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+<p>
+Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="previous">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</a> &nbsp; [<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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+<hr>
+<a name="Generation-of-output-template-in-define_005fsubst"></a>
+<h4 class="subsection">17.21.3 Generation of output template in <code>define_subst</code></h4>
+<a name="index-define_005fsubst-3"></a>
+
+<p>If all necessary checks for <code>define_subst</code> application pass, a new
+RTL-pattern, based on the output-template, is created to replace the old
+template.  Like in input-patterns, meanings of some RTL expressions are
+changed when they are used in output-patterns of a <code>define_subst</code>.
+Thus, <code>match_dup</code> is used for copying the whole expression from the
+original pattern, which matched corresponding <code>match_operand</code> from
+the input pattern.
+</p>
+<p><code>match_dup N</code> is used in the output template to be replaced with
+the expression from the original pattern, which matched
+<code>match_operand N</code> from the input pattern.  As a consequence,
+<code>match_dup</code> cannot be used to point to <code>match_operand</code>s from
+the output pattern, it should always refer to a <code>match_operand</code>
+from the input pattern.  If a <code>match_dup N</code> occurs more than once
+in the output template, its first occurrence is replaced with the
+expression from the original pattern, and the subsequent expressions
+are replaced with <code>match_dup N</code>, i.e., a reference to the first
+expression.
+</p>
+<p>In the output template one can refer to the expressions from the
+original pattern and create new ones.  For instance, some operands could
+be added by means of standard <code>match_operand</code>.
+</p>
+<p>After replacing <code>match_dup</code> with some RTL-subtree from the original
+pattern, it could happen that several <code>match_operand</code>s in the
+output pattern have the same indexes.  It is unknown, how many and what
+indexes would be used in the expression which would replace
+<code>match_dup</code>, so such conflicts in indexes are inevitable.  To
+overcome this issue, <code>match_operands</code> and <code>match_operators</code>,
+which were introduced into the output pattern, are renumerated when all
+<code>match_dup</code>s are replaced.
+</p>
+<p>Number of alternatives in <code>match_operand</code>s introduced into the
+output template <code>M</code> could differ from the number of alternatives in
+the original pattern <code>N</code>, so in the resultant pattern there would
+be <code>N*M</code> alternatives.  Thus, constraints from the original pattern
+would be duplicated <code>N</code> times, constraints from the output pattern
+would be duplicated <code>M</code> times, producing all possible combinations.
+</p>
+<hr>
+<div class="header">
+<p>
+Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="previous">Define Subst Pattern Matching</a>, Up: <a href="Define-Subst.html#Define-Subst" accesskey="u" rel="up">Define Subst</a> &nbsp; [<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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