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6.44.3.4 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.

Each architecture 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 useful for asm. Here is a summary of some of the machine-dependent constraints available on some particular machines; it includes both constraints that are useful for asm and constraints that aren't. The compiler source file mentioned in the table heading for each architecture is the definitive reference for the meanings of that architecture's constraints.

AArch64 family—config/aarch64/constraints.md
k
The stack pointer register (SP)
w
Floating point or SIMD vector register
I
Integer constant that is valid as an immediate operand in an ADD instruction
J
Integer constant that is valid as an immediate operand in a SUB instruction (once negated)
K
Integer constant that can be used with a 32-bit logical instruction
L
Integer constant that can be used with a 64-bit logical instruction
M
Integer constant that is valid as an immediate operand in a 32-bit MOV pseudo instruction. The MOV may be assembled to one of several different machine instructions depending on the value
N
Integer constant that is valid as an immediate operand in a 64-bit MOV pseudo instruction
S
An absolute symbolic address or a label reference
Y
Floating point constant zero
Z
Integer constant zero
Ush
The high part (bits 12 and upwards) of the pc-relative address of a symbol within 4GB of the instruction
Q
A memory address which uses a single base register with no offset
Ump
A memory address suitable for a load/store pair instruction in SI, DI, SF and DF modes

ARC —config/arc/constraints.md
q
Registers usable in ARCompact 16-bit instructions: r0-r3, r12-r15. This constraint can only match when the -mq option is in effect.
e
Registers usable as base-regs of memory addresses in ARCompact 16-bit memory instructions: r0-r3, r12-r15, sp. This constraint can only match when the -mq option is in effect.
D
ARC FPX (dpfp) 64-bit registers. D0, D1.
I
A signed 12-bit integer constant.
Cal
constant for arithmetic/logical operations. This might be any constant that can be put into a long immediate by the assmbler or linker without involving a PIC relocation.
K
A 3-bit unsigned integer constant.
L
A 6-bit unsigned integer constant.
CnL
One's complement of a 6-bit unsigned integer constant.
CmL
Two's complement of a 6-bit unsigned integer constant.
M
A 5-bit unsigned integer constant.
O
A 7-bit unsigned integer constant.
P
A 8-bit unsigned integer constant.
H
Any const_double value.

ARM family—config/arm/constraints.md
h
In Thumb state, the core registers r8-r15.
k
The stack pointer register.
l
In Thumb State the core registers r0-r7. In ARM state this is an alias for the r constraint.
t
VFP floating-point registers s0-s31. Used for 32 bit values.
w
VFP floating-point registers d0-d31 and the appropriate subset d0-d15 based on command line options. Used for 64 bit values only. Not valid for Thumb1.
y
The iWMMX co-processor registers.
z
The iWMMX GR registers.
G
The floating-point constant 0.0
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
Uv
A memory reference suitable for VFP load/store insns (reg+constant offset)
Uy
A memory reference suitable for iWMMXt load/store instructions.
Uq
A memory reference suitable for the ARMv4 ldrsb instruction.

AVR family—config/avr/constraints.md
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
Q
A memory address based on Y or Z pointer with displacement.

Blackfin family—config/bfin/constraints.md
a
P register
d
D register
z
A call clobbered P register.
qn
A single register. If n is in the range 0 to 7, the corresponding D register. If it is A, then the register P0.
D
Even-numbered D register
W
Odd-numbered D register
e
Accumulator register.
A
Even-numbered accumulator register.
B
Odd-numbered accumulator register.
b
I register
v
B register
f
M register
c
Registers used for circular buffering, i.e. I, B, or L registers.
C
The CC register.
t
LT0 or LT1.
k
LC0 or LC1.
u
LB0 or LB1.
x
Any D, P, B, M, I or L register.
y
Additional registers typically used only in prologues and epilogues: RETS, RETN, RETI, RETX, RETE, ASTAT, SEQSTAT and USP.
w
Any register except accumulators or CC.
Ksh
Signed 16 bit integer (in the range −32768 to 32767)
Kuh
Unsigned 16 bit integer (in the range 0 to 65535)
Ks7
Signed 7 bit integer (in the range −64 to 63)
Ku7
Unsigned 7 bit integer (in the range 0 to 127)
Ku5
Unsigned 5 bit integer (in the range 0 to 31)
Ks4
Signed 4 bit integer (in the range −8 to 7)
Ks3
Signed 3 bit integer (in the range −3 to 4)
Ku3
Unsigned 3 bit integer (in the range 0 to 7)
Pn
Constant n, where n is a single-digit constant in the range 0 to 4.
PA
An integer equal to one of the MACFLAG_XXX constants that is suitable for use with either accumulator.
PB
An integer equal to one of the MACFLAG_XXX constants that is suitable for use only with accumulator A1.
M1
Constant 255.
M2
Constant 65535.
J
An integer constant with exactly a single bit set.
L
An integer constant with all bits set except exactly one.
H
Q
Any SYMBOL_REF.

