GNU Compiler Collection (GCC) 4.2 Internals

14.11 Interdependence of Patterns

Every machine description must have a named pattern for each of the conditional branch names `bcond'. The recognition template must always have the form

     (set (pc)
          (if_then_else (cond (cc0) (const_int 0))
                        (label_ref (match_operand 0 "" ""))
                        (pc)))

In addition, every machine description must have an anonymous pattern for each of the possible reverse-conditional branches. Their templates look like

     (set (pc)
          (if_then_else (cond (cc0) (const_int 0))
                        (pc)
                        (label_ref (match_operand 0 "" ""))))

They are necessary because jump optimization can turn direct-conditional branches into reverse-conditional branches.

It is often convenient to use the match_operator construct to reduce the number of patterns that must be specified for branches. For example,

     (define_insn ""
       [(set (pc)
             (if_then_else (match_operator 0 "comparison_operator"
                                           [(cc0) (const_int 0)])
                           (pc)
                           (label_ref (match_operand 1 "" ""))))]
       "condition"
       "...")

In some cases machines support instructions identical except for the machine mode of one or more operands. For example, there may be “sign-extend halfword” and “sign-extend byte” instructions whose patterns are

     (set (match_operand:SI 0 ...)
          (extend:SI (match_operand:HI 1 ...)))
     
     (set (match_operand:SI 0 ...)
          (extend:SI (match_operand:QI 1 ...)))

Constant integers do not specify a machine mode, so an instruction to extend a constant value could match either pattern. The pattern it actually will match is the one that appears first in the file. For correct results, this must be the one for the widest possible mode (HImode, here). If the pattern matches the QImode instruction, the results will be incorrect if the constant value does not actually fit that mode.

Such instructions to extend constants are rarely generated because they are optimized away, but they do occasionally happen in nonoptimized compilations.

If a constraint in a pattern allows a constant, the reload pass may replace a register with a constant permitted by the constraint in some cases. Similarly for memory references. Because of this substitution, you should not provide separate patterns for increment and decrement instructions. Instead, they should be generated from the same pattern that supports register-register add insns by examining the operands and generating the appropriate machine instruction.