visus
/
toolchain-gcc-arm-embedded


			
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2020 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 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Predicates (GNU Compiler Collection (GCC) Internals)</title>

<meta name="description" content="Predicates (GNU Compiler Collection (GCC) Internals)">
<meta name="keywords" content="Predicates (GNU Compiler Collection (GCC) Internals)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<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="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
<link href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" rel="next" title="Machine-Independent Predicates">
<link href="Output-Statement.html#Output-Statement" rel="prev" title="Output Statement">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
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.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="Predicates"></a>
<div class="header">
<p>
Next: <a href="Constraints.html#Constraints" accesskey="n" rel="next">Constraints</a>, Previous: <a href="Output-Statement.html#Output-Statement" accesskey="p" rel="prev">Output Statement</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<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="Predicates-1"></a>
<h3 class="section">17.7 Predicates</h3>
<a name="index-predicates"></a>
<a name="index-operand-predicates"></a>
<a name="index-operator-predicates"></a>

<p>A predicate determines whether a <code>match_operand</code> or
<code>match_operator</code> expression matches, and therefore whether the
surrounding instruction pattern will be used for that combination of
operands.  GCC has a number of machine-independent predicates, and you
can define machine-specific predicates as needed.  By convention,
predicates used with <code>match_operand</code> have names that end in
&lsquo;<samp>_operand</samp>&rsquo;, and those used with <code>match_operator</code> have names
that end in &lsquo;<samp>_operator</samp>&rsquo;.
</p>
<p>All predicates are boolean functions (in the mathematical sense) of
two arguments: the RTL expression that is being considered at that
position in the instruction pattern, and the machine mode that the
<code>match_operand</code> or <code>match_operator</code> specifies.  In this
section, the first argument is called <var>op</var> and the second argument
<var>mode</var>.  Predicates can be called from C as ordinary two-argument
functions; this can be useful in output templates or other
machine-specific code.
</p>
<p>Operand predicates can allow operands that are not actually acceptable
to the hardware, as long as the constraints give reload the ability to
fix them up (see <a href="Constraints.html#Constraints">Constraints</a>).  However, GCC will usually generate
better code if the predicates specify the requirements of the machine
instructions as closely as possible.  Reload cannot fix up operands
that must be constants (&ldquo;immediate operands&rdquo;); you must use a
predicate that allows only constants, or else enforce the requirement
in the extra condition.
</p>
<a name="index-predicates-and-machine-modes"></a>
<a name="index-normal-predicates"></a>
<a name="index-special-predicates"></a>
<p>Most predicates handle their <var>mode</var> argument in a uniform manner.
If <var>mode</var> is <code>VOIDmode</code> (unspecified), then <var>op</var> can have
any mode.  If <var>mode</var> is anything else, then <var>op</var> must have the
same mode, unless <var>op</var> is a <code>CONST_INT</code> or integer
<code>CONST_DOUBLE</code>.  These RTL expressions always have
<code>VOIDmode</code>, so it would be counterproductive to check that their
mode matches.  Instead, predicates that accept <code>CONST_INT</code> and/or
integer <code>CONST_DOUBLE</code> check that the value stored in the
constant will fit in the requested mode.
</p>
<p>Predicates with this behavior are called <em>normal</em>.
<code>genrecog</code> can optimize the instruction recognizer based on
knowledge of how normal predicates treat modes.  It can also diagnose
certain kinds of common errors in the use of normal predicates; for
instance, it is almost always an error to use a normal predicate
without specifying a mode.
</p>
<p>Predicates that do something different with their <var>mode</var> argument
are called <em>special</em>.  The generic predicates
<code>address_operand</code> and <code>pmode_register_operand</code> are special
predicates.  <code>genrecog</code> does not do any optimizations or
diagnosis when special predicates are used.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" accesskey="1">Machine-Independent Predicates</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Predicates available to all back ends.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Defining-Predicates.html#Defining-Predicates" accesskey="2">Defining Predicates</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to write machine-specific predicate
                                    functions.
</td></tr>
</table>

<hr>
<div class="header">
<p>
Next: <a href="Constraints.html#Constraints" accesskey="n" rel="next">Constraints</a>, Previous: <a href="Output-Statement.html#Output-Statement" accesskey="p" rel="prev">Output Statement</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<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>