It is occasionally useful to test a constraint from C code rather than
implicitly via the constraint string in a match_operand
. The
generated file tm_p.h declares a few interfaces for working
with constraints. At present these are defined for all constraints
except g
(which is equivalent to general_operand
).
Some valid constraint names are not valid C identifiers, so there is a mangling scheme for referring to them from C. Constraint names that do not contain angle brackets or underscores are left unchanged. Underscores are doubled, each ‘<’ is replaced with ‘_l’, and each ‘>’ with ‘_g’. Here are some examples:
Original  Mangled 
x  x 
P42x  P42x 
P4_x  P4__x 
P4>x  P4_gx 
P4>>  P4_g_g 
P4_g>  P4__g_g

Throughout this section, the variable c is either a constraint
in the abstract sense, or a constant from enum constraint_num
;
the variable m is a mangled constraint name (usually as part of
a larger identifier).
For each constraint except
g
, there is a corresponding enumeration constant: ‘CONSTRAINT_’ plus the mangled name of the constraint. Functions that take anenum constraint_num
as an argument expect one of these constants.
For each nonregister constraint m except
g
, there is one of these functions; it returnstrue
if exp satisfies the constraint. These functions are only visible if rtl.h was included before tm_p.h.
Like the
satisfies_constraint_
m functions, but the constraint to test is given as an argument, c. If c specifies a register constraint, this function will always returnfalse
.
Returns the register class associated with c. If c is not a register constraint, or those registers are not available for the currently selected subtarget, returns
NO_REGS
.
Here is an example use of satisfies_constraint_
m. In
peephole optimizations (see Peephole Definitions), operand
constraint strings are ignored, so if there are relevant constraints,
they must be tested in the C condition. In the example, the
optimization is applied if operand 2 does not satisfy the
‘K’ constraint. (This is a simplified version of a peephole
definition from the i386 machine description.)
(define_peephole2 [(match_scratch:SI 3 "r") (set (match_operand:SI 0 "register_operand" "") (mult:SI (match_operand:SI 1 "memory_operand" "") (match_operand:SI 2 "immediate_operand" "")))] "!satisfies_constraint_K (operands[2])" [(set (match_dup 3) (match_dup 1)) (set (match_dup 0) (mult:SI (match_dup 3) (match_dup 2)))] "")