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  25. <title>Define Subst Output Template (GNU Compiler Collection (GCC) Internals)</title>

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  70. <a name="Define-Subst-Output-Template"></a>

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  72. <p>

  73. Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="prev">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>

  74. </div>

  75. <hr>

  76. <a name="Generation-of-output-template-in-define_005fsubst"></a>

  77. <h4 class="subsection">17.21.3 Generation of output template in <code>define_subst</code></h4>

  78. <a name="index-define_005fsubst-3"></a>

  79. 

  80. <p>If all necessary checks for <code>define_subst</code> application pass, a new

  81. RTL-pattern, based on the output-template, is created to replace the old

  82. template.  Like in input-patterns, meanings of some RTL expressions are

  83. changed when they are used in output-patterns of a <code>define_subst</code>.

  84. Thus, <code>match_dup</code> is used for copying the whole expression from the

  85. original pattern, which matched corresponding <code>match_operand</code> from

  86. the input pattern.

  87. </p>

  88. <p><code>match_dup N</code> is used in the output template to be replaced with

  89. the expression from the original pattern, which matched

  90. <code>match_operand N</code> from the input pattern.  As a consequence,

  91. <code>match_dup</code> cannot be used to point to <code>match_operand</code>s from

  92. the output pattern, it should always refer to a <code>match_operand</code>

  93. from the input pattern.  If a <code>match_dup N</code> occurs more than once

  94. in the output template, its first occurrence is replaced with the

  95. expression from the original pattern, and the subsequent expressions

  96. are replaced with <code>match_dup N</code>, i.e., a reference to the first

  97. expression.

  98. </p>

  99. <p>In the output template one can refer to the expressions from the

  100. original pattern and create new ones.  For instance, some operands could

  101. be added by means of standard <code>match_operand</code>.

  102. </p>

  103. <p>After replacing <code>match_dup</code> with some RTL-subtree from the original

  104. pattern, it could happen that several <code>match_operand</code>s in the

  105. output pattern have the same indexes.  It is unknown, how many and what

  106. indexes would be used in the expression which would replace

  107. <code>match_dup</code>, so such conflicts in indexes are inevitable.  To

  108. overcome this issue, <code>match_operands</code> and <code>match_operators</code>,

  109. which were introduced into the output pattern, are renumerated when all

  110. <code>match_dup</code>s are replaced.

  111. </p>

  112. <p>Number of alternatives in <code>match_operand</code>s introduced into the

  113. output template <code>M</code> could differ from the number of alternatives in

  114. the original pattern <code>N</code>, so in the resultant pattern there would

  115. be <code>N*M</code> alternatives.  Thus, constraints from the original pattern

  116. would be duplicated <code>N</code> times, constraints from the output pattern

  117. would be duplicated <code>M</code> times, producing all possible combinations.

  118. </p>

  119. <hr>

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  122. Previous: <a href="Define-Subst-Pattern-Matching.html#Define-Subst-Pattern-Matching" accesskey="p" rel="prev">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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