1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
|
<!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: 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="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" rel="next" title="MODE_CC Condition Codes">
<link href="Anchored-Addresses.html#Anchored-Addresses" rel="previous" 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="previous">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">18.15 Condition Code Status</h3>
<a name="index-condition-code-status"></a>
<p>Condition codes in GCC are represented as registers,
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>Implicit clobbering would pose 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
arithmetic 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>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="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" accesskey="1">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="previous">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>
|
