You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
113 lines
5.2 KiB
HTML
113 lines
5.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Multi-Alternative (Using the GNU Compiler Collection (GCC))</title>
|
|
|
|
<meta name="description" content="Multi-Alternative (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="keywords" content="Multi-Alternative (Using the GNU Compiler Collection (GCC))">
|
|
<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="Modifiers.html#Modifiers" rel="next" title="Modifiers">
|
|
<link href="Simple-Constraints.html#Simple-Constraints" rel="prev" title="Simple Constraints">
|
|
<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="Multi_002dAlternative"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Modifiers.html#Modifiers" accesskey="n" rel="next">Modifiers</a>, Previous: <a href="Simple-Constraints.html#Simple-Constraints" accesskey="p" rel="prev">Simple Constraints</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="Multiple-Alternative-Constraints"></a>
|
|
<h4 class="subsubsection">6.45.3.2 Multiple Alternative Constraints</h4>
|
|
<a name="index-multiple-alternative-constraints"></a>
|
|
|
|
<p>Sometimes a single instruction has multiple alternative sets of possible
|
|
operands. For example, on the 68000, a logical-or instruction can combine
|
|
register or an immediate value into memory, or it can combine any kind of
|
|
operand into a register; but it cannot combine one memory location into
|
|
another.
|
|
</p>
|
|
<p>These constraints are represented as multiple alternatives. An alternative
|
|
can be described by a series of letters for each operand. The overall
|
|
constraint for an operand is made from the letters for this operand
|
|
from the first alternative, a comma, the letters for this operand from
|
|
the second alternative, a comma, and so on until the last alternative.
|
|
All operands for a single instruction must have the same number of
|
|
alternatives.
|
|
</p>
|
|
<p>So the first alternative for the 68000’s logical-or could be written as
|
|
<code>"+m" (output) : "ir" (input)</code>. The second could be <code>"+r"
|
|
(output): "irm" (input)</code>. However, the fact that two memory locations
|
|
cannot be used in a single instruction prevents simply using <code>"+rm"
|
|
(output) : "irm" (input)</code>. Using multi-alternatives, this might be
|
|
written as <code>"+m,r" (output) : "ir,irm" (input)</code>. This describes
|
|
all the available alternatives to the compiler, allowing it to choose
|
|
the most efficient one for the current conditions.
|
|
</p>
|
|
<p>There is no way within the template to determine which alternative was
|
|
chosen. However you may be able to wrap your <code>asm</code> statements with
|
|
builtins such as <code>__builtin_constant_p</code> to achieve the desired results.
|
|
</p>
|
|
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|