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Constraints for Particular Machines

Whenever possible, you should use the general-purpose constraint letters in asm arguments, since they will convey meaning more readily to people reading your code. Failing that, use the constraint letters that usually have very similar meanings across architectures. The most commonly used constraints are m and r (for memory and general-purpose registers respectively; see Simple Constraints), and I, usually the letter indicating the most common immediate-constant format. For each machine architecture, the config/ machine.h file defines additional constraints. These constraints are used by the compiler itself for instruction generation, as well as for asm statements; therefore, some of the constraints are not particularly interesting for asm.

The constraints are defined through the following macros.

- REG_CLASS_FROM_LETTER
  Register class constraints (usually lower case).
- CONST_OK_FOR_LETTER_P
  Immediate constant constraints, for non-floating point constants of word size or smaller precision (usually upper case).
- CONST_DOUBLE_OK_FOR_LETTER_P
  Immediate constant constraints, for all floating point constants and for constants of greater than word size precision (usually upper case).
- EXTRA_CONSTRAINT
  Special cases of registers or memory. This macro is not required, and is only defined for some machines.

Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, see the following summary of the machine-dependent constraints available on some particular machines.

- ARM family
- AMD 29000 family
- IBM RS6000 family
- Intel 386 architecture
- Intel 960 architecture
- MIPS family
- Motorola 68k family
- SPARC family

ARM family arm.h

f
Floating-point register

F
One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 or 10.0

G
Floating-point constant that would satisfy the constraint F if it were negated

I
Integer that is valid as an immediate operand in a data processing instruction. That is, an integer in the range 0 to 255 rotated by a multiple of 2

J
Integer in the range -4095 to 4095

K
Integer that satisfies constraint I when inverted (ones complement)

L
Integer that satisfies constraint I when negated (twos complement)

M
Integer in the range 0 to 32

Q
A memory reference where the exact address is in a single register (m is preferable for asm statements)

R
An item in the constant pool

S
A symbol in the text segment of the current file

AMD 29000 family a29k.h

l
Local register 0
b
Byte Pointer (BP) register
q
Q register
h
Special purpose register

A
First accumulator register

a
Other accumulator register

f
Floating point register

I
Constant greater than 0, less than 0x100

J
Constant greater than 0, less than 0x10000

K
Constant whose high 24 bits are on (1)

L
16 bit constant whose high 8 bits are on (1)

M
32 bit constant whose high 16 bits are on (1)

N
32 bit negative constant that fits in 8 bits

O
The constant 0x80000000 or, on the 29050, any 32 bit constant whose low 16 bits are 0

P
16 bit negative constant that fits in 8 bits

G, H
A floating point constant (in asm statements, use the machine independent E or F instead)

IBM RS6000 rs6000.h

b
Address base register

f
Floating point register

h
MQ, CTR, or LINK register

q
MQ register

c
CTR register

l
LINK register

x
CR register (condition register) number 0

y
CR register (condition register)

I
Signed 16 bit constant

J
Constant whose low 16 bits are 0

K
Constant whose high 16 bits are 0

L
Constant suitable as a mask operand

M
Constant larger than 31

N
Exact power of 2

O
Zero

P
Constant whose negation is a signed 16 bit constant

G
Floating point constant that can be loaded into a register with one instruction per word

|

Q
Memory operand that is an offset from a register (m is preferable for asm statements)

R
AIX TOC entry

S
Windows NT SYMBOL REF

T
Windows NT LABEL REF

U
System V Release 4 small data area reference

Intel 386 i386.h

q
a, b, c,or d register

A
a, or d register (for 64-bit ints)

f
Floating point register

t
First (top of stack) floating point register

u
Second floating point register

a
a register

b
b register

c
c register

d
d register
D
di register

S
si register

I
Constant in range 0 to 31 (for 32 bit shifts)

J
Constant in range 0 to 63 (for 64 bit shifts)

K
0xff

L
0xffff

M
0, 1, 2, or 3 (shifts for lea instruction)

N
Constant in range 0 to 255 (for out instruction)

G
Standard 80387 floating point constant

Intel 960 i960.h

f
Floating point register (fp0 to fp3)

l
Local register (r0 to r15)

b
Global register (g0 to g15)

d
Any local or global register

I
Integers from 0 to 31

J
0

K
Integers from -31 to 0

G
Floating point 0

H
Floating point 1

MIPS mips.h

d
General-purpose integer register

f
Floating-point register (if available)

h
Hi register

l
Lo register

x
Hi or Lo register

y
General-purpose integer register

z
Floating-point status register

I
Signed 16 bit constant (for arithmetic instructions)

J
Zero

K
Zero-extended 16-bit constant (for logic instructions)

L
Constant with low 16 bits zero (can be loaded with lui)

M
32 bit constant which requires two instructions to load (a constant which is not I, K, or L)

N
Negative 16 bit constant

O
Exact power of two

P
Positive 16 bit constant

GO
Floating point zero

Q
Memory reference that can be loaded with more than one instruction (m is preferable for asm statements)

R
Memory reference that can be loaded with one instruction (m is preferable for asm statements)

S
Memory reference in external OSF/rose PIC for-mat (m is preferable for asm statements)

Motorola 680x0 m68k.h

a
Address register

d
Data register

f
68881 floating-point register, if available

x
Sun FPA (floating-point) register, if available

y
First 16 Sun FPA registers, if available

I
Integer in the range 1 to 8

J
16 bit signed number

K
Signed number whose magnitude is greater than 0x80

L
Integer in the range -8 to -1

M
Signed number whose magnitude is greater than 0x100

G
Floating point constant that is not a 68881 constant

H
Floating point constant that can be used by Sun FPA

SPARC sparc.h

f
Floating-point register

I
Signed 13 bit constant

J
Zero

K
32 bit constant with the low 12 bits clear (a constant that can be loaded with the sethi instruction)

G
Floating-point zero

H
Signed 13 bit constant, sign-extended to 32 or 64 bits

Q
Memory reference that can be loaded with one instruction (m is more appropriate for asm statements)

S
Constant, or memory address

T
Memory address aligned to an 8-byte boundary

U
Even register