Cleanups.html 5.6 KB

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  68. <a name="Cleanups-1"></a>
  69. <h4 class="subsection">10.7.6 Cleanups</h4>
  70. <a name="index-Cleanups"></a>
  71. <p>Destructors for local C++ objects and similar dynamic cleanups are
  72. represented in GIMPLE by a <code>TRY_FINALLY_EXPR</code>.
  73. <code>TRY_FINALLY_EXPR</code> has two operands, both of which are a sequence
  74. of statements to execute. The first sequence is executed. When it
  75. completes the second sequence is executed.
  76. </p>
  77. <p>The first sequence may complete in the following ways:
  78. </p>
  79. <ol>
  80. <li> Execute the last statement in the sequence and fall off the
  81. end.
  82. </li><li> Execute a goto statement (<code>GOTO_EXPR</code>) to an ordinary
  83. label outside the sequence.
  84. </li><li> Execute a return statement (<code>RETURN_EXPR</code>).
  85. </li><li> Throw an exception. This is currently not explicitly represented in
  86. GIMPLE.
  87. </li></ol>
  88. <p>The second sequence is not executed if the first sequence completes by
  89. calling <code>setjmp</code> or <code>exit</code> or any other function that does
  90. not return. The second sequence is also not executed if the first
  91. sequence completes via a non-local goto or a computed goto (in general
  92. the compiler does not know whether such a goto statement exits the
  93. first sequence or not, so we assume that it doesn&rsquo;t).
  94. </p>
  95. <p>After the second sequence is executed, if it completes normally by
  96. falling off the end, execution continues wherever the first sequence
  97. would have continued, by falling off the end, or doing a goto, etc.
  98. </p>
  99. <p><code>TRY_FINALLY_EXPR</code> complicates the flow graph, since the cleanup
  100. needs to appear on every edge out of the controlled block; this
  101. reduces the freedom to move code across these edges. Therefore, the
  102. EH lowering pass which runs before most of the optimization passes
  103. eliminates these expressions by explicitly adding the cleanup to each
  104. edge. Rethrowing the exception is represented using <code>RESX_EXPR</code>.
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