177 lines
8.4 KiB
HTML
177 lines
8.4 KiB
HTML
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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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<!-- Copyright (C) 1988-2016 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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(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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Conditional Execution</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Conditional Execution">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Conditional Execution">
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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="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="Define-Subst.html#Define-Subst" rel="next" title="Define Subst">
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<link href="Processor-pipeline-description.html#Processor-pipeline-description" rel="prev" title="Processor pipeline description">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Conditional-Execution"></a>
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<div class="header">
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<p>
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Next: <a href="Define-Subst.html#Define-Subst" accesskey="n" rel="next">Define Subst</a>, Previous: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="p" rel="prev">Insn Attributes</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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="Conditional-Execution-1"></a>
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<h3 class="section">16.20 Conditional Execution</h3>
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<a name="index-conditional-execution"></a>
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<a name="index-predication"></a>
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<p>A number of architectures provide for some form of conditional
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execution, or predication. The hallmark of this feature is the
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ability to nullify most of the instructions in the instruction set.
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When the instruction set is large and not entirely symmetric, it
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can be quite tedious to describe these forms directly in the
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<samp>.md</samp> file. An alternative is the <code>define_cond_exec</code> template.
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</p>
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<a name="index-define_005fcond_005fexec"></a>
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<div class="smallexample">
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<pre class="smallexample">(define_cond_exec
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[<var>predicate-pattern</var>]
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"<var>condition</var>"
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"<var>output-template</var>"
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"<var>optional-insn-attribues</var>")
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</pre></div>
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<p><var>predicate-pattern</var> is the condition that must be true for the
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insn to be executed at runtime and should match a relational operator.
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One can use <code>match_operator</code> to match several relational operators
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at once. Any <code>match_operand</code> operands must have no more than one
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alternative.
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</p>
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<p><var>condition</var> is a C expression that must be true for the generated
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pattern to match.
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</p>
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<a name="index-current_005finsn_005fpredicate"></a>
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<p><var>output-template</var> is a string similar to the <code>define_insn</code>
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output template (see <a href="Output-Template.html#Output-Template">Output Template</a>), except that the ‘<samp>*</samp>’
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and ‘<samp>@</samp>’ special cases do not apply. This is only useful if the
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assembly text for the predicate is a simple prefix to the main insn.
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In order to handle the general case, there is a global variable
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<code>current_insn_predicate</code> that will contain the entire predicate
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if the current insn is predicated, and will otherwise be <code>NULL</code>.
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</p>
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<p><var>optional-insn-attributes</var> is an optional vector of attributes that gets
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appended to the insn attributes of the produced cond_exec rtx. It can
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be used to add some distinguishing attribute to cond_exec rtxs produced
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that way. An example usage would be to use this attribute in conjunction
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with attributes on the main pattern to disable particular alternatives under
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certain conditions.
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</p>
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<p>When <code>define_cond_exec</code> is used, an implicit reference to
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the <code>predicable</code> instruction attribute is made.
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See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>. This attribute must be a boolean (i.e. have
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exactly two elements in its <var>list-of-values</var>), with the possible
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values being <code>no</code> and <code>yes</code>. The default and all uses in
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the insns must be a simple constant, not a complex expressions. It
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may, however, depend on the alternative, by using a comma-separated
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list of values. If that is the case, the port should also define an
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<code>enabled</code> attribute (see <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>), which
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should also allow only <code>no</code> and <code>yes</code> as its values.
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</p>
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<p>For each <code>define_insn</code> for which the <code>predicable</code>
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attribute is true, a new <code>define_insn</code> pattern will be
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generated that matches a predicated version of the instruction.
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For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn "addsi"
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[(set (match_operand:SI 0 "register_operand" "r")
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(plus:SI (match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))]
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"<var>test1</var>"
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"add %2,%1,%0")
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(define_cond_exec
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[(ne (match_operand:CC 0 "register_operand" "c")
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(const_int 0))]
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"<var>test2</var>"
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"(%0)")
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</pre></div>
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<p>generates a new pattern
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn ""
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[(cond_exec
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(ne (match_operand:CC 3 "register_operand" "c") (const_int 0))
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(set (match_operand:SI 0 "register_operand" "r")
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(plus:SI (match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r"))))]
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"(<var>test2</var>) && (<var>test1</var>)"
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"(%3) add %2,%1,%0")
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Define-Subst.html#Define-Subst" accesskey="n" rel="next">Define Subst</a>, Previous: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="p" rel="prev">Insn Attributes</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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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