You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
119 lines
4.9 KiB
HTML
119 lines
4.9 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 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. -->
|
||
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Temporaries (GNU Compiler Collection (GCC) Internals)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Temporaries (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="keywords" content="Temporaries (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="GIMPLE.html#GIMPLE" rel="up" title="GIMPLE">
|
||
|
|
<link href="Operands.html#Operands" rel="next" title="Operands">
|
||
|
|
<link href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" rel="prev" title="GIMPLE Exception Handling">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Temporaries"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="Temporaries-1"></a>
|
||
|
|
<h3 class="section">12.5 Temporaries</h3>
|
||
|
|
<a name="index-Temporaries"></a>
|
||
|
|
|
||
|
|
<p>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>expression temporaries</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>
|
||
|
|
<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 name="DOCF3" href="#FOOT3"><sup>3</sup></a>. Other temporaries can be allocated
|
||
|
|
using <code>get_initialized_tmp_var</code> or <code>create_tmp_var</code>.
|
||
|
|
</p>
|
||
|
|
<p>Currently, an expression like <code>a = b + 5</code> is not reduced any
|
||
|
|
further. We tried converting it to something like
|
||
|
|
</p><div class="smallexample">
|
||
|
|
<pre class="smallexample">T1 = b + 5;
|
||
|
|
a = T1;
|
||
|
|
</pre></div>
|
||
|
|
<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.
|
||
|
|
</p>
|
||
|
|
<div class="footnote">
|
||
|
|
<hr>
|
||
|
|
<h4 class="footnotes-heading">Footnotes</h4>
|
||
|
|
|
||
|
|
<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
|
||
|
|
<p>These restrictions are derived
|
||
|
|
from those in Morgan 4.8.</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|