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(gcc-300.info)Peephole Definitions

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Machine-Specific Peephole Optimizers
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   In addition to instruction patterns the `md' file may contain
definitions of machine-specific peephole optimizations.

   The combiner does not notice certain peephole optimizations when the
data flow in the program does not suggest that it should try them.  For
example, sometimes two consecutive insns related in purpose can be
combined even though the second one does not appear to use a register
computed in the first one.  A machine-specific peephole optimizer can
detect such opportunities.

   There are two forms of peephole definitions that may be used.  The
original `define_peephole' is run at assembly output time to match
insns and substitute assembly text.  Use of `define_peephole' is
deprecated.

   A newer `define_peephole2' matches insns and substitutes new insns.
The `peephole2' pass is run after register allocation but before
scheduling, which may result in much better code for targets that do
scheduling.

define_peephole

RTL to Text Peephole Optimizers

define_peephole2

RTL to RTL Peephole Optimizers