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78 lines
3.5 KiB
HTML
78 lines
3.5 KiB
HTML
15 years ago
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<title>Xtensa Branch Relaxation - Using as</title>
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.7">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Xtensa-Relaxation.html#Xtensa-Relaxation" title="Xtensa Relaxation">
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<link rel="next" href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" title="Xtensa Call Relaxation">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 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 1.1
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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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man end-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<p>
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<a name="Xtensa-Branch-Relaxation"></a>Next: <a rel="next" accesskey="n" href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation">Xtensa Call Relaxation</a>,
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Up: <a rel="up" accesskey="u" href="Xtensa-Relaxation.html#Xtensa-Relaxation">Xtensa Relaxation</a>
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<hr><br>
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</div>
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<h5 class="subsubsection">9.36.4.1 Conditional Branch Relaxation</h5>
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<p><a name="index-relaxation-of-branch-instructions-1864"></a><a name="index-branch-instructions_002c-relaxation-1865"></a>
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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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<pre class="smallexample"> beqz a2, L
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</pre>
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<p>may result in:
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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>
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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>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. <span class="command">as</span> 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>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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<span class="samp">--no-transform</span> 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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</body></html>
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