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<a name="Subst-Iterators"></a>
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Previous: <a rel="previous" accesskey="p" href="Int-Iterators.html#Int-Iterators">Int Iterators</a>,
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Up: <a rel="up" accesskey="u" href="Iterators.html#Iterators">Iterators</a>
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<h4 class="subsection">16.23.4 Subst Iterators</h4>
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<p><a name="index-subst-iterators-in-_0040file_007b_002emd_007d-files-3846"></a><a name="index-define_005fsubst-3847"></a><a name="index-define_005fsubst_005fattr-3848"></a>
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Subst iterators are special type of iterators with the following
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restrictions: they could not be declared explicitly, they always have
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only two values, and they do not have explicit dedicated name.
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Subst-iterators are triggered only when corresponding subst-attribute is
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used in RTL-pattern.
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<p>Subst iterators transform templates in the following way: the templates
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are duplicated, the subst-attributes in these templates are replaced
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with the corresponding values, and a new attribute is implicitly added
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to the given <code>define_insn</code>/<code>define_expand</code>. The name of the
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added attribute matches the name of <code>define_subst</code>. Such
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attributes are declared implicitly, and it is not allowed to have a
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<code>define_attr</code> named as a <code>define_subst</code>.
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<p>Each subst iterator is linked to a <code>define_subst</code>. It is declared
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implicitly by the first appearance of the corresponding
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<code>define_subst_attr</code>, and it is not allowed to define it explicitly.
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<p>Declarations of subst-attributes have the following syntax:
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<p><a name="index-define_005fsubst_005fattr-3849"></a>
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<pre class="smallexample"> (define_subst_attr "<var>name</var>"
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"<var>subst-name</var>"
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"<var>no-subst-value</var>"
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"<var>subst-applied-value</var>")
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</pre>
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<p><var>name</var> is a string with which the given subst-attribute could be
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referred to.
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<p><var>subst-name</var> shows which <code>define_subst</code> should be applied to an
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RTL-template if the given subst-attribute is present in the
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RTL-template.
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<p><var>no-subst-value</var> is a value with which subst-attribute would be
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replaced in the first copy of the original RTL-template.
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<p><var>subst-applied-value</var> is a value with which subst-attribute would be
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replaced in the second copy of the original RTL-template.
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