CR16 Architecture—config/cr16/cr16.h
b
Registers from r0 to r14 (registers without stack pointer)
t
Register from r0 to r11 (all 16-bit registers)
p
Register from r12 to r15 (all 32-bit registers)
I
Signed constant that fits in 4 bits
J
Signed constant that fits in 5 bits
K
Signed constant that fits in 6 bits
L
Unsigned constant that fits in 4 bits
M
Signed constant that fits in 32 bits
N
Check for 64 bits wide constants for add/sub instructions
G
Floating point constant that is legal for store immediate

Epiphany—config/epiphany/constraints.md
U16
An unsigned 16-bit constant.
K
An unsigned 5-bit constant.
L
A signed 11-bit constant.
Cm1
A signed 11-bit constant added to −1. Can only match when the -m1reg-reg option is active.
Cl1
Left-shift of −1, i.e., a bit mask with a block of leading ones, the rest being a block of trailing zeroes. Can only match when the -m1reg-reg option is active.
Cr1
Right-shift of −1, i.e., a bit mask with a trailing block of ones, the rest being zeroes. Or to put it another way, one less than a power of two. Can only match when the -m1reg-reg option is active.
Cal
Constant for arithmetic/logical operations. This is like i, except that for position independent code, no symbols / expressions needing relocations are allowed.
Csy
Symbolic constant for call/jump instruction.
Rcs
The register class usable in short insns. This is a register class constraint, and can thus drive register allocation. This constraint won't match unless -mprefer-short-insn-regs is in effect.
Rsc
The the register class of registers that can be used to hold a sibcall call address. I.e., a caller-saved register.
Rct
Core control register class.
Rgs
The register group usable in short insns. This constraint does not use a register class, so that it only passively matches suitable registers, and doesn't drive register allocation.
Rra
Matches the return address if it can be replaced with the link register.
Rcc
Matches the integer condition code register.
Sra
Matches the return address if it is in a stack slot.
Cfm
Matches control register values to switch fp mode, which are encapsulated in UNSPEC_FP_MODE.

FRV—config/frv/frv.h
a
Register in the class ACC_REGS (acc0 to acc7).
b
Register in the class EVEN_ACC_REGS (acc0 to acc7).
c
Register in the class CC_REGS (fcc0 to fcc3 and icc0 to icc3).
d
Register in the class GPR_REGS (gr0 to gr63).
e
Register in the class EVEN_REGS (gr0 to gr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.
f
Register in the class FPR_REGS (fr0 to fr63).
h
Register in the class FEVEN_REGS (fr0 to fr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.
l
Register in the class LR_REG (the lr register).
q
Register in the class QUAD_REGS (gr2 to gr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.
t
Register in the class ICC_REGS (icc0 to icc3).
u
Register in the class FCC_REGS (fcc0 to fcc3).
v
Register in the class ICR_REGS (cc4 to cc7).
w
Register in the class FCR_REGS (cc0 to cc3).
x
Register in the class QUAD_FPR_REGS (fr0 to fr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.
z
Register in the class SPR_REGS (lcr and lr).
A
Register in the class QUAD_ACC_REGS (acc0 to acc7).
B
Register in the class ACCG_REGS (accg0 to accg7).
C
Register in the class CR_REGS (cc0 to cc7).
G
Floating point constant zero
I
6-bit signed integer constant
J
10-bit signed integer constant
L
16-bit signed integer constant
M
16-bit unsigned integer constant
N
12-bit signed integer constant that is negative—i.e. in the range of −2048 to −1
O
Constant zero
P
12-bit signed integer constant that is greater than zero—i.e. in the range of 1 to 2047.

FT32—config/ft32/constraints.md
A
An absolute address
B
An offset address
W
A register indirect memory operand
e
An offset address.
f
An offset address.
O
The constant zero or one
I
A 16-bit signed constant (−32768 ... 32767)
w
A bitfield mask suitable for bext or bins
x
An inverted bitfield mask suitable for bext or bins
L
A 16-bit unsigned constant, multiple of 4 (0 ... 65532)
S
A 20-bit signed constant (−524288 ... 524287)
b
A constant for a bitfield width (1 ... 16)
KA
A 10-bit signed constant (−512 ... 511)

Hewlett-Packard PA-RISC—config/pa/pa.h
a
General register 1
f
Floating point register
q
Shift amount register
x
Floating point register (deprecated)
y
Upper floating point register (32-bit), floating point register (64-bit)
Z
Any register
I
Signed 11-bit integer constant
J
Signed 14-bit integer constant
K
Integer constant that can be deposited with a zdepi instruction
L
Signed 5-bit integer constant
M
Integer constant 0
N
Integer constant that can be loaded with a ldil instruction
O
Integer constant whose value plus one is a power of 2
P
Integer constant that can be used for and operations in depi and extru instructions
S
Integer constant 31
U
Integer constant 63
G
Floating-point constant 0.0
A
A lo_sum data-linkage-table memory operand
Q
A memory operand that can be used as the destination operand of an integer store instruction
R
A scaled or unscaled indexed memory operand
T
A memory operand for floating-point loads and stores
W
A register indirect memory operand

