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160 lines
7.3 KiB
HTML
160 lines
7.3 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>Modifiers (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Modifiers (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Modifiers (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="Machine-Constraints.html#Machine-Constraints" rel="next" title="Machine Constraints">
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<link href="Multi_002dAlternative.html#Multi_002dAlternative" rel="prev" title="Multi-Alternative">
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<style type="text/css">
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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="Modifiers"></a>
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<div class="header">
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<p>
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Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="p" rel="prev">Multi-Alternative</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="Constraint-Modifier-Characters"></a>
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<h4 class="subsubsection">6.45.3.3 Constraint Modifier Characters</h4>
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<a name="index-modifiers-in-constraints"></a>
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<a name="index-constraint-modifier-characters"></a>
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<p>Here are constraint modifier characters.
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</p>
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<dl compact="compact">
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<dd><a name="index-_003d-in-constraint"></a>
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</dd>
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<dt>‘<samp>=</samp>’</dt>
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<dd><p>Means that this operand is written to by this instruction:
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the previous value is discarded and replaced by new data.
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</p>
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<a name="index-_002b-in-constraint"></a>
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</dd>
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<dt>‘<samp>+</samp>’</dt>
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<dd><p>Means that this operand is both read and written by the instruction.
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</p>
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<p>When the compiler fixes up the operands to satisfy the constraints,
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it needs to know which operands are read by the instruction and
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which are written by it. ‘<samp>=</samp>’ identifies an operand which is only
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written; ‘<samp>+</samp>’ identifies an operand that is both read and written; all
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other operands are assumed to only be read.
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</p>
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<p>If you specify ‘<samp>=</samp>’ or ‘<samp>+</samp>’ in a constraint, you put it in the
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first character of the constraint string.
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</p>
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<a name="index-_0026-in-constraint"></a>
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<a name="index-earlyclobber-operand"></a>
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</dd>
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<dt>‘<samp>&</samp>’</dt>
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<dd><p>Means (in a particular alternative) that this operand is an
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<em>earlyclobber</em> operand, which is written before the instruction is
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finished using the input operands. Therefore, this operand may not lie
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in a register that is read by the instruction or as part of any memory
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address.
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</p>
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<p>‘<samp>&</samp>’ applies only to the alternative in which it is written. In
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constraints with multiple alternatives, sometimes one alternative
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requires ‘<samp>&</samp>’ while others do not. See, for example, the
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‘<samp>movdf</samp>’ insn of the 68000.
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</p>
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<p>A operand which is read by the instruction can be tied to an earlyclobber
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operand if its only use as an input occurs before the early result is
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written. Adding alternatives of this form often allows GCC to produce
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better code when only some of the read operands can be affected by the
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earlyclobber. See, for example, the ‘<samp>mulsi3</samp>’ insn of the ARM.
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</p>
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<p>Furthermore, if the <em>earlyclobber</em> operand is also a read/write
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operand, then that operand is written only after it’s used.
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</p>
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<p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’ or ‘<samp>+</samp>’. As
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<em>earlyclobber</em> operands are always written, a read-only
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<em>earlyclobber</em> operand is ill-formed and will be rejected by the
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compiler.
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</p>
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<a name="index-_0025-in-constraint"></a>
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</dd>
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<dt>‘<samp>%</samp>’</dt>
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<dd><p>Declares the instruction to be commutative for this operand and the
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following operand. This means that the compiler may interchange the
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two operands if that is the cheapest way to make all operands fit the
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constraints. ‘<samp>%</samp>’ applies to all alternatives and must appear as
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the first character in the constraint. Only read-only operands can use
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‘<samp>%</samp>’.
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</p>
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<p>GCC can only handle one commutative pair in an asm; if you use more,
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the compiler may fail. Note that you need not use the modifier if
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the two alternatives are strictly identical; this would only waste
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time in the reload pass.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="p" rel="prev">Multi-Alternative</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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