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  66. <hr>
  67. <a name="Temporaries-1"></a>
  68. <h3 class="section">12.5 Temporaries</h3>
  69. <a name="index-Temporaries"></a>
  70. <p>When gimplification encounters a subexpression that is too
  71. complex, it creates a new temporary variable to hold the value of
  72. the subexpression, and adds a new statement to initialize it
  73. before the current statement. These special temporaries are known
  74. as &lsquo;<samp>expression temporaries</samp>&rsquo;, and are allocated using
  75. <code>get_formal_tmp_var</code>. The compiler tries to always evaluate
  76. identical expressions into the same temporary, to simplify
  77. elimination of redundant calculations.
  78. </p>
  79. <p>We can only use expression temporaries when we know that it will
  80. not be reevaluated before its value is used, and that it will not
  81. be otherwise modified<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>. Other temporaries can be allocated
  82. using <code>get_initialized_tmp_var</code> or <code>create_tmp_var</code>.
  83. </p>
  84. <p>Currently, an expression like <code>a = b + 5</code> is not reduced any
  85. further. We tried converting it to something like
  86. </p><div class="smallexample">
  87. <pre class="smallexample">T1 = b + 5;
  88. a = T1;
  89. </pre></div>
  90. <p>but this bloated the representation for minimal benefit. However, a
  91. variable which must live in memory cannot appear in an expression; its
  92. value is explicitly loaded into a temporary first. Similarly, storing
  93. the value of an expression to a memory variable goes through a
  94. temporary.
  95. </p>
  96. <div class="footnote">
  97. <hr>
  98. <h4 class="footnotes-heading">Footnotes</h4>
  99. <h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
  100. <p>These restrictions are derived
  101. from those in Morgan 4.8.</p>
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