Intel IA-64—config/ia64/ia64.h
a
General register r0 to r3 for addl instruction
b
Branch register
c
Predicate register (‘c’ as in “conditional”)
d
Application register residing in M-unit
e
Application register residing in I-unit
f
Floating-point register
m
Memory operand. If used together with ‘<’ or ‘>’, the operand can have postincrement and postdecrement which require printing with ‘%Pn’ on IA-64.
G
Floating-point constant 0.0 or 1.0
I
14-bit signed integer constant
J
22-bit signed integer constant
K
8-bit signed integer constant for logical instructions
L
8-bit adjusted signed integer constant for compare pseudo-ops
M
6-bit unsigned integer constant for shift counts
N
9-bit signed integer constant for load and store postincrements
O
The constant zero
P
0 or −1 for dep instruction
Q
Non-volatile memory for floating-point loads and stores
R
Integer constant in the range 1 to 4 for shladd instruction
S
Memory operand except postincrement and postdecrement. This is now roughly the same as ‘m’ when not used together with ‘<’ or ‘>’.

M32C—config/m32c/m32c.c
Rsp
Rfb
Rsb
‘$sp’, ‘$fb’, ‘$sb’.
Rcr
Any control register, when they're 16 bits wide (nothing if control registers are 24 bits wide)
Rcl
Any control register, when they're 24 bits wide.
R0w
R1w
R2w
R3w
$r0, $r1, $r2, $r3.
R02
$r0 or $r2, or $r2r0 for 32 bit values.
R13
$r1 or $r3, or $r3r1 for 32 bit values.
Rdi
A register that can hold a 64 bit value.
Rhl
$r0 or $r1 (registers with addressable high/low bytes)
R23
$r2 or $r3
Raa
Address registers
Raw
Address registers when they're 16 bits wide.
Ral
Address registers when they're 24 bits wide.
Rqi
Registers that can hold QI values.
Rad
Registers that can be used with displacements ($a0, $a1, $sb).
Rsi
Registers that can hold 32 bit values.
Rhi
Registers that can hold 16 bit values.
Rhc
Registers chat can hold 16 bit values, including all control registers.
Rra
$r0 through R1, plus $a0 and $a1.
Rfl
The flags register.
Rmm
The memory-based pseudo-registers $mem0 through $mem15.
Rpi
Registers that can hold pointers (16 bit registers for r8c, m16c; 24 bit registers for m32cm, m32c).
Rpa
Matches multiple registers in a PARALLEL to form a larger register. Used to match function return values.
Is3
−8 ... 7
IS1
−128 ... 127
IS2
−32768 ... 32767
IU2
0 ... 65535
In4
−8 ... −1 or 1 ... 8
In5
−16 ... −1 or 1 ... 16
In6
−32 ... −1 or 1 ... 32
IM2
−65536 ... −1
Ilb
An 8 bit value with exactly one bit set.
Ilw
A 16 bit value with exactly one bit set.
Sd
The common src/dest memory addressing modes.
Sa
Memory addressed using $a0 or $a1.
Si
Memory addressed with immediate addresses.
Ss
Memory addressed using the stack pointer ($sp).
Sf
Memory addressed using the frame base register ($fb).
Ss
Memory addressed using the small base register ($sb).
S1
$r1h

MeP—config/mep/constraints.md
a
The $sp register.
b
The $tp register.
c
Any control register.
d
Either the $hi or the $lo register.
em
Coprocessor registers that can be directly loaded ($c0-$c15).
ex
Coprocessor registers that can be moved to each other.
er
Coprocessor registers that can be moved to core registers.
h
The $hi register.
j
The $rpc register.
l
The $lo register.
t
Registers which can be used in $tp-relative addressing.
v
The $gp register.
x
The coprocessor registers.
y
The coprocessor control registers.
z
The $0 register.
A
User-defined register set A.
B
User-defined register set B.
C
User-defined register set C.
D
User-defined register set D.
I
Offsets for $gp-rel addressing.
J
Constants that can be used directly with boolean insns.
K
Constants that can be moved directly to registers.
L
Small constants that can be added to registers.
M
Long shift counts.
N
Small constants that can be compared to registers.
O
Constants that can be loaded into the top half of registers.
S
Signed 8-bit immediates.
T
Symbols encoded for $tp-rel or $gp-rel addressing.
U
Non-constant addresses for loading/saving coprocessor registers.
W
The top half of a symbol's value.
Y
A register indirect address without offset.
Z
Symbolic references to the control bus.

