117 lines
4.5 KiB
HTML
117 lines
4.5 KiB
HTML
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<html lang="en">
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<head>
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<title>Xtensa Jump Relaxation - Using as</title>
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<meta name="description" content="Using as">
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<link rel="up" href="Xtensa-Relaxation.html#Xtensa-Relaxation" title="Xtensa Relaxation">
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<link rel="prev" 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) 1991-2019 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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Xtensa-Jump-Relaxation"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Xtensa-Immediate-Relaxation.html#Xtensa-Immediate-Relaxation">Xtensa Immediate Relaxation</a>,
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Previous: <a rel="previous" accesskey="p" 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>
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</div>
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<h5 class="subsubsection">9.55.4.3 Jump Relaxation</h5>
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<p><a name="index-relaxation-of-jump-instructions-2638"></a><a name="index-jump-instructions_002c-relaxation-2639"></a>
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Jump instruction may require relaxation because the Xtensa jump instruction
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(<code>J</code>) provide a PC-relative offset of only 128 Kbytes in either
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direction. One option is to use jump long (<code>J.L</code>) instruction, which
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depending on jump distance may be assembled as jump (<code>J</code>) or indirect
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jump (<code>JX</code>). However it needs a free register. When there's no spare
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register it is possible to plant intermediate jump sites (trampolines)
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between the jump instruction and its target. These sites may be located in
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areas unreachable by normal code execution flow, in that case they only
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contain intermediate jumps, or they may be inserted in the middle of code
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block, in which case there's an additional jump from the beginning of the
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trampoline to the instruction past its end. So, for example:
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<pre class="smallexample"> j 1f
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...
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retw
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...
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mov a10, a2
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call8 func
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...
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1:
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...
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</pre>
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<p>might be relaxed to:
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<pre class="smallexample"> j .L0_TR_1
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...
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retw
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.L0_TR_1:
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j 1f
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...
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mov a10, a2
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call8 func
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...
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1:
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...
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</pre>
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<p>or to:
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<pre class="smallexample"> j .L0_TR_1
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...
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retw
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...
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mov a10, a2
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j .L0_TR_0
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.L0_TR_1:
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j 1f
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.L0_TR_0:
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call8 func
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...
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1:
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...
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</pre>
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<p>The Xtensa assembler uses trampolines with jump around only when it cannot
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find suitable unreachable trampoline. There may be multiple trampolines
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between the jump instruction and its target.
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<p>This relaxation does not apply to jumps to undefined symbols, assuming they
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will reach their targets once resolved.
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<p>Jump relaxation is enabled by default because it does not affect code size
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or performance while the code itself is small. This relaxation may be
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disabled completely with ‘<samp><span class="samp">--no-trampolines</span></samp>’ or ‘<samp><span class="samp">--no-transform</span></samp>’
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command-line options (see <a href="Xtensa-Options.html#Xtensa-Options">Command-line Options</a>).
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</body></html>
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