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diff --git a/share/doc/gccint/Multi_002dAlternative.html b/share/doc/gccint/Multi_002dAlternative.html new file mode 100644 index 0000000..f6e4564 --- /dev/null +++ b/share/doc/gccint/Multi_002dAlternative.html @@ -0,0 +1,168 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- 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>GNU Compiler Collection (GCC) Internals: Multi-Alternative</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Multi-Alternative"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: 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="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="Class-Preferences.html#Class-Preferences" rel="next" title="Class Preferences"> +<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" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Multi_002dAlternative"></a> +<div class="header"> +<p> +Next: <a href="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</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="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Multiple-Alternative-Constraints"></a> +<h4 class="subsection">17.8.2 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. +Here is how it is done for fullword logical-or on the 68000: +</p> +<div class="smallexample"> +<pre class="smallexample">(define_insn "iorsi3" + [(set (match_operand:SI 0 "general_operand" "=m,d") + (ior:SI (match_operand:SI 1 "general_operand" "%0,0") + (match_operand:SI 2 "general_operand" "dKs,dmKs")))] + …) +</pre></div> + +<p>The first alternative has ‘<samp>m</samp>’ (memory) for operand 0, ‘<samp>0</samp>’ for +operand 1 (meaning it must match operand 0), and ‘<samp>dKs</samp>’ for operand +2. The second alternative has ‘<samp>d</samp>’ (data register) for operand 0, +‘<samp>0</samp>’ for operand 1, and ‘<samp>dmKs</samp>’ for operand 2. The ‘<samp>=</samp>’ and +‘<samp>%</samp>’ in the constraints apply to all the alternatives; their +meaning is explained in a later section (see <a href="Modifiers.html#Modifiers">Modifiers</a>). +</p> +<p>If all the operands fit any one alternative, the instruction is valid. +Otherwise, for each alternative, the compiler counts how many instructions +must be added to copy the operands so that that alternative applies. +The alternative requiring the least copying is chosen. If two alternatives +need the same amount of copying, the one that comes first is chosen. +These choices can be altered with the ‘<samp>?</samp>’ and ‘<samp>!</samp>’ characters: +</p> +<dl compact="compact"> +<dd><a name="index-_003f-in-constraint"></a> +<a name="index-question-mark"></a> +</dd> +<dt><code>?</code></dt> +<dd><p>Disparage slightly the alternative that the ‘<samp>?</samp>’ appears in, +as a choice when no alternative applies exactly. The compiler regards +this alternative as one unit more costly for each ‘<samp>?</samp>’ that appears +in it. +</p> +<a name="index-_0021-in-constraint"></a> +<a name="index-exclamation-point"></a> +</dd> +<dt><code>!</code></dt> +<dd><p>Disparage severely the alternative that the ‘<samp>!</samp>’ appears in. +This alternative can still be used if it fits without reloading, +but if reloading is needed, some other alternative will be used. +</p> +<a name="index-_005e-in-constraint"></a> +<a name="index-caret"></a> +</dd> +<dt><code>^</code></dt> +<dd><p>This constraint is analogous to ‘<samp>?</samp>’ but it disparages slightly +the alternative only if the operand with the ‘<samp>^</samp>’ needs a reload. +</p> +<a name="index-_0024-in-constraint"></a> +<a name="index-dollar-sign"></a> +</dd> +<dt><code>$</code></dt> +<dd><p>This constraint is analogous to ‘<samp>!</samp>’ but it disparages severely +the alternative only if the operand with the ‘<samp>$</samp>’ needs a reload. +</p></dd> +</dl> + +<p>When an insn pattern has multiple alternatives in its constraints, often +the appearance of the assembler code is determined mostly by which +alternative was matched. When this is so, the C code for writing the +assembler code can use the variable <code>which_alternative</code>, which is +the ordinal number of the alternative that was actually satisfied (0 for +the first, 1 for the second alternative, etc.). See <a href="Output-Statement.html#Output-Statement">Output Statement</a>. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</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="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |