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85 lines
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85 lines
4.1 KiB
HTML
15 years ago
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<title>Xtensa Call Relaxation - Using as</title>
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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-Branch-Relaxation.html#Xtensa-Branch-Relaxation" title="Xtensa Branch Relaxation">
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<link rel="next" href="Xtensa-Immediate-Relaxation.html#Xtensa-Immediate-Relaxation" title="Xtensa Immediate Relaxation">
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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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<body>
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<div class="node">
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<p>
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<a name="Xtensa-Call-Relaxation"></a>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-Branch-Relaxation.html#Xtensa-Branch-Relaxation">Xtensa Branch 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">9.36.4.2 Function Call Relaxation</h5>
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<p><a name="index-relaxation-of-call-instructions-1866"></a><a name="index-call-instructions_002c-relaxation-1867"></a>
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Function calls may require relaxation because the Xtensa immediate call
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instructions (<code>CALL0</code>, <code>CALL4</code>, <code>CALL8</code> and
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<code>CALL12</code>) provide a PC-relative offset of only 512 Kbytes in either
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direction. For larger programs, it may be necessary to use indirect
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calls (<code>CALLX0</code>, <code>CALLX4</code>, <code>CALLX8</code> and <code>CALLX12</code>)
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where the target address is specified in a register. The Xtensa
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assembler can automatically relax immediate call instructions into
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indirect call instructions. This relaxation is done by loading the
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address of the called function into the callee's return address register
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and then using a <code>CALLX</code> instruction. So, for example:
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<pre class="smallexample"> call8 func
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</pre>
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<p>might be relaxed to:
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<pre class="smallexample"> .literal .L1, func
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l32r a8, .L1
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callx8 a8
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</pre>
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<p>Because the addresses of targets of function calls are not generally
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known until link-time, the assembler must assume the worst and relax all
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the calls to functions in other source files, not just those that really
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will be out of range. The linker can recognize calls that were
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unnecessarily relaxed, and it will remove the overhead introduced by the
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assembler for those cases where direct calls are sufficient.
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<p>Call relaxation is disabled by default because it can have a negative
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effect on both code size and performance, although the linker can
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usually eliminate the unnecessary overhead. If a program is too large
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and some of the calls are out of range, function call relaxation can be
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enabled using the <span class="samp">--longcalls</span> command-line option or the
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<code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>).
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</body></html>
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