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.

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

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
 

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)

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

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

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

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)

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

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