1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- This file documents the GNU Assembler "as". 4 5Copyright (C) 1991-2021 Free Software Foundation, Inc. 6 7Permission is granted to copy, distribute and/or modify this document 8under the terms of the GNU Free Documentation License, Version 1.3 9or any later version published by the Free Software Foundation; 10with no Invariant Sections, with no Front-Cover Texts, and with no 11Back-Cover Texts. A copy of the license is included in the 12section entitled "GNU Free Documentation License". 13 --> 14<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 15<head> 16<title>Using as: Xtensa Call Relaxation</title> 17 18<meta name="description" content="Using as: Xtensa Call Relaxation"> 19<meta name="keywords" content="Using as: Xtensa Call Relaxation"> 20<meta name="resource-type" content="document"> 21<meta name="distribution" content="global"> 22<meta name="Generator" content="makeinfo"> 23<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 24<link href="index.html#Top" rel="start" title="Top"> 25<link href="AS-Index.html#AS-Index" rel="index" title="AS Index"> 26<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 27<link href="Xtensa-Relaxation.html#Xtensa-Relaxation" rel="up" title="Xtensa Relaxation"> 28<link href="Xtensa-Jump-Relaxation.html#Xtensa-Jump-Relaxation" rel="next" title="Xtensa Jump Relaxation"> 29<link href="Xtensa-Branch-Relaxation.html#Xtensa-Branch-Relaxation" rel="previous" title="Xtensa Branch Relaxation"> 30<style type="text/css"> 31<!-- 32a.summary-letter {text-decoration: none} 33blockquote.smallquotation {font-size: smaller} 34div.display {margin-left: 3.2em} 35div.example {margin-left: 3.2em} 36div.indentedblock {margin-left: 3.2em} 37div.lisp {margin-left: 3.2em} 38div.smalldisplay {margin-left: 3.2em} 39div.smallexample {margin-left: 3.2em} 40div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 41div.smalllisp {margin-left: 3.2em} 42kbd {font-style:oblique} 43pre.display {font-family: inherit} 44pre.format {font-family: inherit} 45pre.menu-comment {font-family: serif} 46pre.menu-preformatted {font-family: serif} 47pre.smalldisplay {font-family: inherit; font-size: smaller} 48pre.smallexample {font-size: smaller} 49pre.smallformat {font-family: inherit; font-size: smaller} 50pre.smalllisp {font-size: smaller} 51span.nocodebreak {white-space:nowrap} 52span.nolinebreak {white-space:nowrap} 53span.roman {font-family:serif; font-weight:normal} 54span.sansserif {font-family:sans-serif; font-weight:normal} 55ul.no-bullet {list-style: none} 56--> 57</style> 58 59 60</head> 61 62<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 63<a name="Xtensa-Call-Relaxation"></a> 64<div class="header"> 65<p> 66Next: <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="previous">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> 67</div> 68<hr> 69<a name="Function-Call-Relaxation"></a> 70<h4 class="subsubsection">9.55.4.2 Function Call Relaxation</h4> 71<a name="index-relaxation-of-call-instructions"></a> 72<a name="index-call-instructions_002c-relaxation"></a> 73 74<p>Function calls may require relaxation because the Xtensa immediate call 75instructions (<code>CALL0</code>, <code>CALL4</code>, <code>CALL8</code> and 76<code>CALL12</code>) provide a PC-relative offset of only 512 Kbytes in either 77direction. For larger programs, it may be necessary to use indirect 78calls (<code>CALLX0</code>, <code>CALLX4</code>, <code>CALLX8</code> and <code>CALLX12</code>) 79where the target address is specified in a register. The Xtensa 80assembler can automatically relax immediate call instructions into 81indirect call instructions. This relaxation is done by loading the 82address of the called function into the callee’s return address register 83and then using a <code>CALLX</code> instruction. So, for example: 84</p> 85<div class="smallexample"> 86<pre class="smallexample"> call8 func 87</pre></div> 88 89<p>might be relaxed to: 90</p> 91<div class="smallexample"> 92<pre class="smallexample"> .literal .L1, func 93 l32r a8, .L1 94 callx8 a8 95</pre></div> 96 97<p>Because the addresses of targets of function calls are not generally 98known until link-time, the assembler must assume the worst and relax all 99the calls to functions in other source files, not just those that really 100will be out of range. The linker can recognize calls that were 101unnecessarily relaxed, and it will remove the overhead introduced by the 102assembler for those cases where direct calls are sufficient. 103</p> 104<p>Call relaxation is disabled by default because it can have a negative 105effect on both code size and performance, although the linker can 106usually eliminate the unnecessary overhead. If a program is too large 107and some of the calls are out of range, function call relaxation can be 108enabled using the ‘<samp>--longcalls</samp>’ command-line option or the 109<code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>). 110</p> 111 112 113 114</body> 115</html> 116