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78 lines
3.5 KiB
HTML
78 lines
3.5 KiB
HTML
4 years ago
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<title>Pattern Ordering - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Standard-Names.html#Standard-Names" title="Standard Names">
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<link rel="next" href="Dependent-Patterns.html#Dependent-Patterns" title="Dependent Patterns">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Pattern-Ordering"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Dependent-Patterns.html#Dependent-Patterns">Dependent Patterns</a>,
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Previous: <a rel="previous" accesskey="p" href="Standard-Names.html#Standard-Names">Standard Names</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.10 When the Order of Patterns Matters</h3>
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<p><a name="index-Pattern-Ordering-3671"></a><a name="index-Ordering-of-Patterns-3672"></a>
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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>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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</body></html>
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