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| <a name="Generation-of-output-template-in-define_005fsubst"></a> |
| <h4 class="subsection">16.21.3 Generation of output template in <code>define_subst</code></h4> |
| <a name="index-define_005fsubst-3"></a> |
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| <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. |
| </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. |
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