210 lines
9.7 KiB
HTML
210 lines
9.7 KiB
HTML
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2016 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
||
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
||
|
|
(see below). A copy of the license is included in the section entitled
|
||
|
|
"GNU Free Documentation License".
|
||
|
|
|
||
|
|
(a) The FSF's Front-Cover Text is:
|
||
|
|
|
||
|
|
A GNU Manual
|
||
|
|
|
||
|
|
(b) The FSF's Back-Cover Text is:
|
||
|
|
|
||
|
|
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. -->
|
||
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>GNU Compiler Collection (GCC) Internals: Modifiers</title>
|
||
|
|
|
||
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals: Modifiers">
|
||
|
|
<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Modifiers">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Constraints.html#Constraints" rel="up" title="Constraints">
|
||
|
|
<link href="Machine-Constraints.html#Machine-Constraints" rel="next" title="Machine Constraints">
|
||
|
|
<link href="Class-Preferences.html#Class-Preferences" rel="prev" title="Class Preferences">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.indentedblock {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style:oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nocodebreak {white-space:nowrap}
|
||
|
|
span.nolinebreak {white-space:nowrap}
|
||
|
|
span.roman {font-family:serif; font-weight:normal}
|
||
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
||
|
|
<a name="Modifiers"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="prev">Class Preferences</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="Constraint-Modifier-Characters"></a>
|
||
|
|
<h4 class="subsection">16.8.4 Constraint Modifier Characters</h4>
|
||
|
|
<a name="index-modifiers-in-constraints"></a>
|
||
|
|
<a name="index-constraint-modifier-characters"></a>
|
||
|
|
|
||
|
|
<p>Here are constraint modifier characters.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dd><a name="index-_003d-in-constraint"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>=</samp>’</dt>
|
||
|
|
<dd><p>Means that this operand is written to by this instruction:
|
||
|
|
the previous value is discarded and replaced by new data.
|
||
|
|
</p>
|
||
|
|
<a name="index-_002b-in-constraint"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>+</samp>’</dt>
|
||
|
|
<dd><p>Means that this operand is both read and written by the instruction.
|
||
|
|
</p>
|
||
|
|
<p>When the compiler fixes up the operands to satisfy the constraints,
|
||
|
|
it needs to know which operands are read by the instruction and
|
||
|
|
which are written by it. ‘<samp>=</samp>’ identifies an operand which is only
|
||
|
|
written; ‘<samp>+</samp>’ identifies an operand that is both read and written; all
|
||
|
|
other operands are assumed to only be read.
|
||
|
|
</p>
|
||
|
|
<p>If you specify ‘<samp>=</samp>’ or ‘<samp>+</samp>’ in a constraint, you put it in the
|
||
|
|
first character of the constraint string.
|
||
|
|
</p>
|
||
|
|
<a name="index-_0026-in-constraint"></a>
|
||
|
|
<a name="index-earlyclobber-operand"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>&</samp>’</dt>
|
||
|
|
<dd><p>Means (in a particular alternative) that this operand is an
|
||
|
|
<em>earlyclobber</em> operand, which is written before the instruction is
|
||
|
|
finished using the input operands. Therefore, this operand may not lie
|
||
|
|
in a register that is read by the instruction or as part of any memory
|
||
|
|
address.
|
||
|
|
</p>
|
||
|
|
<p>‘<samp>&</samp>’ applies only to the alternative in which it is written. In
|
||
|
|
constraints with multiple alternatives, sometimes one alternative
|
||
|
|
requires ‘<samp>&</samp>’ while others do not. See, for example, the
|
||
|
|
‘<samp>movdf</samp>’ insn of the 68000.
|
||
|
|
</p>
|
||
|
|
<p>A operand which is read by the instruction can be tied to an earlyclobber
|
||
|
|
operand if its only use as an input occurs before the early result is
|
||
|
|
written. Adding alternatives of this form often allows GCC to produce
|
||
|
|
better code when only some of the read operands can be affected by the
|
||
|
|
earlyclobber. See, for example, the ‘<samp>mulsi3</samp>’ insn of the ARM.
