ a[ i % 16 + 1 ] ]  2.80   1.78 ->  1.57
   [ T034: x := a[ i % 16 + 1 ] == s ]___________________  1.17   0.69 ->  1.71
   [ T035: x := a[ i % 16 + 1 ] = s ]____________________  1.21   0.57 ->  2.10
   [ T036: x := a[ i % 16 + 1 ] >= s ]___________________  1.15   0.62 ->  1.87
   [ T037: x := a[ i % 16 + 1 ] <= s ]___________________  1.10   0.60 ->  1.86
   [ T038: x := a[ i % 16 + 1 ] < s ]____________________  1.11   0.63 ->  1.75
   [ T039: x := a[ i % 16 + 1 ] > s ]____________________  1.14   0.61 ->  1.88
   [ T040: ascan( a, i % 16 ) ]__________________________  1.66   1.17 ->  1.41
   [ T041: ascan( a, { |x| x == i % 16 } ) ]_____________ 12.39  13.63 ->  0.91
   [ T042: iif( i%1000==0, a:={}, ) , aadd(a,{i,1,.T.,s ]  2.95   2.79 ->  1.06
   [ T043: x := a ]______________________________________  0.37   0.19 ->  1.94
   [ T044: x := {} ]_____________________________________  0.70   2.29 ->  0.31
   [ T045: f0() ]________________________________________  0.78   0.73 ->  1.07
   [ T046: f1( i ) ]_____________________________________  0.91   0.87 ->  1.05
   [ T047: f2( c[1...8] ) ]______________________________  0.92   0.85 ->  1.08
   [ T048: f2( c[1...40000] ) ]__________________________  0.91   0.84 ->  1.08
   [ T049: f2( @c[1...40000] ) ]_________________________  0.89   0.83 ->  1.07
   [ T050: f2( @c[1...40000] ), c2 := c ]________________  1.02   0.90 ->  1.13
   [ T051: f3( a, a2, s, i, s2, bc, i, n, x ) ]__________  1.50   1.14 ->  1.32
   [ T052: f2( a ) ]_____________________________________  0.92   0.85 ->  1.08
   [ T053: x := f4() ]___________________________________  2.45   1.95 ->  1.26
   [ T054: x := f5() ]___________________________________  1.75   1.75 ->  1.00
   [ T055: x := space(16) ]______________________________  1.19   1.02 ->  1.16
   [ T056: f_prv( c ) ]__________________________________  4.51   4.36 ->  1.03
   ============================================================================
   [   TOTAL   ]_________________________________________106.08 105.94 ->  1.00
   ============================================================================
   [ total application time: ]...................................287.58
   [ total real time: ]..........................................212.02

As we can see in xHarbour we do not have any improvement executing MT
programs on MultiCPU machines while in Harbour the speed is noticeably
better. It means that Harbour is quite well scalable and users should
expect speed improvement executing MT parallel programs on multi CPU
machines. For some programs like MT servers it may be critical - programs
compiled by Harbour can be quite well improved by simple hardware
upgrade to 4, 8, 16 or more CPU machines.
