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113 lines
5.2 KiB
HTML
113 lines
5.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Multi-Alternative (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Multi-Alternative (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Multi-Alternative (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Constraints.html#Constraints" rel="up" title="Constraints">
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<link href="Modifiers.html#Modifiers" rel="next" title="Modifiers">
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<link href="Simple-Constraints.html#Simple-Constraints" rel="prev" title="Simple Constraints">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Multi_002dAlternative"></a>
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<div class="header">
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<p>
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Next: <a href="Modifiers.html#Modifiers" accesskey="n" rel="next">Modifiers</a>, Previous: <a href="Simple-Constraints.html#Simple-Constraints" accesskey="p" rel="prev">Simple Constraints</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Multiple-Alternative-Constraints"></a>
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<h4 class="subsubsection">6.45.3.2 Multiple Alternative Constraints</h4>
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<a name="index-multiple-alternative-constraints"></a>
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<p>Sometimes a single instruction has multiple alternative sets of possible
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operands. For example, on the 68000, a logical-or instruction can combine
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register or an immediate value into memory, or it can combine any kind of
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operand into a register; but it cannot combine one memory location into
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another.
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</p>
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<p>These constraints are represented as multiple alternatives. An alternative
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can be described by a series of letters for each operand. The overall
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constraint for an operand is made from the letters for this operand
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from the first alternative, a comma, the letters for this operand from
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the second alternative, a comma, and so on until the last alternative.
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All operands for a single instruction must have the same number of
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alternatives.
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</p>
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<p>So the first alternative for the 68000’s logical-or could be written as
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<code>"+m" (output) : "ir" (input)</code>. The second could be <code>"+r"
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(output): "irm" (input)</code>. However, the fact that two memory locations
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cannot be used in a single instruction prevents simply using <code>"+rm"
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(output) : "irm" (input)</code>. Using multi-alternatives, this might be
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written as <code>"+m,r" (output) : "ir,irm" (input)</code>. This describes
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all the available alternatives to the compiler, allowing it to choose
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the most efficient one for the current conditions.
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</p>
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<p>There is no way within the template to determine which alternative was
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chosen. However you may be able to wrap your <code>asm</code> statements with
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builtins such as <code>__builtin_constant_p</code> to achieve the desired results.
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</p>
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</body>
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</html>
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