|
||
|
|
</p>
|
||
|
|
<p>Furthermore, if the <em>earlyclobber</em> operand is also a read/write
|
||
|
|
operand, then that operand is written only after it’s used.
|
||
|
|
</p>
|
||
|
|
<p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’ or ‘<samp>+</samp>’. As
|
||
|
|
<em>earlyclobber</em> operands are always written, a read-only
|
||
|
|
<em>earlyclobber</em> operand is ill-formed and will be rejected by the
|
||
|
|
compiler.
|
||
|
|
</p>
|
||
|
|
<a name="index-_0025-in-constraint"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>%</samp>’</dt>
|
||
|
|
<dd><p>Declares the instruction to be commutative for this operand and the
|
||
|
|
following operand. This means that the compiler may interchange the
|
||
|
|
two operands if that is the cheapest way to make all operands fit the
|
||
|
|
constraints. ‘<samp>%</samp>’ applies to all alternatives and must appear as
|
||
|
|
the first character in the constraint. Only read-only operands can use
|
||
|
|
‘<samp>%</samp>’.
|
||
|
|
</p>
|
||
|
|
<p>This is often used in patterns for addition instructions
|
||
|
|
that really have only two operands: the result must go in one of the
|
||
|
|
arguments. Here for example, is how the 68000 halfword-add
|
||
|
|
instruction is defined:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(define_insn "addhi3"
|
||
|
|
[(set (match_operand:HI 0 "general_operand" "=m,r")
|
||
|
|
(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
|
||
|
|
(match_operand:HI 2 "general_operand" "di,g")))]
|
||
|
|
…)
|
||
|
|
</pre></div>
|
||
|
|
<p>GCC can only handle one commutative pair in an asm; if you use more,
|
||
|
|
the compiler may fail. Note that you need not use the modifier if
|
||
|
|
the two alternatives are strictly identical; this would only waste
|
||
|
|
time in the reload pass.
|
||
|
|
The modifier is not operational after
|
||
|
|
register allocation, so the result of <code>define_peephole2</code>
|
||
|
|
and <code>define_split</code>s performed after reload cannot rely on
|
||
|
|
‘<samp>%</samp>’ to make the intended insn match.
|
||
|
|
</p>
|
||
|
|
<a name="index-_0023-in-constraint"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>#</samp>’</dt>
|
||
|
|
<dd><p>Says that all following characters, up to the next comma, are to be
|
||
|
|
ignored as a constraint. They are significant only for choosing
|
||
|
|
register preferences.
|
||
|
|
</p>
|
||
|
|
<a name="index-_002a-in-constraint"></a>
|
||
|
|
</dd>
|
||
|
|
<dt>‘<samp>*</samp>’</dt>
|
||
|
|
<dd><p>Says that the following character should be ignored when choosing
|
||
|
|
register preferences. ‘<samp>*</samp>’ has no effect on the meaning of the
|
||
|
|
constraint as a constraint, and no effect on reloading. For LRA
|
||
|
|
‘<samp>*</samp>’ additionally disparages slightly the alternative if the
|
||
|
|
following character matches the operand.
|
||
|
|
</p>
|
||
|
|
<p>Here is an example: the 68000 has an instruction to sign-extend a
|
||
|
|
halfword in a data register, and can also sign-extend a value by
|
||
|
|
copying it into an address register. While either kind of register is
|
||
|
|
acceptable, the constraints on an address-register destination are
|
||
|
|
less strict, so it is best if register allocation makes an address
|
||
|
|
register its goal. Therefore, ‘<samp>*</samp>’ is used so that the ‘<samp>d</samp>’
|
||
|
|
constraint letter (for data register) is ignored when computing
|
||
|
|
register preferences.
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(define_insn "extendhisi2"
|
||
|
|
[(set (match_operand:SI 0 "general_operand" "=*d,a")
|
||
|
|
(sign_extend:SI
|
||
|
|
(match_operand:HI 1 "general_operand" "0,g")))]
|
||
|
|
…)
|
||
|
|
</pre></div>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="prev">Class Preferences</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|