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<h4 class="subsection">16.21.3 Generation of output template in <code>define_subst</code></h4>
<p><a name="index-define_005fsubst-3827"></a>
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><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.
<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>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>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.
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