1<html lang="en"> 2<head> 3<title>Constraints - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Extended-Asm.html#Extended-Asm" title="Extended Asm"> 10<link rel="next" href="Asm-Labels.html#Asm-Labels" title="Asm Labels"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Constraints"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Asm-Labels.html#Asm-Labels">Asm Labels</a>, 53Previous: <a rel="previous" accesskey="p" href="Extended-Asm.html#Extended-Asm">Extended Asm</a>, 54Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">6.42 Constraints for <code>asm</code> Operands</h3> 59 60<p><a name="index-operand-constraints_002c-_0040code_007basm_007d-2625"></a><a name="index-constraints_002c-_0040code_007basm_007d-2626"></a><a name="index-g_t_0040code_007basm_007d-constraints-2627"></a> 61Here are specific details on what constraint letters you can use with 62<code>asm</code> operands. 63Constraints can say whether 64an operand may be in a register, and which kinds of register; whether the 65operand can be a memory reference, and which kinds of address; whether the 66operand may be an immediate constant, and which possible values it may 67have. Constraints can also require two operands to match. 68Side-effects aren't allowed in operands of inline <code>asm</code>, unless 69‘<samp><span class="samp"><</span></samp>’ or ‘<samp><span class="samp">></span></samp>’ constraints are used, because there is no guarantee 70that the side-effects will happen exactly once in an instruction that can update 71the addressing register. 72 73<ul class="menu"> 74<li><a accesskey="1" href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>: Basic use of constraints. 75<li><a accesskey="2" href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>: When an insn has two alternative constraint-patterns. 76<li><a accesskey="3" href="Modifiers.html#Modifiers">Modifiers</a>: More precise control over effects of constraints. 77<li><a accesskey="4" href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>: Special constraints for some particular machines. 78</ul> 79 80 </body></html> 81 82