MicroBlaze—config/microblaze/constraints.md
d
A general register (r0 to r31).
z
A status register (rmsr, $fcc1 to $fcc7).

MIPS—config/mips/constraints.md
d
An address register. This is equivalent to r unless generating MIPS16 code.
f
A floating-point register (if available).
h
Formerly the hi register. This constraint is no longer supported.
l
The lo register. Use this register to store values that are no bigger than a word.
x
The concatenated hi and lo registers. Use this register to store doubleword values.
c
A register suitable for use in an indirect jump. This will always be $25 for -mabicalls.
v
Register $3. Do not use this constraint in new code; it is retained only for compatibility with glibc.
y
Equivalent to r; retained for backwards compatibility.
z
A floating-point condition code register.
I
A signed 16-bit constant (for arithmetic instructions).
J
Integer zero.
K
An unsigned 16-bit constant (for logic instructions).
L
A signed 32-bit constant in which the lower 16 bits are zero. Such constants can be loaded using lui.
M
A constant that cannot be loaded using lui, addiu or ori.
N
A constant in the range −65535 to −1 (inclusive).
O
A signed 15-bit constant.
P
A constant in the range 1 to 65535 (inclusive).
G
Floating-point zero.
R
An address that can be used in a non-macro load or store.
ZC
A memory operand whose address is formed by a base register and offset that is suitable for use in instructions with the same addressing mode as ll and sc.
ZD
An address suitable for a prefetch instruction, or for any other instruction with the same addressing mode as prefetch.

Motorola 680x0—config/m68k/constraints.md
a
Address register
d
Data register
f
68881 floating-point register, 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
N
Range 24 to 31, rotatert:SI 8 to 1 expressed as rotate
O
16 (for rotate using swap)
P
Range 8 to 15, rotatert:HI 8 to 1 expressed as rotate
R
Numbers that mov3q can handle
G
Floating point constant that is not a 68881 constant
S
Operands that satisfy 'm' when -mpcrel is in effect
T
Operands that satisfy 's' when -mpcrel is not in effect
Q
Address register indirect addressing mode
U
Register offset addressing
W
const_call_operand
Cs
symbol_ref or const
Ci
const_int
C0
const_int 0
Cj
Range of signed numbers that don't fit in 16 bits
Cmvq
Integers valid for mvq
Capsw
Integers valid for a moveq followed by a swap
Cmvz
Integers valid for mvz
Cmvs
Integers valid for mvs
Ap
push_operand
Ac
Non-register operands allowed in clr

Moxie—config/moxie/constraints.md
A
An absolute address
B
An offset address
W
A register indirect memory operand
I
A constant in the range of 0 to 255.
N
A constant in the range of 0 to −255.

MSP430–config/msp430/constraints.md
R12
Register R12.
R13
Register R13.
K
Integer constant 1.
L
Integer constant -1^20..1^19.
M
Integer constant 1-4.
Ya
Memory references which do not require an extended MOVX instruction.
Yl
Memory reference, labels only.
Ys
Memory reference, stack only.

NDS32—config/nds32/constraints.md
w
LOW register class $r0 to $r7 constraint for V3/V3M ISA.
l
LOW register class $r0 to $r7.
d
MIDDLE register class $r0 to $r11, $r16 to $r19.
h
HIGH register class $r12 to $r14, $r20 to $r31.
t
Temporary assist register $ta (i.e. $r15).
k
Stack register $sp.
Iu03
Unsigned immediate 3-bit value.
In03
Negative immediate 3-bit value in the range of −7–0.
Iu04
Unsigned immediate 4-bit value.
Is05
Signed immediate 5-bit value.
Iu05
Unsigned immediate 5-bit value.
In05
Negative immediate 5-bit value in the range of −31–0.
Ip05
Unsigned immediate 5-bit value for movpi45 instruction with range 16–47.
Iu06
Unsigned immediate 6-bit value constraint for addri36.sp instruction.
Iu08
Unsigned immediate 8-bit value.
Iu09
Unsigned immediate 9-bit value.
Is10
Signed immediate 10-bit value.
Is11
Signed immediate 11-bit value.
Is15
Signed immediate 15-bit value.
Iu15
Unsigned immediate 15-bit value.
Ic15
A constant which is not in the range of imm15u but ok for bclr instruction.
Ie15
A constant which is not in the range of imm15u but ok for bset instruction.
It15
A constant which is not in the range of imm15u but ok for btgl instruction.
Ii15
A constant whose compliment value is in the range of imm15u and ok for bitci instruction.
Is16
Signed immediate 16-bit value.
Is17
Signed immediate 17-bit value.
Is19
Signed immediate 19-bit value.
Is20
Signed immediate 20-bit value.
Ihig
The immediate value that can be simply set high 20-bit.
Izeb
The immediate value 0xff.
Izeh
The immediate value 0xffff.
Ixls
The immediate value 0x01.
Ix11
The immediate value 0x7ff.
Ibms
The immediate value with power of 2.
Ifex
The immediate value with power of 2 minus 1.
U33
Memory constraint for 333 format.
U45
Memory constraint for 45 format.
U37
Memory constraint for 37 format.

