visus
/
toolchain-gcc-arm-embedded


			
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the GNU Assembler "as".

Copyright (C) 1991-2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".
 -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Xtensa Call Relaxation (Using as)</title>

<meta name="description" content="Xtensa Call Relaxation (Using as)">
<meta name="keywords" content="Xtensa Call Relaxation (Using as)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="up" title="Xtensa Relaxation">
<link href="Xtensa-Jump-Relaxation.html#Xtensa-Jump-Relaxation" rel="next" title="Xtensa Jump Relaxation">
<link href="Xtensa-Branch-Relaxation.html#Xtensa-Branch-Relaxation" rel="prev" title="Xtensa Branch Relaxation">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="Xtensa-Call-Relaxation"></a>
<div class="header">
<p>
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> &nbsp; [<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>
</div>
<hr>
<a name="Function-Call-Relaxation"></a>
<h4 class="subsubsection">9.55.4.2 Function Call Relaxation</h4>
<a name="index-relaxation-of-call-instructions"></a>
<a name="index-call-instructions_002c-relaxation"></a>

<p>Function calls may require relaxation because the Xtensa immediate call
instructions (<code>CALL0</code>, <code>CALL4</code>, <code>CALL8</code> and
<code>CALL12</code>) provide a PC-relative offset of only 512 Kbytes in either
direction.  For larger programs, it may be necessary to use indirect
calls (<code>CALLX0</code>, <code>CALLX4</code>, <code>CALLX8</code> and <code>CALLX12</code>)
where the target address is specified in a register.  The Xtensa
assembler can automatically relax immediate call instructions into
indirect call instructions.  This relaxation is done by loading the
address of the called function into the callee&rsquo;s return address register
and then using a <code>CALLX</code> instruction.  So, for example:
</p>
<div class="smallexample">
<pre class="smallexample">    call8 func
</pre></div>

<p>might be relaxed to:
</p>
<div class="smallexample">
<pre class="smallexample">    .literal .L1, func
    l32r    a8, .L1
    callx8  a8
</pre></div>

<p>Because the addresses of targets of function calls are not generally
known until link-time, the assembler must assume the worst and relax all
the calls to functions in other source files, not just those that really
will be out of range.  The linker can recognize calls that were
unnecessarily relaxed, and it will remove the overhead introduced by the
assembler for those cases where direct calls are sufficient.
</p>
<p>Call relaxation is disabled by default because it can have a negative
effect on both code size and performance, although the linker can
usually eliminate the unnecessary overhead.  If a program is too large
and some of the calls are out of range, function call relaxation can be
enabled using the &lsquo;<samp>--longcalls</samp>&rsquo; command-line option or the
<code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>).
</p>


</body>
</html>