116 lines
5.3 KiB
HTML
116 lines
5.3 KiB
HTML
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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) 1991-2016 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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using as: Xtensa Call Relaxation</title>
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<meta name="description" content="Using as: Xtensa Call Relaxation">
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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-Jump-Relaxation.html#Xtensa-Jump-Relaxation" rel="next" title="Xtensa Jump Relaxation">
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<link href="Xtensa-Branch-Relaxation.html#Xtensa-Branch-Relaxation" rel="prev" title="Xtensa Branch Relaxation">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Xtensa-Call-Relaxation"></a>
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<div class="header">
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<p>
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Next: <a href="Xtensa-Jump-Relaxation.html#Xtensa-Jump-Relaxation" accesskey="n" rel="next">Xtensa Jump Relaxation</a>, Previous: <a href="Xtensa-Branch-Relaxation.html#Xtensa-Branch-Relaxation" accesskey="p" rel="prev">Xtensa Branch 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="Function-Call-Relaxation"></a>
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<h4 class="subsubsection">9.52.4.2 Function Call Relaxation</h4>
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<a name="index-relaxation-of-call-instructions"></a>
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<a name="index-call-instructions_002c-relaxation"></a>
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<p>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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</p>
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<div class="smallexample">
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<pre class="smallexample"> call8 func
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</pre></div>
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<p>might be relaxed to:
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</p>
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<div class="smallexample">
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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></div>
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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>
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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 ‘<samp>--longcalls</samp>’ 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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</p>
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</body>
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</html>
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