gccint: Peephole Definitions
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1 17.18 Machine-Specific Peephole Optimizers
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1 In addition to instruction patterns the 'md' file may contain
1 definitions of machine-specific peephole optimizations.
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1 The combiner does not notice certain peephole optimizations when the
1 data flow in the program does not suggest that it should try them. For
1 example, sometimes two consecutive insns related in purpose can be
1 combined even though the second one does not appear to use a register
1 computed in the first one. A machine-specific peephole optimizer can
1 detect such opportunities.
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1 There are two forms of peephole definitions that may be used. The
1 original 'define_peephole' is run at assembly output time to match insns
1 and substitute assembly text. Use of 'define_peephole' is deprecated.
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1 A newer 'define_peephole2' matches insns and substitutes new insns.
1 The 'peephole2' pass is run after register allocation but before
1 scheduling, which may result in much better code for targets that do
1 scheduling.
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