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>Constraints (GNU Compiler Collection (GCC) Internals)</title>

<meta name="description" content="Constraints (GNU Compiler Collection (GCC) Internals)">
<meta name="keywords" content="Constraints (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="Simple-Constraints.html#Simple-Constraints" rel="next" title="Simple Constraints">
<link href="Defining-Predicates.html#Defining-Predicates" rel="prev" title="Defining Predicates">
<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="Constraints"></a>
<div class="header">
<p>
Next: <a href="Standard-Names.html#Standard-Names" accesskey="n" rel="next">Standard Names</a>, Previous: <a href="Predicates.html#Predicates" accesskey="p" rel="prev">Predicates</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="Operand-Constraints"></a>
<h3 class="section">17.8 Operand Constraints</h3>
<a name="index-operand-constraints"></a>
<a name="index-constraints"></a>

<p>Each <code>match_operand</code> in an instruction pattern can specify
constraints for the operands allowed.  The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
</p>
<p>Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have.  Constraints can also require two operands to match.
Side-effects aren&rsquo;t allowed in operands of inline <code>asm</code>, unless
&lsquo;<samp>&lt;</samp>&rsquo; or &lsquo;<samp>&gt;</samp>&rsquo; constraints are used, because there is no guarantee
that the side effects will happen exactly once in an instruction that can update
the addressing register.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Simple-Constraints.html#Simple-Constraints" accesskey="1">Simple Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Basic use of constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="2">Multi-Alternative</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">When an insn has two alternative constraint-patterns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Class-Preferences.html#Class-Preferences" accesskey="3">Class Preferences</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Constraints guide which hard register to put things in.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Modifiers.html#Modifiers" accesskey="4">Modifiers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">More precise control over effects of constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Machine-Constraints.html#Machine-Constraints" accesskey="5">Machine Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Existing constraints for some particular machines.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives" accesskey="6">Disable Insn Alternatives</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Disable insn alternatives using attributes.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Define-Constraints.html#Define-Constraints" accesskey="7">Define Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to define machine-specific constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="C-Constraint-Interface.html#C-Constraint-Interface" accesskey="8">C Constraint Interface</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to test constraints from C code.
</td></tr>
</table>


</body>
</html>