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106 lines
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106 lines
5.0 KiB
HTML
4 years ago
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<title>Define Subst - GNU Compiler Collection (GCC) Internals</title>
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<a name="Define-Subst"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Constant-Definitions.html#Constant-Definitions">Constant Definitions</a>,
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Previous: <a rel="previous" accesskey="p" href="Conditional-Execution.html#Conditional-Execution">Conditional Execution</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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</div>
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<h3 class="section">16.21 RTL Templates Transformations</h3>
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<p><a name="index-define_005fsubst-3823"></a>
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For some hardware architectures there are common cases when the RTL
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templates for the instructions can be derived from the other RTL
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templates using simple transformations. E.g., <samp><span class="file">i386.md</span></samp> contains
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an RTL template for the ordinary <code>sub</code> instruction—
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<code>*subsi_1</code>, and for the <code>sub</code> instruction with subsequent
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zero-extension—<code>*subsi_1_zext</code>. Such cases can be easily
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implemented by a single meta-template capable of generating a modified
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case based on the initial one:
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<p><a name="index-define_005fsubst-3824"></a>
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<pre class="smallexample"> (define_subst "<var>name</var>"
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[<var>input-template</var>]
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"<var>condition</var>"
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[<var>output-template</var>])
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</pre>
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<p><var>input-template</var> is a pattern describing the source RTL template,
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which will be transformed.
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<p><var>condition</var> is a C expression that is conjunct with the condition
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from the input-template to generate a condition to be used in the
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output-template.
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<p><var>output-template</var> is a pattern that will be used in the resulting
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template.
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<p><code>define_subst</code> mechanism is tightly coupled with the notion of the
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subst attribute (see <a href="Subst-Iterators.html#Subst-Iterators">Subst Iterators</a>). The use of
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<code>define_subst</code> is triggered by a reference to a subst attribute in
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the transforming RTL template. This reference initiates duplication of
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the source RTL template and substitution of the attributes with their
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values. The source RTL template is left unchanged, while the copy is
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transformed by <code>define_subst</code>. This transformation can fail in the
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case when the source RTL template is not matched against the
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input-template of the <code>define_subst</code>. In such case the copy is
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deleted.
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<p><code>define_subst</code> can be used only in <code>define_insn</code> and
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<code>define_expand</code>, it cannot be used in other expressions (e.g. in
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<code>define_insn_and_split</code>).
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<ul class="menu">
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<li><a accesskey="1" href="Define-Subst-Example.html#Define-Subst-Example">Define Subst Example</a>: Example of <code>define_subst</code> work.
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<li><a accesskey="2" href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching">Define Subst Pattern Matching</a>: Process of template comparison.
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<li><a accesskey="3" href="Define-Subst-Output-Template.html#Define-Subst-Output-Template">Define Subst Output Template</a>: Generation of output template.
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</ul>
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</body></html>
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