120 lines
5.0 KiB
HTML
120 lines
5.0 KiB
HTML
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Temporaries</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Temporaries">
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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="GIMPLE.html#GIMPLE" rel="up" title="GIMPLE">
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<link href="Operands.html#Operands" rel="next" title="Operands">
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<a name="Temporaries"></a>
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<div class="header">
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<p>
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Next: <a href="Operands.html#Operands" accesskey="n" rel="next">Operands</a>, Previous: <a href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" accesskey="p" rel="prev">GIMPLE Exception Handling</a>, Up: <a href="GIMPLE.html#GIMPLE" accesskey="u" rel="up">GIMPLE</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="Temporaries-1"></a>
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<h3 class="section">11.5 Temporaries</h3>
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<a name="index-Temporaries"></a>
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<p>When gimplification encounters a subexpression that is too
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complex, it creates a new temporary variable to hold the value of
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the subexpression, and adds a new statement to initialize it
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before the current statement. These special temporaries are known
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as ‘<samp>expression temporaries</samp>’, and are allocated using
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<code>get_formal_tmp_var</code>. The compiler tries to always evaluate
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identical expressions into the same temporary, to simplify
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elimination of redundant calculations.
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</p>
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<p>We can only use expression temporaries when we know that it will
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not be reevaluated before its value is used, and that it will not
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be otherwise modified<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>. Other temporaries can be allocated
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using <code>get_initialized_tmp_var</code> or <code>create_tmp_var</code>.
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</p>
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<p>Currently, an expression like <code>a = b + 5</code> is not reduced any
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further. We tried converting it to something like
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</p><div class="smallexample">
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<pre class="smallexample">T1 = b + 5;
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a = T1;
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</pre></div>
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<p>but this bloated the representation for minimal benefit. However, a
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variable which must live in memory cannot appear in an expression; its
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value is explicitly loaded into a temporary first. Similarly, storing
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the value of an expression to a memory variable goes through a
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temporary.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
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<p>These restrictions are derived
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from those in Morgan 4.8.</p>
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</div>
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