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| <a name="Machine_002dSpecific-Peephole-Optimizers"></a> |
| <h3 class="section">16.18 Machine-Specific Peephole Optimizers</h3> |
| <a name="index-peephole-optimizer-definitions"></a> |
| <a name="index-defining-peephole-optimizers"></a> |
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| <p>In addition to instruction patterns the <samp>md</samp> file may contain |
| definitions of machine-specific peephole optimizations. |
| </p> |
| <p>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. |
| </p> |
| <p>There are two forms of peephole definitions that may be used. The |
| original <code>define_peephole</code> is run at assembly output time to |
| match insns and substitute assembly text. Use of <code>define_peephole</code> |
| is deprecated. |
| </p> |
| <p>A newer <code>define_peephole2</code> matches insns and substitutes new |
| insns. The <code>peephole2</code> pass is run after register allocation |
| but before scheduling, which may result in much better code for |
| targets that do scheduling. |
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