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141 lines
6.5 KiB
HTML
141 lines
6.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Attr Example (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Attr Example (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Attr Example (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Insn-Attributes.html#Insn-Attributes" rel="up" title="Insn Attributes">
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<link href="Insn-Lengths.html#Insn-Lengths" rel="next" title="Insn Lengths">
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<link href="Tagging-Insns.html#Tagging-Insns" rel="prev" title="Tagging Insns">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Attr-Example"></a>
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<div class="header">
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<p>
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Next: <a href="Insn-Lengths.html#Insn-Lengths" accesskey="n" rel="next">Insn Lengths</a>, Previous: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="p" rel="prev">Tagging Insns</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</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>
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</div>
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<hr>
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<a name="Example-of-Attribute-Specifications"></a>
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<h4 class="subsection">17.19.4 Example of Attribute Specifications</h4>
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<a name="index-attribute-specifications-example"></a>
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<a name="index-attribute-specifications"></a>
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<p>The judicious use of defaulting is important in the efficient use of
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insn attributes. Typically, insns are divided into <em>types</em> and an
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attribute, customarily called <code>type</code>, is used to represent this
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value. This attribute is normally used only to define the default value
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for other attributes. An example will clarify this usage.
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</p>
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<p>Assume we have a RISC machine with a condition code and in which only
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full-word operations are performed in registers. Let us assume that we
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can divide all insns into loads, stores, (integer) arithmetic
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operations, floating point operations, and branches.
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</p>
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<p>Here we will concern ourselves with determining the effect of an insn on
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the condition code and will limit ourselves to the following possible
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effects: The condition code can be set unpredictably (clobbered), not
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be changed, be set to agree with the results of the operation, or only
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changed if the item previously set into the condition code has been
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modified.
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</p>
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<p>Here is part of a sample <samp>md</samp> file for such a machine:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_attr "type" "load,store,arith,fp,branch" (const_string "arith"))
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(define_attr "cc" "clobber,unchanged,set,change0"
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(cond [(eq_attr "type" "load")
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(const_string "change0")
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(eq_attr "type" "store,branch")
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(const_string "unchanged")
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(eq_attr "type" "arith")
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(if_then_else (match_operand:SI 0 "" "")
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(const_string "set")
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(const_string "clobber"))]
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(const_string "clobber")))
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(define_insn ""
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[(set (match_operand:SI 0 "general_operand" "=r,r,m")
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(match_operand:SI 1 "general_operand" "r,m,r"))]
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""
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"@
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move %0,%1
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load %0,%1
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store %0,%1"
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[(set_attr "type" "arith,load,store")])
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</pre></div>
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<p>Note that we assume in the above example that arithmetic operations
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performed on quantities smaller than a machine word clobber the condition
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code since they will set the condition code to a value corresponding to the
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full-word result.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Insn-Lengths.html#Insn-Lengths" accesskey="n" rel="next">Insn Lengths</a>, Previous: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="p" rel="prev">Tagging Insns</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</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>
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</div>
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</body>
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</html>
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