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diff --git a/share/doc/gcc/Multi_002dAlternative.html b/share/doc/gcc/Multi_002dAlternative.html new file mode 100644 index 0000000..ee4a312 --- /dev/null +++ b/share/doc/gcc/Multi_002dAlternative.html @@ -0,0 +1,115 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 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 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Multi-Alternative</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Multi-Alternative"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Multi-Alternative"> +<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="Indices.html#Indices" rel="index" title="Indices"> +<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="previous" title="Simple Constraints"> +<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_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<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="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Multiple-Alternative-Constraints"></a> +<h4 class="subsubsection">6.47.3.2 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> |