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<a name="Subst-Iterators-1"></a>
<h4 class="subsection">17.23.4 Subst Iterators</h4>
<a name="index-subst-iterators-in-_002emd-files"></a>
<a name="index-define_005fsubst-5"></a>
<a name="index-define_005fsubst_005fattr"></a>

<p>Subst iterators are special type of iterators with the following
restrictions: they could not be declared explicitly, they always have
only two values, and they do not have explicit dedicated name.
Subst-iterators are triggered only when corresponding subst-attribute is
used in RTL-pattern.
</p>
<p>Subst iterators transform templates in the following way: the templates
are duplicated, the subst-attributes in these templates are replaced
with the corresponding values, and a new attribute is implicitly added
to the given <code>define_insn</code>/<code>define_expand</code>.  The name of the
added attribute matches the name of <code>define_subst</code>.  Such
attributes are declared implicitly, and it is not allowed to have a
<code>define_attr</code> named as a <code>define_subst</code>.
</p>
<p>Each subst iterator is linked to a <code>define_subst</code>.  It is declared
implicitly by the first appearance of the corresponding
<code>define_subst_attr</code>, and it is not allowed to define it explicitly.
</p>
<p>Declarations of subst-attributes have the following syntax:
</p>
<a name="index-define_005fsubst_005fattr-1"></a>
<div class="smallexample">
<pre class="smallexample">(define_subst_attr &quot;<var>name</var>&quot;
  &quot;<var>subst-name</var>&quot;
  &quot;<var>no-subst-value</var>&quot;
  &quot;<var>subst-applied-value</var>&quot;)
</pre></div>

<p><var>name</var> is a string with which the given subst-attribute could be
referred to.
</p>
<p><var>subst-name</var> shows which <code>define_subst</code> should be applied to an
RTL-template if the given subst-attribute is present in the
RTL-template.
</p>
<p><var>no-subst-value</var> is a value with which subst-attribute would be
replaced in the first copy of the original RTL-template.
</p>
<p><var>subst-applied-value</var> is a value with which subst-attribute would be
replaced in the second copy of the original RTL-template.
</p>


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