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110 lines
4.7 KiB
HTML
110 lines
4.7 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Xtensa Branch Relaxation (Using as)</title>
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<meta name="description" content="Xtensa Branch Relaxation (Using as)">
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<meta name="keywords" content="Xtensa Branch Relaxation (Using as)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="up" title="Xtensa Relaxation">
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<link href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" rel="next" title="Xtensa Call Relaxation">
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<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="prev" title="Xtensa Relaxation">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Xtensa-Branch-Relaxation"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Conditional-Branch-Relaxation"></a>
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<h4 class="subsubsection">9.54.4.1 Conditional Branch Relaxation</h4>
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<a name="index-relaxation-of-branch-instructions"></a>
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<a name="index-branch-instructions_002c-relaxation"></a>
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<p>When the target of a branch is too far away from the branch itself,
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i.e., when the offset from the branch to the target is too large to fit
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in the immediate field of the branch instruction, it may be necessary to
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replace the branch with a branch around a jump. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample"> beqz a2, L
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</pre></div>
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<p>may result in:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> bnez.n a2, M
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j L
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M:
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</pre></div>
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<p>(The <code>BNEZ.N</code> instruction would be used in this example only if the
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density option is available. Otherwise, <code>BNEZ</code> would be used.)
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</p>
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<p>This relaxation works well because the unconditional jump instruction
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has a much larger offset range than the various conditional branches.
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However, an error will occur if a branch target is beyond the range of a
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jump instruction. <code>as</code> cannot relax unconditional jumps.
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Similarly, an error will occur if the original input contains an
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unconditional jump to a target that is out of range.
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</p>
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<p>Branch relaxation is enabled by default. It can be disabled by using
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underscore prefixes (see <a href="Xtensa-Opcodes.html#Xtensa-Opcodes">Opcode Names</a>), the
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‘<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
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(see <a href="Transform-Directive.html#Transform-Directive">transform</a>).
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</p>
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</body>
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</html>
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