Nios II family—config/nios2/constraints.md
I
Integer that is valid as an immediate operand in an instruction taking a signed 16-bit number. Range −32768 to 32767.
J
Integer that is valid as an immediate operand in an instruction taking an unsigned 16-bit number. Range 0 to 65535.
K
Integer that is valid as an immediate operand in an instruction taking only the upper 16-bits of a 32-bit number. Range 32-bit numbers with the lower 16-bits being 0.
L
Integer that is valid as an immediate operand for a shift instruction. Range 0 to 31.
M
Integer that is valid as an immediate operand for only the value 0. Can be used in conjunction with the format modifier z to use r0 instead of 0 in the assembly output.
N
Integer that is valid as an immediate operand for a custom instruction opcode. Range 0 to 255.
P
An immediate operand for R2 andchi/andci instructions.
S
Matches immediates which are addresses in the small data section and therefore can be added to gp as a 16-bit immediate to re-create their 32-bit value.
U
Matches constants suitable as an operand for the rdprs and cache instructions.
v
A memory operand suitable for Nios II R2 load/store exclusive instructions.
w
A memory operand suitable for load/store IO and cache instructions.

PDP-11—config/pdp11/constraints.md
a
Floating point registers AC0 through AC3. These can be loaded from/to memory with a single instruction.
d
Odd numbered general registers (R1, R3, R5). These are used for 16-bit multiply operations.
f
Any of the floating point registers (AC0 through AC5).
G
Floating point constant 0.
I
An integer constant that fits in 16 bits.
J
An integer constant whose low order 16 bits are zero.
K
An integer constant that does not meet the constraints for codes ‘I’ or ‘J’.
L
The integer constant 1.
M
The integer constant −1.
N
The integer constant 0.
O
Integer constants −4 through −1 and 1 through 4; shifts by these amounts are handled as multiple single-bit shifts rather than a single variable-length shift.
Q
A memory reference which requires an additional word (address or offset) after the opcode.
R
A memory reference that is encoded within the opcode.

PowerPC and IBM RS6000—config/rs6000/constraints.md
b
Address base register
d
Floating point register (containing 64-bit value)
f
Floating point register (containing 32-bit value)
v
Altivec vector register
wa
Any VSX register if the -mvsx option was used or NO_REGS.

When using any of the register constraints (wa, wd, wf, wg, wh, wi, wj, wk, wl, wm, wo, wp, wq, ws, wt, wu, wv, ww, or wy) that take VSX registers, you must use %x<n> in the template so that the correct register is used. Otherwise the register number output in the assembly file will be incorrect if an Altivec register is an operand of a VSX instruction that expects VSX register numbering. 

               asm ("xvadddp %x0,%x1,%x2" : "=wa" (v1) : "wa" (v2), "wa" (v3));

is correct, but: 

               asm ("xvadddp %0,%1,%2" : "=wa" (v1) : "wa" (v2), "wa" (v3));

is not correct.

If an instruction only takes Altivec registers, you do not want to use %x<n>. 

               asm ("xsaddqp %0,%1,%2" : "=v" (v1) : "v" (v2), "v" (v3));

is correct because the xsaddqp instruction only takes Altivec registers, while: 

               asm ("xsaddqp %x0,%x1,%x2" : "=v" (v1) : "v" (v2), "v" (v3));

is incorrect.

