GNU Compiler Collection (GCC) Internals: C Constraint Interface 

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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 












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 an enum constraint_num
as an
argument expect one of these constants.
For each nonregister constraint m except g
, there is
one of these functions; it returns true
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 return
false
.
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)))] "")
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