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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
familyarm.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 familya29k.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
RS6000rs6000.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
386i386.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
960i960.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
MIPSmips.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
680x0m68k.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
SPARCsparc.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