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| <a name="Jump-Patterns"></a> |
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| Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent Patterns</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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| <a name="Defining-Jump-Instruction-Patterns"></a> |
| <h3 class="section">16.12 Defining Jump Instruction Patterns</h3> |
| <a name="index-jump-instruction-patterns"></a> |
| <a name="index-defining-jump-instruction-patterns"></a> |
| |
| <p>GCC does not assume anything about how the machine realizes jumps. |
| The machine description should define a single pattern, usually |
| a <code>define_expand</code>, which expands to all the required insns. |
| </p> |
| <p>Usually, this would be a comparison insn to set the condition code |
| and a separate branch insn testing the condition code and branching |
| or not according to its value. For many machines, however, |
| separating compares and branches is limiting, which is why the |
| more flexible approach with one <code>define_expand</code> is used in GCC. |
| The machine description becomes clearer for architectures that |
| have compare-and-branch instructions but no condition code. It also |
| works better when different sets of comparison operators are supported |
| by different kinds of conditional branches (e.g. integer vs. floating-point), |
| or by conditional branches with respect to conditional stores. |
| </p> |
| <p>Two separate insns are always used if the machine description represents |
| a condition code register using the legacy RTL expression <code>(cc0)</code>, |
| and on most machines that use a separate condition code register |
| (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>). For machines that use <code>(cc0)</code>, in |
| fact, the set and use of the condition code must be separate and |
| adjacent<a name="DOCF4" href="#FOOT4"><sup>4</sup></a>, thus |
| allowing flags in <code>cc_status</code> to be used (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>) and |
| so that the comparison and branch insns could be located from each other |
| by using the functions <code>prev_cc0_setter</code> and <code>next_cc0_user</code>. |
| </p> |
| <p>Even in this case having a single entry point for conditional branches |
| is advantageous, because it handles equally well the case where a single |
| comparison instruction records the results of both signed and unsigned |
| comparison of the given operands (with the branch insns coming in distinct |
| signed and unsigned flavors) as in the x86 or SPARC, and the case where |
| there are distinct signed and unsigned compare instructions and only |
| one set of conditional branch instructions as in the PowerPC. |
| </p> |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT4" href="#DOCF4">(4)</a></h3> |
| <p><code>note</code> insns can separate them, though.</p> |
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| Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent Patterns</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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