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