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104 lines
4.6 KiB
HTML
104 lines
4.6 KiB
HTML
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<head>
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<title>Pattern Ordering (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Pattern Ordering (GNU Compiler Collection (GCC) Internals)">
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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="Dependent-Patterns.html#Dependent-Patterns" rel="next" title="Dependent Patterns">
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<link href="Standard-Names.html#Standard-Names" rel="prev" title="Standard Names">
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<a name="Pattern-Ordering"></a>
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<div class="header">
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<p>
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Next: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="n" rel="next">Dependent Patterns</a>, Previous: <a href="Standard-Names.html#Standard-Names" accesskey="p" rel="prev">Standard Names</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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</div>
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<hr>
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<a name="When-the-Order-of-Patterns-Matters"></a>
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<h3 class="section">17.10 When the Order of Patterns Matters</h3>
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<a name="index-Pattern-Ordering"></a>
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<a name="index-Ordering-of-Patterns"></a>
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<p>Sometimes an insn can match more than one instruction pattern. Then the
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pattern that appears first in the machine description is the one used.
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Therefore, more specific patterns (patterns that will match fewer things)
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and faster instructions (those that will produce better code when they
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do match) should usually go first in the description.
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</p>
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<p>In some cases the effect of ordering the patterns can be used to hide
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a pattern when it is not valid. For example, the 68000 has an
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instruction for converting a fullword to floating point and another
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for converting a byte to floating point. An instruction converting
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an integer to floating point could match either one. We put the
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pattern to convert the fullword first to make sure that one will
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be used rather than the other. (Otherwise a large integer might
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be generated as a single-byte immediate quantity, which would not work.)
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Instead of using this pattern ordering it would be possible to make the
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pattern for convert-a-byte smart enough to deal properly with any
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constant value.
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</p>
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</body>
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</html>
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