Dependent-Patterns.html 5.2 KB

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  69. <h3 class="section">16.11 Interdependence of Patterns</h3>
  70. <a name="index-Dependent-Patterns"></a>
  71. <a name="index-Interdependence-of-Patterns"></a>
  72. <p>In some cases machines support instructions identical except for the
  73. machine mode of one or more operands. For example, there may be
  74. &ldquo;sign-extend halfword&rdquo; and &ldquo;sign-extend byte&rdquo; instructions whose
  75. patterns are
  76. </p>
  77. <div class="smallexample">
  78. <pre class="smallexample">(set (match_operand:SI 0 &hellip;)
  79. (extend:SI (match_operand:HI 1 &hellip;)))
  80. (set (match_operand:SI 0 &hellip;)
  81. (extend:SI (match_operand:QI 1 &hellip;)))
  82. </pre></div>
  83. <p>Constant integers do not specify a machine mode, so an instruction to
  84. extend a constant value could match either pattern. The pattern it
  85. actually will match is the one that appears first in the file. For correct
  86. results, this must be the one for the widest possible mode (<code>HImode</code>,
  87. here). If the pattern matches the <code>QImode</code> instruction, the results
  88. will be incorrect if the constant value does not actually fit that mode.
  89. </p>
  90. <p>Such instructions to extend constants are rarely generated because they are
  91. optimized away, but they do occasionally happen in nonoptimized
  92. compilations.
  93. </p>
  94. <p>If a constraint in a pattern allows a constant, the reload pass may
  95. replace a register with a constant permitted by the constraint in some
  96. cases. Similarly for memory references. Because of this substitution,
  97. you should not provide separate patterns for increment and decrement
  98. instructions. Instead, they should be generated from the same pattern
  99. that supports register-register add insns by examining the operands and
  100. generating the appropriate machine instruction.
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