119 lines
5.2 KiB
HTML
119 lines
5.2 KiB
HTML
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Dependent Patterns</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Dependent Patterns">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="Jump-Patterns.html#Jump-Patterns" rel="next" title="Jump Patterns">
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<a name="Dependent-Patterns"></a>
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<p>
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Next: <a href="Jump-Patterns.html#Jump-Patterns" accesskey="n" rel="next">Jump Patterns</a>, Previous: <a href="Pattern-Ordering.html#Pattern-Ordering" accesskey="p" rel="prev">Pattern Ordering</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Interdependence-of-Patterns"></a>
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<h3 class="section">16.11 Interdependence of Patterns</h3>
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<a name="index-Dependent-Patterns"></a>
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<a name="index-Interdependence-of-Patterns"></a>
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<p>In some cases machines support instructions identical except for the
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machine mode of one or more operands. For example, there may be
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“sign-extend halfword” and “sign-extend byte” instructions whose
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patterns are
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</p>
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<div class="smallexample">
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<pre class="smallexample">(set (match_operand:SI 0 …)
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(extend:SI (match_operand:HI 1 …)))
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(set (match_operand:SI 0 …)
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(extend:SI (match_operand:QI 1 …)))
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</pre></div>
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<p>Constant integers do not specify a machine mode, so an instruction to
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extend a constant value could match either pattern. The pattern it
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actually will match is the one that appears first in the file. For correct
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results, this must be the one for the widest possible mode (<code>HImode</code>,
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here). If the pattern matches the <code>QImode</code> instruction, the results
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will be incorrect if the constant value does not actually fit that mode.
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</p>
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<p>Such instructions to extend constants are rarely generated because they are
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optimized away, but they do occasionally happen in nonoptimized
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compilations.
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</p>
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<p>If a constraint in a pattern allows a constant, the reload pass may
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replace a register with a constant permitted by the constraint in some
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cases. Similarly for memory references. Because of this substitution,
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you should not provide separate patterns for increment and decrement
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instructions. Instead, they should be generated from the same pattern
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that supports register-register add insns by examining the operands and
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generating the appropriate machine instruction.
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</p>
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