| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- 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. --> |
| <!-- Created by GNU Texinfo 5.2, 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="prev" 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="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">11.4 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> |
