<html lang="en">
<head>
<title>Temporaries - GNU Compiler Collection (GCC) Internals</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="GIMPLE.html#GIMPLE" title="GIMPLE">
<link rel="prev" href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" title="GIMPLE Exception Handling">
<link rel="next" href="Operands.html#Operands" title="Operands">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Funding Free Software'', the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
pre.display { font-family:inherit }
pre.format { font-family:inherit }
pre.smalldisplay { font-family:inherit; font-size:smaller }
pre.smallformat { font-family:inherit; font-size:smaller }
pre.smallexample { font-size:smaller }
pre.smalllisp { font-size:smaller }
span.sc { font-variant:small-caps }
span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<a name="Temporaries"></a>
<p>
Next: <a rel="next" accesskey="n" href="Operands.html#Operands">Operands</a>,
Previous: <a rel="previous" accesskey="p" href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling">GIMPLE Exception Handling</a>,
Up: <a rel="up" accesskey="u" href="GIMPLE.html#GIMPLE">GIMPLE</a>
<hr>
</div>
<h3 class="section">11.5 Temporaries</h3>
<p><a name="index-Temporaries-2139"></a>
When gimplification encounters a subexpression that is too
complex, it creates a new temporary variable to hold the value of
the subexpression, and adds a new statement to initialize it
before the current statement. These special temporaries are known
as ‘<samp><span class="samp">expression temporaries</span></samp>’, and are allocated using
<code>get_formal_tmp_var</code>. The compiler tries to always evaluate
identical expressions into the same temporary, to simplify
elimination of redundant calculations.
<p>We can only use expression temporaries when we know that it will
not be reevaluated before its value is used, and that it will not
be otherwise modified<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>. Other temporaries can be allocated
using <code>get_initialized_tmp_var</code> or <code>create_tmp_var</code>.
<p>Currently, an expression like <code>a = b + 5</code> is not reduced any
further. We tried converting it to something like
<pre class="smallexample"> T1 = b + 5;
a = T1;
</pre>
<p>but this bloated the representation for minimal benefit. However, a
variable which must live in memory cannot appear in an expression; its
value is explicitly loaded into a temporary first. Similarly, storing
the value of an expression to a memory variable goes through a
temporary.
<div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> These restrictions are derived
from those in Morgan 4.8.</p>
<hr></div>
</body></html>