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92 lines
3.9 KiB
HTML
92 lines
3.9 KiB
HTML
4 years ago
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<title>Temporaries - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link rel="up" href="GIMPLE.html#GIMPLE" title="GIMPLE">
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<link rel="prev" href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" title="GIMPLE Exception Handling">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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(a) The FSF's Front-Cover Text is:
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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="Temporaries"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Operands.html#Operands">Operands</a>,
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Previous: <a rel="previous" accesskey="p" href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling">GIMPLE Exception Handling</a>,
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Up: <a rel="up" accesskey="u" href="GIMPLE.html#GIMPLE">GIMPLE</a>
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<hr>
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</div>
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<h3 class="section">11.5 Temporaries</h3>
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<p><a name="index-Temporaries-2139"></a>
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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><span class="samp">expression temporaries</span></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>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 rel="footnote" href="#fn-1" name="fnd-1"><sup>1</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>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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<pre class="smallexample"> T1 = b + 5;
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a = T1;
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</pre>
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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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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> These restrictions are derived
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from those in Morgan 4.8.</p>
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<hr></div>
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</body></html>
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