diff options
Diffstat (limited to 'share/doc/gccint/Temporaries.html')
| -rw-r--r-- | share/doc/gccint/Temporaries.html | 119 |
1 files changed, 119 insertions, 0 deletions
diff --git a/share/doc/gccint/Temporaries.html b/share/doc/gccint/Temporaries.html new file mode 100644 index 0000000..8cca0a7 --- /dev/null +++ b/share/doc/gccint/Temporaries.html @@ -0,0 +1,119 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Temporaries</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Temporaries"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Temporaries"> +<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="previous" title="GIMPLE Exception Handling"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +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.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<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="previous">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> |