wb
Altivec register if -mpower9-dform is used or NO_REGS.
wd
VSX vector register to hold vector double data or NO_REGS.
we
VSX register if the -mpower9-vector and -m64 options were used or NO_REGS.
wf
VSX vector register to hold vector float data or NO_REGS.
wg
If -mmfpgpr was used, a floating point register or NO_REGS.
wh
Floating point register if direct moves are available, or NO_REGS.
wi
FP or VSX register to hold 64-bit integers for VSX insns or NO_REGS.
wj
FP or VSX register to hold 64-bit integers for direct moves or NO_REGS.
wk
FP or VSX register to hold 64-bit doubles for direct moves or NO_REGS.
wl
Floating point register if the LFIWAX instruction is enabled or NO_REGS.
wm
VSX register if direct move instructions are enabled, or NO_REGS.
wn
No register (NO_REGS).
wo
VSX register to use for ISA 3.0 vector instructions, or NO_REGS.
wp
VSX register to use for IEEE 128-bit floating point TFmode, or NO_REGS.
wq
VSX register to use for IEEE 128-bit floating point, or NO_REGS.
wr
General purpose register if 64-bit instructions are enabled or NO_REGS.
ws
VSX vector register to hold scalar double values or NO_REGS.
wt
VSX vector register to hold 128 bit integer or NO_REGS.
wu
Altivec register to use for float/32-bit int loads/stores or NO_REGS.
wv
Altivec register to use for double loads/stores or NO_REGS.
ww
FP or VSX register to perform float operations under -mvsx or NO_REGS.
wx
Floating point register if the STFIWX instruction is enabled or NO_REGS.
wy
FP or VSX register to perform ISA 2.07 float ops or NO_REGS.
wz
Floating point register if the LFIWZX instruction is enabled or NO_REGS.
wD
Int constant that is the element number of the 64-bit scalar in a vector.
wE
Vector constant that can be loaded with the XXSPLTIB instruction.
wF
Memory operand suitable for power9 fusion load/stores.
wG
Memory operand suitable for TOC fusion memory references.
wL
Int constant that is the element number that the MFVSRLD instruction. targets.
wM
Match vector constant with all 1's if the XXLORC instruction is available.
wO
A memory operand suitable for the ISA 3.0 vector d-form instructions.
wQ
A memory address that will work with the lq and stq instructions.
wS
Vector constant that can be loaded with XXSPLTIB & sign extension.
h
‘MQ’, ‘CTR’, or ‘LINK’ register
c
‘CTR’ register
l
‘LINK’ register
x
‘CR’ register (condition register) number 0
y
‘CR’ register (condition register)
z
‘XER[CA]’ carry bit (part of the XER register)
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
H
Integer/Floating point constant that can be loaded into a register using three instructions
m
Memory operand. Normally, m does not allow addresses that update the base register. If ‘<’ or ‘>’ constraint is also used, they are allowed and therefore on PowerPC targets in that case it is only safe to use ‘m<>’ in an asm statement if that asm statement accesses the operand exactly once. The asm statement must also use ‘%U<opno>’ as a placeholder for the “update” flag in the corresponding load or store instruction. For example: 
               asm ("st%U0 %1,%0" : "=m<>" (mem) : "r" (val));

is correct but: 

               asm ("st %1,%0" : "=m<>" (mem) : "r" (val));

is not.

es
A “stable” memory operand; that is, one which does not include any automodification of the base register. This used to be useful when ‘m’ allowed automodification of the base register, but as those are now only allowed when ‘<’ or ‘>’ is used, ‘es’ is basically the same as ‘m’ without ‘<’ and ‘>’.
Q
Memory operand that is an offset from a register (it is usually better to use ‘m’ or ‘es’ in asm statements)
Z
Memory operand that is an indexed or indirect from a register (it is usually better to use ‘m’ or ‘es’ in asm statements)
R
AIX TOC entry
a
Address operand that is an indexed or indirect from a register (‘p’ is preferable for asm statements)
U
System V Release 4 small data area reference
W
Vector constant that does not require memory
j
Vector constant that is all zeros.

RL78—config/rl78/constraints.md
Int3
An integer constant in the range 1 ... 7.
Int8
An integer constant in the range 0 ... 255.
J
An integer constant in the range −255 ... 0
K
The integer constant 1.
L
The integer constant -1.
M
The integer constant 0.
N
The integer constant 2.
O
The integer constant -2.
P
An integer constant in the range 1 ... 15.
Qbi
The built-in compare types–eq, ne, gtu, ltu, geu, and leu.
Qsc
The synthetic compare types–gt, lt, ge, and le.
Wab
A memory reference with an absolute address.
Wbc
A memory reference using BC as a base register, with an optional offset.
Wca
A memory reference using AX, BC, DE, or HL for the address, for calls.
Wcv
A memory reference using any 16-bit register pair for the address, for calls.
Wd2
A memory reference using DE as a base register, with an optional offset.
Wde
A memory reference using DE as a base register, without any offset.
Wfr
Any memory reference to an address in the far address space.
Wh1
A memory reference using HL as a base register, with an optional one-byte offset.
Whb
A memory reference using HL as a base register, with B or C as the index register.
Whl
A memory reference using HL as a base register, without any offset.
Ws1
A memory reference using SP as a base register, with an optional one-byte offset.
Y
Any memory reference to an address in the near address space.
A
The AX register.
B
The BC register.
D
The DE register.
R
A through L registers.
S
The SP register.
T
The HL register.
Z08W
The 16-bit R8 register.
Z10W
The 16-bit R10 register.
Zint
The registers reserved for interrupts (R24 to R31).
a
The A register.
b
The B register.
c
The C register.
d
The D register.
e
The E register.
h
The H register.
l
The L register.
v
The virtual registers.
w
The PSW register.
x
The X register.

RX—config/rx/constraints.md
Q
An address which does not involve register indirect addressing or pre/post increment/decrement addressing.
Symbol
A symbol reference.
Int08
A constant in the range −256 to 255, inclusive.
Sint08
A constant in the range −128 to 127, inclusive.
Sint16
A constant in the range −32768 to 32767, inclusive.
Sint24
A constant in the range −8388608 to 8388607, inclusive.
Uint04
A constant in the range 0 to 15, inclusive.

