135 lines
6.7 KiB
HTML
135 lines
6.7 KiB
HTML
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2016 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: Condition Code</title>
|
||
|
|
|
||
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals: Condition Code">
|
||
|
|
<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Condition Code">
|
||
|
|
<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="Target-Macros.html#Target-Macros" rel="up" title="Target Macros">
|
||
|
|
<link href="CC0-Condition-Codes.html#CC0-Condition-Codes" rel="next" title="CC0 Condition Codes">
|
||
|
|
<link href="Anchored-Addresses.html#Anchored-Addresses" rel="prev" title="Anchored Addresses">
|
||
|
|
<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="Condition-Code"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Costs.html#Costs" accesskey="n" rel="next">Costs</a>, Previous: <a href="Anchored-Addresses.html#Anchored-Addresses" accesskey="p" rel="prev">Anchored Addresses</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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="Condition-Code-Status"></a>
|
||
|
|
<h3 class="section">17.15 Condition Code Status</h3>
|
||
|
|
<a name="index-condition-code-status"></a>
|
||
|
|
|
||
|
|
<p>The macros in this section can be split in two families, according to the
|
||
|
|
two ways of representing condition codes in GCC.
|
||
|
|
</p>
|
||
|
|
<p>The first representation is the so called <code>(cc0)</code> representation
|
||
|
|
(see <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>), where all instructions can have an implicit
|
||
|
|
clobber of the condition codes. The second is the condition code
|
||
|
|
register representation, which provides better schedulability for
|
||
|
|
architectures that do have a condition code register, but on which
|
||
|
|
most instructions do not affect it. The latter category includes
|
||
|
|
most RISC machines.
|
||
|
|
</p>
|
||
|
|
<p>The implicit clobbering poses a strong restriction on the placement of
|
||
|
|
the definition and use of the condition code. In the past the definition
|
||
|
|
and use were always adjacent. However, recent changes to support trapping
|
||
|
|
arithmatic may result in the definition and user being in different blocks.
|
||
|
|
Thus, there may be a <code>NOTE_INSN_BASIC_BLOCK</code> between them. Additionally,
|
||
|
|
the definition may be the source of exception handling edges.
|
||
|
|
</p>
|
||
|
|
<p>These restrictions can prevent important
|
||
|
|
optimizations on some machines. For example, on the IBM RS/6000, there
|
||
|
|
is a delay for taken branches unless the condition code register is set
|
||
|
|
three instructions earlier than the conditional branch. The instruction
|
||
|
|
scheduler cannot perform this optimization if it is not permitted to
|
||
|
|
separate the definition and use of the condition code register.
|
||
|
|
</p>
|
||
|
|
<p>For this reason, it is possible and suggested to use a register to
|
||
|
|
represent the condition code for new ports. If there is a specific
|
||
|
|
condition code register in the machine, use a hard register. If the
|
||
|
|
condition code or comparison result can be placed in any general register,
|
||
|
|
or if there are multiple condition registers, use a pseudo register.
|
||
|
|
Registers used to store the condition code value will usually have a mode
|
||
|
|
that is in class <code>MODE_CC</code>.
|
||
|
|
</p>
|
||
|
|
<p>Alternatively, you can use <code>BImode</code> if the comparison operator is
|
||
|
|
specified already in the compare instruction. In this case, you are not
|
||
|
|
interested in most macros in this section.
|
||
|
|
</p>
|
||
|
|
<table class="menu" border="0" cellspacing="0">
|
||
|
|
<tr><td align="left" valign="top">• <a href="CC0-Condition-Codes.html#CC0-Condition-Codes" accesskey="1">CC0 Condition Codes</a>:</td><td> </td><td align="left" valign="top">Old style representation of condition codes.
|
||
|
|
</td></tr>
|
||
|
|
<tr><td align="left" valign="top">• <a href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" accesskey="2">MODE_CC Condition Codes</a>:</td><td> </td><td align="left" valign="top">Modern representation of condition codes.
|
||
|
|
</td></tr>
|
||
|
|
</table>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Costs.html#Costs" accesskey="n" rel="next">Costs</a>, Previous: <a href="Anchored-Addresses.html#Anchored-Addresses" accesskey="p" rel="prev">Anchored Addresses</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|