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<h4 class="subsection">16.21.2 Pattern Matching in <code>define_subst</code></h4>
<p><a name="index-define_005fsubst-3826"></a>
All expressions, allowed in <code>define_insn</code> or <code>define_expand</code>,
are allowed in the input-template of <code>define_subst</code>, except
<code>match_par_dup</code>, <code>match_scratch</code>, <code>match_parallel</code>. The
meanings of expressions in the input-template were changed:
<p><code>match_operand</code> matches any expression (possibly, a subtree in
RTL-template), if modes of the <code>match_operand</code> and this expression
are the same, or mode of the <code>match_operand</code> is <code>VOIDmode</code>, or
this expression is <code>match_dup</code>, <code>match_op_dup</code>. If the
expression is <code>match_operand</code> too, and predicate of
<code>match_operand</code> from the input pattern is not empty, then the
predicates are compared. That can be used for more accurate filtering
of accepted RTL-templates.
<p><code>match_operator</code> matches common operators (like <code>plus</code>,
<code>minus</code>), <code>unspec</code>, <code>unspec_volatile</code> operators and
<code>match_operator</code>s from the original pattern if the modes match and
<code>match_operator</code> from the input pattern has the same number of
operands as the operator from the original pattern.
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