117 lines
5.2 KiB
HTML
117 lines
5.2 KiB
HTML
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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: Blocks</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Blocks">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Blocks">
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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="Statement-Sequences.html#Statement-Sequences" rel="next" title="Statement Sequences">
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<link href="Basic-Statements.html#Basic-Statements" rel="prev" title="Basic Statements">
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<a name="Blocks"></a>
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<div class="header">
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<p>
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Next: <a href="Statement-Sequences.html#Statement-Sequences" accesskey="n" rel="next">Statement Sequences</a>, Previous: <a href="Basic-Statements.html#Basic-Statements" accesskey="p" rel="prev">Basic Statements</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="Blocks-1"></a>
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<h4 class="subsection">10.7.2 Blocks</h4>
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<a name="index-Blocks"></a>
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<p>Block scopes and the variables they declare in GENERIC are
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expressed using the <code>BIND_EXPR</code> code, which in previous
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versions of GCC was primarily used for the C statement-expression
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extension.
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</p>
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<p>Variables in a block are collected into <code>BIND_EXPR_VARS</code> in
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declaration order through their <code>TREE_CHAIN</code> field. Any runtime
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initialization is moved out of <code>DECL_INITIAL</code> and into a
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statement in the controlled block. When gimplifying from C or C++,
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this initialization replaces the <code>DECL_STMT</code>. These variables
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will never require cleanups. The scope of these variables is just the
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body
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</p>
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<p>Variable-length arrays (VLAs) complicate this process, as their size
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often refers to variables initialized earlier in the block and their
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initialization involves an explicit stack allocation. To handle this,
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we add an indirection and replace them with a pointer to stack space
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allocated by means of <code>alloca</code>. In most cases, we also arrange
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for this space to be reclaimed when the enclosing <code>BIND_EXPR</code> is
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exited, the exception to this being when there is an explicit call to
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<code>alloca</code> in the source code, in which case the stack is left
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depressed on exit of the <code>BIND_EXPR</code>.
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</p>
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<p>A C++ program will usually contain more <code>BIND_EXPR</code>s than
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there are syntactic blocks in the source code, since several C++
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constructs have implicit scopes associated with them. On the
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other hand, although the C++ front end uses pseudo-scopes to
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handle cleanups for objects with destructors, these don’t
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translate into the GIMPLE form; multiple declarations at the same
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level use the same <code>BIND_EXPR</code>.
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</p>
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</body>
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</html>
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