Define-Subst-Output-Template.html 6.2 KB

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  65. 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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  68. <a name="Generation-of-output-template-in-define_005fsubst"></a>
  69. <h4 class="subsection">16.21.3 Generation of output template in <code>define_subst</code></h4>
  70. <a name="index-define_005fsubst-3"></a>
  71. <p>If all necessary checks for <code>define_subst</code> application pass, a new
  72. RTL-pattern, based on the output-template, is created to replace the old
  73. template. Like in input-patterns, meanings of some RTL expressions are
  74. changed when they are used in output-patterns of a <code>define_subst</code>.
  75. Thus, <code>match_dup</code> is used for copying the whole expression from the
  76. original pattern, which matched corresponding <code>match_operand</code> from
  77. the input pattern.
  78. </p>
  79. <p><code>match_dup N</code> is used in the output template to be replaced with
  80. the expression from the original pattern, which matched
  81. <code>match_operand N</code> from the input pattern. As a consequence,
  82. <code>match_dup</code> cannot be used to point to <code>match_operand</code>s from
  83. the output pattern, it should always refer to a <code>match_operand</code>
  84. from the input pattern.
  85. </p>
  86. <p>In the output template one can refer to the expressions from the
  87. original pattern and create new ones. For instance, some operands could
  88. be added by means of standard <code>match_operand</code>.
  89. </p>
  90. <p>After replacing <code>match_dup</code> with some RTL-subtree from the original
  91. pattern, it could happen that several <code>match_operand</code>s in the
  92. output pattern have the same indexes. It is unknown, how many and what
  93. indexes would be used in the expression which would replace
  94. <code>match_dup</code>, so such conflicts in indexes are inevitable. To
  95. overcome this issue, <code>match_operands</code> and <code>match_operators</code>,
  96. which were introduced into the output pattern, are renumerated when all
  97. <code>match_dup</code>s are replaced.
  98. </p>
  99. <p>Number of alternatives in <code>match_operand</code>s introduced into the
  100. output template <code>M</code> could differ from the number of alternatives in
  101. the original pattern <code>N</code>, so in the resultant pattern there would
  102. be <code>N*M</code> alternatives. Thus, constraints from the original pattern
  103. would be duplicated <code>N</code> times, constraints from the output pattern
  104. would be duplicated <code>M</code> times, producing all possible combinations.
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