169 lines
8.2 KiB
HTML
169 lines
8.2 KiB
HTML
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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) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Multi-Alternative</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Multi-Alternative">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Multi-Alternative">
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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="Class-Preferences.html#Class-Preferences" rel="next" title="Class Preferences">
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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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a.summary-letter {text-decoration: none}
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kbd {font-style:oblique}
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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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="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</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="subsection">16.8.2 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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Here is how it is done for fullword logical-or on the 68000:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn "iorsi3"
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[(set (match_operand:SI 0 "general_operand" "=m,d")
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(ior:SI (match_operand:SI 1 "general_operand" "%0,0")
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(match_operand:SI 2 "general_operand" "dKs,dmKs")))]
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…)
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</pre></div>
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<p>The first alternative has ‘<samp>m</samp>’ (memory) for operand 0, ‘<samp>0</samp>’ for
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operand 1 (meaning it must match operand 0), and ‘<samp>dKs</samp>’ for operand
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2. The second alternative has ‘<samp>d</samp>’ (data register) for operand 0,
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‘<samp>0</samp>’ for operand 1, and ‘<samp>dmKs</samp>’ for operand 2. The ‘<samp>=</samp>’ and
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‘<samp>%</samp>’ in the constraints apply to all the alternatives; their
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meaning is explained in the next section (see <a href="Class-Preferences.html#Class-Preferences">Class Preferences</a>).
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</p>
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<p>If all the operands fit any one alternative, the instruction is valid.
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Otherwise, for each alternative, the compiler counts how many instructions
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must be added to copy the operands so that that alternative applies.
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The alternative requiring the least copying is chosen. If two alternatives
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need the same amount of copying, the one that comes first is chosen.
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These choices can be altered with the ‘<samp>?</samp>’ and ‘<samp>!</samp>’ characters:
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</p>
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<dl compact="compact">
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<dd><a name="index-_003f-in-constraint"></a>
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<a name="index-question-mark"></a>
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</dd>
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<dt><code>?</code></dt>
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<dd><p>Disparage slightly the alternative that the ‘<samp>?</samp>’ appears in,
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as a choice when no alternative applies exactly. The compiler regards
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this alternative as one unit more costly for each ‘<samp>?</samp>’ that appears
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in it.
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</p>
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<a name="index-_0021-in-constraint"></a>
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<a name="index-exclamation-point"></a>
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</dd>
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<dt><code>!</code></dt>
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<dd><p>Disparage severely the alternative that the ‘<samp>!</samp>’ appears in.
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This alternative can still be used if it fits without reloading,
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but if reloading is needed, some other alternative will be used.
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</p>
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<a name="index-_005e-in-constraint"></a>
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<a name="index-caret"></a>
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</dd>
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<dt><code>^</code></dt>
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<dd><p>This constraint is analogous to ‘<samp>?</samp>’ but it disparages slightly
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the alternative only if the operand with the ‘<samp>^</samp>’ needs a reload.
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</p>
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<a name="index-_0024-in-constraint"></a>
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<a name="index-dollar-sign"></a>
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</dd>
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<dt><code>$</code></dt>
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<dd><p>This constraint is analogous to ‘<samp>!</samp>’ but it disparages severely
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the alternative only if the operand with the ‘<samp>$</samp>’ needs a reload.
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</p></dd>
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</dl>
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<p>When an insn pattern has multiple alternatives in its constraints, often
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the appearance of the assembler code is determined mostly by which
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alternative was matched. When this is so, the C code for writing the
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assembler code can use the variable <code>which_alternative</code>, which is
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the ordinal number of the alternative that was actually satisfied (0 for
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the first, 1 for the second alternative, etc.). See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</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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</body>
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</html>
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