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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/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 these 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, here is a summary of the machine-dependent constraints available on some particular machines.

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)

AVR family--avr.h
l
Registers from r0 to r15
a
Registers from r16 to r23
d
Registers from r16 to r31
w
Registers from r24 to r31. These registers can be used in adiw command
e
Pointer register (r26-r31)
b
Base pointer register (r28-r31)
q
Stack pointer register (SPH:SPL)
t
Temporary register r0
x
Register pair X (r27:r26)
y
Register pair Y (r29:r28)
z
Register pair Z (r31:r30)
I
Constant greater than -1, less than 64
J
Constant greater than -64, less than 1
K
Constant integer 2
L
Constant integer 0
M
Constant that fits in 8 bits
N
Constant integer -1
O
Constant integer 8, 16, or 24
P
Constant integer 1
G
A floating point constant 0.0

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)
z
FPMEM stack memory for FPR-GPR transfers
I
Signed 16-bit constant
J
Unsigned 16-bit constant shifted left 16 bits (use L instead for SImode constants)
K
Unsigned 16-bit constant
L
Signed 16-bit constant shifted left 16 bits
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
Constant suitable as a 64-bit mask operand
T
Constant suitable as a 32-bit mask operand
U
System V Release 4 small data area reference

Intel 386--i386.h
q
a, b, c, or d register for the i386. For x86-64 it is equivalent to r class. (for 8-bit instructions that do not use upper halves)
Q
a, b, c, or d register. (for 8-bit instructions, that do use upper halves)
R
Legacy register--equivalent to r class in i386 mode. (for non-8-bit registers used together with 8-bit upper halves in a single instruction)
A
Specifies the a or d registers. This is primarily useful for 64-bit integer values (when in 32-bit mode) intended to be returned with the d register holding the most significant bits and the a register holding the least significant bits.
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
x
xmm SSE register
y
MMX 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)
Z
Constant in range 0 to 0xffffffff or symbolic reference known to fit specified range. (for using immediates in zero extending 32-bit to 64-bit x86-64 instructions)
e
Constant in range -2147483648 to 2147483647 or symbolic reference known to fit specified range. (for using immediates in 64-bit x86-64 instructions)
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
G
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 format (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

Motorola 68HC11 & 68HC12 families--m68hc11.h
a
Register 'a'
b
Register 'b'
d
Register 'd'
q
An 8-bit register
t
Temporary soft register _.tmp
u
A soft register _.d1 to _.d31
w
Stack pointer register
x
Register 'x'
y
Register 'y'
z
Pseudo register 'z' (replaced by 'x' or 'y' at the end)
A
An address register: x, y or z
B
An address register: x or y
D
Register pair (x:d) to form a 32-bit value
L
Constants in the range -65536 to 65535
M
Constants whose 16-bit low part is zero
N
Constant integer 1 or -1
O
Constant integer 16
P
Constants in the range -8 to 2

SPARC--sparc.h
f
Floating-point register that can hold 32- or 64-bit values.
e
Floating-point register that can hold 64- or 128-bit values.
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)
L
A constant in the range supported by movcc instructions
M
A constant in the range supported by movrcc instructions
N
Same as K, except that it verifies that bits that are not in the lower 32-bits range are all zero. Must be used instead of K for modes wider than SImode
G
Floating-point zero
H
Signed 13-bit constant, sign-extended to 32 or 64 bits
Q
Floating-point constant whose integral representation can be moved into an integer register using a single sethi instruction
R
Floating-point constant whose integral representation can be moved into an integer register using a single mov instruction
S
Floating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence
T
Memory address aligned to an 8-byte boundary
U
Even register
W
Memory address for e constraint registers.

TMS320C3x/C4x--c4x.h
a
Auxiliary (address) register (ar0-ar7)
b
Stack pointer register (sp)
c
Standard (32-bit) precision integer register
f
Extended (40-bit) precision register (r0-r11)
k
Block count register (bk)
q
Extended (40-bit) precision low register (r0-r7)
t
Extended (40-bit) precision register (r0-r1)
u
Extended (40-bit) precision register (r2-r3)
v
Repeat count register (rc)
x
Index register (ir0-ir1)
y
Status (condition code) register (st)
z
Data page register (dp)
G
Floating-point zero
H
Immediate 16-bit floating-point constant
I
Signed 16-bit constant
J
Signed 8-bit constant
K
Signed 5-bit constant
L
Unsigned 16-bit constant
M
Unsigned 8-bit constant
N
Ones complement of unsigned 16-bit constant
O
High 16-bit constant (32-bit constant with 16 LSBs zero)
Q
Indirect memory reference with signed 8-bit or index register displacement
R
Indirect memory reference with unsigned 5-bit displacement
S
Indirect memory reference with 1 bit or index register displacement
T
Direct memory reference
U
Symbolic address

S/390 and zSeries--s390.h
a
Address register (general purpose register except r0)
d
Data register (arbitrary general purpose register)
f
Floating-point register
I
Unsigned 8-bit constant (0-255)
J
Unsigned 12-bit constant (0-4095)
K
Signed 16-bit constant (-32768-32767)
L
Unsigned 16-bit constant (0-65535)
Q
Memory reference without index register
S
Symbolic constant suitable for use with the larl instruction

Xstormy16--stormy16.h
a
Register r0.
b
Register r1.
c
Register r2.
d
Register r8.
e
Registers r0 through r7.
t
Registers r0 and r1.
y
The carry register.
z
Registers r8 and r9.
I
A constant between 0 and 3 inclusive.
J
A constant that has exactly one bit set.
K
A constant that has exactly one bit clear.
L
A constant between 0 and 255 inclusive.
M
A constant between -255 and 0 inclusive.
N
A constant between -3 and 0 inclusive.
O
A constant between 1 and 4 inclusive.
P
A constant between -4 and -1 inclusive.
Q
A memory reference that is a stack push.
R
A memory reference that is a stack pop.
S
A memory reference that refers to an constant address of known value.
T
The register indicated by Rx (not implemented yet).
U
A constant that is not between 2 and 15 inclusive.

Xtensa--xtensa.h
a
General-purpose 32-bit register
b
One-bit boolean register
A
MAC16 40-bit accumulator register
I
Signed 12-bit integer constant, for use in MOVI instructions
J
Signed 8-bit integer constant, for use in ADDI instructions
K
Integer constant valid for BccI instructions
L
Unsigned constant valid for BccUI instructions