S/390 and zSeries—config/s390/s390.h
a
Address register (general purpose register except r0)
c
Condition code register
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
Value appropriate as displacement.
(0..4095)
for short displacement
(−524288..524287)
for long displacement

M
Constant integer with a value of 0x7fffffff.
N
Multiple letter constraint followed by 4 parameter letters.
0..9:
number of the part counting from most to least significant
H,Q:
mode of the part
D,S,H:
mode of the containing operand
0,F:
value of the other parts (F—all bits set)
The constraint matches if the specified part of a constant has a value different from its other parts.
Q
Memory reference without index register and with short displacement.
R
Memory reference with index register and short displacement.
S
Memory reference without index register but with long displacement.
T
Memory reference with index register and long displacement.
U
Pointer with short displacement.
W
Pointer with long displacement.
Y
Shift count operand.

SPARC—config/sparc/sparc.h
f
Floating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture.
e
Floating-point register. It is equivalent to ‘f’ on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture.
c
Floating-point condition code register.
d
Lower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
b
Floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
h
64-bit global or out register for the SPARC-V8+ architecture.
C
The constant all-ones, for floating-point.
A
Signed 5-bit constant
D
A vector constant
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 (11-bit signed immediate)
M
A constant in the range supported by movrcc instructions (10-bit signed immediate)
N
Same as ‘K’, except that it verifies that bits that are not in the lower 32-bit range are all zero. Must be used instead of ‘K’ for modes wider than SImode
O
The constant 4096
G
Floating-point zero
H
Signed 13-bit constant, sign-extended to 32 or 64 bits
P
The constant -1
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
w
Memory address with only a base register
Y
Vector zero

SPU—config/spu/spu.h
a
An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 64 bit value.
c
An immediate for and/xor/or instructions. const_int is treated as a 64 bit value.
d
An immediate for the iohl instruction. const_int is treated as a 64 bit value.
f
An immediate which can be loaded with fsmbi.
A
An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 32 bit value.
B
An immediate for most arithmetic instructions. const_int is treated as a 32 bit value.
C
An immediate for and/xor/or instructions. const_int is treated as a 32 bit value.
D
An immediate for the iohl instruction. const_int is treated as a 32 bit value.
I
A constant in the range [−64, 63] for shift/rotate instructions.
J
An unsigned 7-bit constant for conversion/nop/channel instructions.
K
A signed 10-bit constant for most arithmetic instructions.
M
A signed 16 bit immediate for stop.
N
An unsigned 16-bit constant for iohl and fsmbi.
O
An unsigned 7-bit constant whose 3 least significant bits are 0.
P
An unsigned 3-bit constant for 16-byte rotates and shifts
R
Call operand, reg, for indirect calls
S
Call operand, symbol, for relative calls.
T
Call operand, const_int, for absolute calls.
U
An immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is sign extended to 128 bit.
W
An immediate for shift and rotate instructions. const_int is treated as a 32 bit value.
Y
An immediate for and/xor/or instructions. const_int is sign extended as a 128 bit.
Z
An immediate for the iohl instruction. const_int is sign extended to 128 bit.

TI C6X family—config/c6x/constraints.md
a
Register file A (A0–A31).
b
Register file B (B0–B31).
A
Predicate registers in register file A (A0–A2 on C64X and higher, A1 and A2 otherwise).
B
Predicate registers in register file B (B0–B2).
C
A call-used register in register file B (B0–B9, B16–B31).
Da
Register file A, excluding predicate registers (A3–A31, plus A0 if not C64X or higher).
Db
Register file B, excluding predicate registers (B3–B31).
Iu4
Integer constant in the range 0 ... 15.
Iu5
Integer constant in the range 0 ... 31.
In5
Integer constant in the range −31 ... 0.
Is5
Integer constant in the range −16 ... 15.
I5x
Integer constant that can be the operand of an ADDA or a SUBA insn.
IuB
Integer constant in the range 0 ... 65535.
IsB
Integer constant in the range −32768 ... 32767.
IsC
Integer constant in the range -2^20 ... 2^20 - 1.
Jc
Integer constant that is a valid mask for the clr instruction.
Js
Integer constant that is a valid mask for the set instruction.
Q
Memory location with A base register.
R
Memory location with B base register.
Z
Register B14 (aka DP).

