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101 lines
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101 lines
4.0 KiB
HTML
4 years ago
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<title>Cleanups - GNU Compiler Collection (GCC) Internals</title>
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<link rel="up" href="Statements.html#Statements" title="Statements">
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<link rel="prev" href="Jumps.html#Jumps" title="Jumps">
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A GNU Manual
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<body>
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<div class="node">
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<a name="Cleanups"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="OpenMP.html#OpenMP">OpenMP</a>,
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Previous: <a rel="previous" accesskey="p" href="Jumps.html#Jumps">Jumps</a>,
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Up: <a rel="up" accesskey="u" href="Statements.html#Statements">Statements</a>
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<hr>
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</div>
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<h4 class="subsection">10.7.6 Cleanups</h4>
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<p><a name="index-Cleanups-1942"></a>
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Destructors for local C++ objects and similar dynamic cleanups are
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represented in GIMPLE by a <code>TRY_FINALLY_EXPR</code>.
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<code>TRY_FINALLY_EXPR</code> has two operands, both of which are a sequence
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of statements to execute. The first sequence is executed. When it
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completes the second sequence is executed.
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<p>The first sequence may complete in the following ways:
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<ol type=1 start=1>
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<li>Execute the last statement in the sequence and fall off the
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end.
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<li>Execute a goto statement (<code>GOTO_EXPR</code>) to an ordinary
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label outside the sequence.
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<li>Execute a return statement (<code>RETURN_EXPR</code>).
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<li>Throw an exception. This is currently not explicitly represented in
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GIMPLE.
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</ol>
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<p>The second sequence is not executed if the first sequence completes by
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calling <code>setjmp</code> or <code>exit</code> or any other function that does
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not return. The second sequence is also not executed if the first
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sequence completes via a non-local goto or a computed goto (in general
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the compiler does not know whether such a goto statement exits the
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first sequence or not, so we assume that it doesn't).
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<p>After the second sequence is executed, if it completes normally by
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falling off the end, execution continues wherever the first sequence
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would have continued, by falling off the end, or doing a goto, etc.
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<p><code>TRY_FINALLY_EXPR</code> complicates the flow graph, since the cleanup
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needs to appear on every edge out of the controlled block; this
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reduces the freedom to move code across these edges. Therefore, the
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EH lowering pass which runs before most of the optimization passes
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eliminates these expressions by explicitly adding the cleanup to each
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edge. Rethrowing the exception is represented using <code>RESX_EXPR</code>.
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</body></html>
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