131 lines
5.6 KiB
HTML
131 lines
5.6 KiB
HTML
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Cleanups</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Cleanups">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Statements.html#Statements" rel="up" title="Statements">
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<link href="OpenMP.html#OpenMP" rel="next" title="OpenMP">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Cleanups"></a>
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<div class="header">
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<p>
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Next: <a href="OpenMP.html#OpenMP" accesskey="n" rel="next">OpenMP</a>, Previous: <a href="Jumps.html#Jumps" accesskey="p" rel="prev">Jumps</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> [<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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</div>
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<hr>
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<a name="Cleanups-1"></a>
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<h4 class="subsection">10.7.6 Cleanups</h4>
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<a name="index-Cleanups"></a>
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<p>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>
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<p>The first sequence may complete in the following ways:
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</p>
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<ol>
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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><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><li> Execute a return statement (<code>RETURN_EXPR</code>).
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</li><li> Throw an exception. This is currently not explicitly represented in
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GIMPLE.
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</li></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>
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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>
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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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="OpenMP.html#OpenMP" accesskey="n" rel="next">OpenMP</a>, Previous: <a href="Jumps.html#Jumps" accesskey="p" rel="prev">Jumps</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> [<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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</div>
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</body>
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</html>
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