Red Hat ENTERPRISE LINUX 4 Manual page 191

Chapter 6. Extensions to the C Language Family
REG_CLASS_FROM_LETTER
Register class constraints (usually lowercase).
CONST_OK_FOR_LETTER_P
Immediate constant constraints, for non-floating point constants of word size or smaller precision
(usually uppercase).
CONST_DOUBLE_OK_FOR_LETTER_P
Immediate constant constraints, for all floating point constants and for constants of greater than
word size precision (usually uppercase).
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
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
L
Integer that satisfies constraint
M
Integer in the range 0 to 32
Q
A memory reference where the exact address is in a single register ('
statements)
when inverted (ones complement)
I
when negated (twos complement)
I
if it were negated
F
' is preferable for
m
183
asm