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>Dependent Patterns (GNU Compiler Collection (GCC) Internals)</title>

<meta name="description" content="Dependent Patterns (GNU Compiler Collection (GCC) Internals)">
<meta name="keywords" content="Dependent Patterns (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="Jump-Patterns.html#Jump-Patterns" rel="next" title="Jump Patterns">
<link href="Pattern-Ordering.html#Pattern-Ordering" rel="prev" title="Pattern Ordering">
<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="Dependent-Patterns"></a>
<div class="header">
<p>
Next: <a href="Jump-Patterns.html#Jump-Patterns" accesskey="n" rel="next">Jump Patterns</a>, Previous: <a href="Pattern-Ordering.html#Pattern-Ordering" accesskey="p" rel="prev">Pattern Ordering</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="Interdependence-of-Patterns"></a>
<h3 class="section">17.11 Interdependence of Patterns</h3>
<a name="index-Dependent-Patterns"></a>
<a name="index-Interdependence-of-Patterns"></a>

<p>In some cases machines support instructions identical except for the
machine mode of one or more operands.  For example, there may be
&ldquo;sign-extend halfword&rdquo; and &ldquo;sign-extend byte&rdquo; instructions whose
patterns are
</p>
<div class="smallexample">
<pre class="smallexample">(set (match_operand:SI 0 &hellip;)
     (extend:SI (match_operand:HI 1 &hellip;)))

(set (match_operand:SI 0 &hellip;)
     (extend:SI (match_operand:QI 1 &hellip;)))
</pre></div>

<p>Constant integers do not specify a machine mode, so an instruction to
extend a constant value could match either pattern.  The pattern it
actually will match is the one that appears first in the file.  For correct
results, this must be the one for the widest possible mode (<code>HImode</code>,
here).  If the pattern matches the <code>QImode</code> instruction, the results
will be incorrect if the constant value does not actually fit that mode.
</p>
<p>Such instructions to extend constants are rarely generated because they are
optimized away, but they do occasionally happen in nonoptimized
compilations.
</p>
<p>If a constraint in a pattern allows a constant, the reload pass may
replace a register with a constant permitted by the constraint in some
cases.  Similarly for memory references.  Because of this substitution,
you should not provide separate patterns for increment and decrement
instructions.  Instead, they should be generated from the same pattern
that supports register-register add insns by examining the operands and
generating the appropriate machine instruction.
</p>


</body>
</html>