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99 lines
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99 lines
4.8 KiB
HTML
4 years ago
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<title>Jump Patterns - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Dependent-Patterns.html#Dependent-Patterns" title="Dependent Patterns">
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<link rel="next" href="Looping-Patterns.html#Looping-Patterns" title="Looping 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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A GNU Manual
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<body>
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<div class="node">
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<a name="Jump-Patterns"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Looping-Patterns.html#Looping-Patterns">Looping Patterns</a>,
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Previous: <a rel="previous" accesskey="p" href="Dependent-Patterns.html#Dependent-Patterns">Dependent Patterns</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">16.12 Defining Jump Instruction Patterns</h3>
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<p><a name="index-jump-instruction-patterns-3675"></a><a name="index-defining-jump-instruction-patterns-3676"></a>
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GCC does not assume anything about how the machine realizes jumps.
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The machine description should define a single pattern, usually
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a <code>define_expand</code>, which expands to all the required insns.
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<p>Usually, this would be a comparison insn to set the condition code
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and a separate branch insn testing the condition code and branching
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or not according to its value. For many machines, however,
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separating compares and branches is limiting, which is why the
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more flexible approach with one <code>define_expand</code> is used in GCC.
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The machine description becomes clearer for architectures that
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have compare-and-branch instructions but no condition code. It also
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works better when different sets of comparison operators are supported
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by different kinds of conditional branches (e.g. integer vs. floating-point),
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or by conditional branches with respect to conditional stores.
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<p>Two separate insns are always used if the machine description represents
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a condition code register using the legacy RTL expression <code>(cc0)</code>,
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and on most machines that use a separate condition code register
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(see <a href="Condition-Code.html#Condition-Code">Condition Code</a>). For machines that use <code>(cc0)</code>, in
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fact, the set and use of the condition code must be separate and
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adjacent<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>, thus
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allowing flags in <code>cc_status</code> to be used (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>) and
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so that the comparison and branch insns could be located from each other
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by using the functions <code>prev_cc0_setter</code> and <code>next_cc0_user</code>.
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<p>Even in this case having a single entry point for conditional branches
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is advantageous, because it handles equally well the case where a single
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comparison instruction records the results of both signed and unsigned
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comparison of the given operands (with the branch insns coming in distinct
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signed and unsigned flavors) as in the x86 or SPARC, and the case where
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there are distinct signed and unsigned compare instructions and only
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one set of conditional branch instructions as in the PowerPC.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> <code>note</code> insns can separate them, though.</p>
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<hr></div>
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</body></html>
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