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.
110 lines
4.7 KiB
HTML
110 lines
4.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- This file documents the GNU Assembler "as".
|
|
|
|
Copyright (C) 1991-2019 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 no Invariant Sections, with no Front-Cover Texts, and with no
|
|
Back-Cover Texts. A copy of the license is included in the
|
|
section entitled "GNU Free Documentation License".
|
|
-->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Xtensa Branch Relaxation (Using as)</title>
|
|
|
|
<meta name="description" content="Xtensa Branch Relaxation (Using as)">
|
|
<meta name="keywords" content="Xtensa Branch Relaxation (Using as)">
|
|
<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="AS-Index.html#AS-Index" rel="index" title="AS Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="up" title="Xtensa Relaxation">
|
|
<link href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" rel="next" title="Xtensa Call Relaxation">
|
|
<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="prev" title="Xtensa Relaxation">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Xtensa-Branch-Relaxation"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="n" rel="next">Xtensa Call Relaxation</a>, Up: <a href="Xtensa-Relaxation.html#Xtensa-Relaxation" accesskey="u" rel="up">Xtensa Relaxation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
<hr>
|
|
<a name="Conditional-Branch-Relaxation"></a>
|
|
<h4 class="subsubsection">9.54.4.1 Conditional Branch Relaxation</h4>
|
|
<a name="index-relaxation-of-branch-instructions"></a>
|
|
<a name="index-branch-instructions_002c-relaxation"></a>
|
|
|
|
<p>When the target of a branch is too far away from the branch itself,
|
|
i.e., when the offset from the branch to the target is too large to fit
|
|
in the immediate field of the branch instruction, it may be necessary to
|
|
replace the branch with a branch around a jump. For example,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample"> beqz a2, L
|
|
</pre></div>
|
|
|
|
<p>may result in:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample"> bnez.n a2, M
|
|
j L
|
|
M:
|
|
</pre></div>
|
|
|
|
<p>(The <code>BNEZ.N</code> instruction would be used in this example only if the
|
|
density option is available. Otherwise, <code>BNEZ</code> would be used.)
|
|
</p>
|
|
<p>This relaxation works well because the unconditional jump instruction
|
|
has a much larger offset range than the various conditional branches.
|
|
However, an error will occur if a branch target is beyond the range of a
|
|
jump instruction. <code>as</code> cannot relax unconditional jumps.
|
|
Similarly, an error will occur if the original input contains an
|
|
unconditional jump to a target that is out of range.
|
|
</p>
|
|
<p>Branch relaxation is enabled by default. It can be disabled by using
|
|
underscore prefixes (see <a href="Xtensa-Opcodes.html#Xtensa-Opcodes">Opcode Names</a>), the
|
|
‘<samp>--no-transform</samp>’ command-line option (see <a href="Xtensa-Options.html#Xtensa-Options">Command-line Options</a>), or the <code>no-transform</code> directive
|
|
(see <a href="Transform-Directive.html#Transform-Directive">transform</a>).
|
|
</p>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|