1<html lang="en"> 2<head> 3<title>Modifiers - 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="Constraints.html#Constraints" title="Constraints"> 9<link rel="prev" href="Multi_002dAlternative.html#Multi_002dAlternative" title="Multi-Alternative"> 10<link rel="next" href="Machine-Constraints.html#Machine-Constraints" title="Machine Constraints"> 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). 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Therefore, this operand may not lie 83in a register that is used as an input operand or as part of any memory 84address. 85 86 <p>‘<samp><span class="samp">&</span></samp>’ applies only to the alternative in which it is written. In 87constraints with multiple alternatives, sometimes one alternative 88requires ‘<samp><span class="samp">&</span></samp>’ while others do not. See, for example, the 89‘<samp><span class="samp">movdf</span></samp>’ insn of the 68000. 90 91 <p>An input operand can be tied to an earlyclobber operand if its only 92use as an input occurs before the early result is written. Adding 93alternatives of this form often allows GCC to produce better code 94when only some of the inputs can be affected by the earlyclobber. 95See, for example, the ‘<samp><span class="samp">mulsi3</span></samp>’ insn of the ARM. 96 97 <p>‘<samp><span class="samp">&</span></samp>’ does not obviate the need to write ‘<samp><span class="samp">=</span></samp>’. 98 99 <p><a name="index-g_t_0040samp_007b_0025_007d-in-constraint-2672"></a><br><dt>‘<samp><span class="samp">%</span></samp>’<dd>Declares the instruction to be commutative for this operand and the 100following operand. This means that the compiler may interchange the 101two operands if that is the cheapest way to make all operands fit the 102constraints. 103GCC can only handle one commutative pair in an asm; if you use more, 104the compiler may fail. Note that you need not use the modifier if 105the two alternatives are strictly identical; this would only waste 106time in the reload pass. The modifier is not operational after 107register allocation, so the result of <code>define_peephole2</code> 108and <code>define_split</code>s performed after reload cannot rely on 109‘<samp><span class="samp">%</span></samp>’ to make the intended insn match. 110 111 <p><a name="index-g_t_0040samp_007b_0023_007d-in-constraint-2673"></a><br><dt>‘<samp><span class="samp">#</span></samp>’<dd>Says that all following characters, up to the next comma, are to be 112ignored as a constraint. They are significant only for choosing 113register preferences. 114 115 <p><a name="index-g_t_0040samp_007b_002a_007d-in-constraint-2674"></a><br><dt>‘<samp><span class="samp">*</span></samp>’<dd>Says that the following character should be ignored when choosing 116register preferences. ‘<samp><span class="samp">*</span></samp>’ has no effect on the meaning of the 117constraint as a constraint, and no effect on reloading. 118 119 </dl> 120 121 </body></html> 122 123