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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gccint/Modifiers.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
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diff --git a/share/doc/gccint/Modifiers.html b/share/doc/gccint/Modifiers.html new file mode 100644 index 0000000..40ed5b6 --- /dev/null +++ b/share/doc/gccint/Modifiers.html @@ -0,0 +1,209 @@ +<!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: Modifiers</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Modifiers"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Modifiers"> +<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="Machine-Constraints.html#Machine-Constraints" rel="next" title="Machine Constraints"> +<link href="Class-Preferences.html#Class-Preferences" rel="previous" title="Class Preferences"> +<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="Modifiers"></a> +<div class="header"> +<p> +Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="previous">Class Preferences</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="Constraint-Modifier-Characters"></a> +<h4 class="subsection">17.8.4 Constraint Modifier Characters</h4> +<a name="index-modifiers-in-constraints"></a> +<a name="index-constraint-modifier-characters"></a> + +<p>Here are constraint modifier characters. +</p> +<dl compact="compact"> +<dd><a name="index-_003d-in-constraint"></a> +</dd> +<dt>‘<samp>=</samp>’</dt> +<dd><p>Means that this operand is written to by this instruction: +the previous value is discarded and replaced by new data. +</p> +<a name="index-_002b-in-constraint"></a> +</dd> +<dt>‘<samp>+</samp>’</dt> +<dd><p>Means that this operand is both read and written by the instruction. +</p> +<p>When the compiler fixes up the operands to satisfy the constraints, +it needs to know which operands are read by the instruction and +which are written by it. ‘<samp>=</samp>’ identifies an operand which is only +written; ‘<samp>+</samp>’ identifies an operand that is both read and written; all +other operands are assumed to only be read. +</p> +<p>If you specify ‘<samp>=</samp>’ or ‘<samp>+</samp>’ in a constraint, you put it in the +first character of the constraint string. +</p> +<a name="index-_0026-in-constraint"></a> +<a name="index-earlyclobber-operand"></a> +</dd> +<dt>‘<samp>&</samp>’</dt> +<dd><p>Means (in a particular alternative) that this operand is an +<em>earlyclobber</em> operand, which is written before the instruction is +finished using the input operands. Therefore, this operand may not lie +in a register that is read by the instruction or as part of any memory +address. +</p> +<p>‘<samp>&</samp>’ applies only to the alternative in which it is written. In +constraints with multiple alternatives, sometimes one alternative +requires ‘<samp>&</samp>’ while others do not. See, for example, the +‘<samp>movdf</samp>’ insn of the 68000. +</p> +<p>An operand which is read by the instruction can be tied to an earlyclobber +operand if its only use as an input occurs before the early result is +written. Adding alternatives of this form often allows GCC to produce +better code when only some of the read operands can be affected by the +earlyclobber. See, for example, the ‘<samp>mulsi3</samp>’ insn of the ARM. +</p> +<p>Furthermore, if the <em>earlyclobber</em> operand is also a read/write +operand, then that operand is written only after it’s used. +</p> +<p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’ or ‘<samp>+</samp>’. As +<em>earlyclobber</em> operands are always written, a read-only +<em>earlyclobber</em> operand is ill-formed and will be rejected by the +compiler. +</p> +<a name="index-_0025-in-constraint"></a> +</dd> +<dt>‘<samp>%</samp>’</dt> +<dd><p>Declares the instruction to be commutative for this operand and the +following operand. This means that the compiler may interchange the +two operands if that is the cheapest way to make all operands fit the +constraints. ‘<samp>%</samp>’ applies to all alternatives and must appear as +the first character in the constraint. Only read-only operands can use +‘<samp>%</samp>’. +</p> +<p>This is often used in patterns for addition instructions +that really have only two operands: the result must go in one of the +arguments. Here for example, is how the 68000 halfword-add +instruction is defined: +</p> +<div class="smallexample"> +<pre class="smallexample">(define_insn "addhi3" + [(set (match_operand:HI 0 "general_operand" "=m,r") + (plus:HI (match_operand:HI 1 "general_operand" "%0,0") + (match_operand:HI 2 "general_operand" "di,g")))] + …) +</pre></div> +<p>GCC can only handle one commutative pair in an asm; if you use more, +the compiler may fail. Note that you need not use the modifier if +the two alternatives are strictly identical; this would only waste +time in the reload pass. +The modifier is not operational after +register allocation, so the result of <code>define_peephole2</code> +and <code>define_split</code>s performed after reload cannot rely on +‘<samp>%</samp>’ to make the intended insn match. +</p> +<a name="index-_0023-in-constraint"></a> +</dd> +<dt>‘<samp>#</samp>’</dt> +<dd><p>Says that all following characters, up to the next comma, are to be +ignored as a constraint. They are significant only for choosing +register preferences. +</p> +<a name="index-_002a-in-constraint"></a> +</dd> +<dt>‘<samp>*</samp>’</dt> +<dd><p>Says that the following character should be ignored when choosing +register preferences. ‘<samp>*</samp>’ has no effect on the meaning of the +constraint as a constraint, and no effect on reloading. For LRA +‘<samp>*</samp>’ additionally disparages slightly the alternative if the +following character matches the operand. +</p> +<p>Here is an example: the 68000 has an instruction to sign-extend a +halfword in a data register, and can also sign-extend a value by +copying it into an address register. While either kind of register is +acceptable, the constraints on an address-register destination are +less strict, so it is best if register allocation makes an address +register its goal. Therefore, ‘<samp>*</samp>’ is used so that the ‘<samp>d</samp>’ +constraint letter (for data register) is ignored when computing +register preferences. +</p> +<div class="smallexample"> +<pre class="smallexample">(define_insn "extendhisi2" + [(set (match_operand:SI 0 "general_operand" "=*d,a") + (sign_extend:SI + (match_operand:HI 1 "general_operand" "0,g")))] + …) +</pre></div> +</dd> +</dl> + +<hr> +<div class="header"> +<p> +Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="previous">Class Preferences</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> |