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16.8.8 Testing constraints from C

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).

— Enum: constraint_num

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.

— Function: inline bool satisfies_constraint_m (rtx exp)

For each non-register 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.

— Function: bool constraint_satisfied_p (rtx exp, enum constraint_num c)

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.

— Function: enum reg_class reg_class_for_constraint (enum constraint_num c)

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)))]
     
       "")