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.
99 lines
4.8 KiB
HTML
99 lines
4.8 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Jump Patterns - GNU Compiler Collection (GCC) Internals</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
|
|
<meta name="generator" content="makeinfo 4.13">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
|
|
<link rel="prev" href="Dependent-Patterns.html#Dependent-Patterns" title="Dependent Patterns">
|
|
<link rel="next" href="Looping-Patterns.html#Looping-Patterns" title="Looping Patterns">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
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.-->
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<style type="text/css"><!--
|
|
pre.display { font-family:inherit }
|
|
pre.format { font-family:inherit }
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
|
pre.smallexample { font-size:smaller }
|
|
pre.smalllisp { font-size:smaller }
|
|
span.sc { font-variant:small-caps }
|
|
span.roman { font-family:serif; font-weight:normal; }
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<a name="Jump-Patterns"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Looping-Patterns.html#Looping-Patterns">Looping Patterns</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Dependent-Patterns.html#Dependent-Patterns">Dependent Patterns</a>,
|
|
Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<h3 class="section">16.12 Defining Jump Instruction Patterns</h3>
|
|
|
|
<p><a name="index-jump-instruction-patterns-3675"></a><a name="index-defining-jump-instruction-patterns-3676"></a>
|
|
GCC does not assume anything about how the machine realizes jumps.
|
|
The machine description should define a single pattern, usually
|
|
a <code>define_expand</code>, which expands to all the required insns.
|
|
|
|
<p>Usually, this would be a comparison insn to set the condition code
|
|
and a separate branch insn testing the condition code and branching
|
|
or not according to its value. For many machines, however,
|
|
separating compares and branches is limiting, which is why the
|
|
more flexible approach with one <code>define_expand</code> is used in GCC.
|
|
The machine description becomes clearer for architectures that
|
|
have compare-and-branch instructions but no condition code. It also
|
|
works better when different sets of comparison operators are supported
|
|
by different kinds of conditional branches (e.g. integer vs. floating-point),
|
|
or by conditional branches with respect to conditional stores.
|
|
|
|
<p>Two separate insns are always used if the machine description represents
|
|
a condition code register using the legacy RTL expression <code>(cc0)</code>,
|
|
and on most machines that use a separate condition code register
|
|
(see <a href="Condition-Code.html#Condition-Code">Condition Code</a>). For machines that use <code>(cc0)</code>, in
|
|
fact, the set and use of the condition code must be separate and
|
|
adjacent<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>, thus
|
|
allowing flags in <code>cc_status</code> to be used (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>) and
|
|
so that the comparison and branch insns could be located from each other
|
|
by using the functions <code>prev_cc0_setter</code> and <code>next_cc0_user</code>.
|
|
|
|
<p>Even in this case having a single entry point for conditional branches
|
|
is advantageous, because it handles equally well the case where a single
|
|
comparison instruction records the results of both signed and unsigned
|
|
comparison of the given operands (with the branch insns coming in distinct
|
|
signed and unsigned flavors) as in the x86 or SPARC, and the case where
|
|
there are distinct signed and unsigned compare instructions and only
|
|
one set of conditional branch instructions as in the PowerPC.
|
|
|
|
<div class="footnote">
|
|
<hr>
|
|
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> <code>note</code> insns can separate them, though.</p>
|
|
|
|
<hr></div>
|
|
|
|
</body></html>
|
|
|