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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller }--></style></head><body><div class="node"><p>Node: <a name="Multi-Alternative">Multi-Alternative</a>,Next: <a rel="next" accesskey="n" href="Modifiers.html#Modifiers">Modifiers</a>,Previous: <a rel="previous" accesskey="p" href="Simple-Constraints.html#Simple%20Constraints">Simple Constraints</a>,Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a><hr><br></div><h3 class="subsection">Multiple Alternative Constraints</h4><p>Sometimes a single instruction has multiple alternative sets of possibleoperands. For example, on the 68000, a logical-or instruction can combineregister or an immediate value into memory, or it can combine any kind ofoperand into a register; but it cannot combine one memory location intoanother. <p>These constraints are represented as multiple alternatives. An alternativecan be described by a series of letters for each operand. The overallconstraint for an operand is made from the letters for this operandfrom the first alternative, a comma, the letters for this operand fromthe second alternative, a comma, and so on until the last alternative. <p>If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructionsmust be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternativesneed the same amount of copying, the one that comes first is chosen. These choices can be altered with the <code>?</code> and <code>!</code> characters: <dl><dt><code>?</code> <dd>Disparage slightly the alternative that the <code>?</code> appears in,as a choice when no alternative applies exactly. The compiler regardsthis alternative as one unit more costly for each <code>?</code> that appearsin it. <br><dt><code>!</code> <dd>Disparage severely the alternative that the <code>!</code> appears in. This alternative can still be used if it fits without reloading,but if reloading is needed, some other alternative will be used. </dl> </body></html>?? 快捷鍵說明
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