TILE-Gx—config/tilegx/constraints.md
R00
R01
R02
R03
R04
R05
R06
R07
R08
R09
R10
Each of these represents a register constraint for an individual register, from r0 to r10.
I
Signed 8-bit integer constant.
J
Signed 16-bit integer constant.
K
Unsigned 16-bit integer constant.
L
Integer constant that fits in one signed byte when incremented by one (−129 ... 126).
m
Memory operand. If used together with ‘<’ or ‘>’, the operand can have postincrement which requires printing with ‘%In’ and ‘%in’ on TILE-Gx. For example: 
               asm ("st_add %I0,%1,%i0" : "=m<>" (*mem) : "r" (val));

M
A bit mask suitable for the BFINS instruction.
N
Integer constant that is a byte tiled out eight times.
O
The integer zero constant.
P
Integer constant that is a sign-extended byte tiled out as four shorts.
Q
Integer constant that fits in one signed byte when incremented (−129 ... 126), but excluding -1.
S
Integer constant that has all 1 bits consecutive and starting at bit 0.
T
A 16-bit fragment of a got, tls, or pc-relative reference.
U
Memory operand except postincrement. This is roughly the same as ‘m’ when not used together with ‘<’ or ‘>’.
W
An 8-element vector constant with identical elements.
Y
A 4-element vector constant with identical elements.
Z0
The integer constant 0xffffffff.
Z1
The integer constant 0xffffffff00000000.

TILEPro—config/tilepro/constraints.md
R00
R01
R02
R03
R04
R05
R06
R07
R08
R09
R10
Each of these represents a register constraint for an individual register, from r0 to r10.
I
Signed 8-bit integer constant.
J
Signed 16-bit integer constant.
K
Nonzero integer constant with low 16 bits zero.
L
Integer constant that fits in one signed byte when incremented by one (−129 ... 126).
m
Memory operand. If used together with ‘<’ or ‘>’, the operand can have postincrement which requires printing with ‘%In’ and ‘%in’ on TILEPro. For example: 
               asm ("swadd %I0,%1,%i0" : "=m<>" (mem) : "r" (val));

M
A bit mask suitable for the MM instruction.
N
Integer constant that is a byte tiled out four times.
O
The integer zero constant.
P
Integer constant that is a sign-extended byte tiled out as two shorts.
Q
Integer constant that fits in one signed byte when incremented (−129 ... 126), but excluding -1.
T
A symbolic operand, or a 16-bit fragment of a got, tls, or pc-relative reference.
U
Memory operand except postincrement. This is roughly the same as ‘m’ when not used together with ‘<’ or ‘>’.
W
A 4-element vector constant with identical elements.
Y
A 2-element vector constant with identical elements.

Visium—config/visium/constraints.md
b
EAM register mdb
c
EAM register mdc
f
Floating point register
l
General register, but not r29, r30 and r31
t
Register r1
u
Register r2
v
Register r3
G
Floating-point constant 0.0
J
Integer constant in the range 0 .. 65535 (16-bit immediate)
K
Integer constant in the range 1 .. 31 (5-bit immediate)
L
Integer constant in the range −65535 .. −1 (16-bit negative immediate)
M
Integer constant −1
O
Integer constant 0
P
Integer constant 32

x86 family—config/i386/constraints.md
R
Legacy register—the eight integer registers available on all i386 processors (a, b, c, d, si, di, bp, sp).
q
Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 64-bit mode, any integer register.
Q
Any register accessible as rh: a, b, c, and d.
a
The a register.
b
The b register.
c
The c register.
d
The d register.
S
The si register.
D
The di register.
A
The a and d registers. This class is used for instructions that return double word results in the ax:dx register pair. Single word values will be allocated either in ax or dx. For example on i386 the following implements rdtsc: 
               unsigned long long rdtsc (void)
               {
                 unsigned long long tick;
                 __asm__ __volatile__("rdtsc":"=A"(tick));
                 return tick;
               }

This is not correct on x86-64 as it would allocate tick in either ax or dx. You have to use the following variant instead: 

               unsigned long long rdtsc (void)
               {
                 unsigned int tickl, tickh;
                 __asm__ __volatile__("rdtsc":"=a"(tickl),"=d"(tickh));
                 return ((unsigned long long)tickh << 32)|tickl;
               }

f
Any 80387 floating-point (stack) register.
t
Top of 80387 floating-point stack (%st(0)).
u
Second from top of 80387 floating-point stack (%st(1)).
y
Any MMX register.
x
Any SSE register.
Yz
First SSE register (%xmm0).
I
Integer constant in the range 0 ... 31, for 32-bit shifts.
J
Integer constant in the range 0 ... 63, for 64-bit shifts.
K
Signed 8-bit integer constant.
L
0xFF or 0xFFFF, for andsi as a zero-extending move.
M
0, 1, 2, or 3 (shifts for the lea instruction).
N
Unsigned 8-bit integer constant (for in and out instructions).
G
Standard 80387 floating point constant.
C
SSE constant zero operand.
e
32-bit signed integer constant, or a symbolic reference known to fit that range (for immediate operands in sign-extending x86-64 instructions).
Z
32-bit unsigned integer constant, or a symbolic reference known to fit that range (for immediate operands in zero-extending x86-64 instructions).

Xstormy16—config/stormy16/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 a 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.
Z
The constant 0.

Xtensa—config/xtensa/constraints.md
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