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201 lines
9.2 KiB
HTML
201 lines
9.2 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>Output Statement (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Output Statement (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Output Statement (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="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="Predicates.html#Predicates" rel="next" title="Predicates">
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<link href="Output-Template.html#Output-Template" rel="prev" title="Output Template">
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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="Output-Statement"></a>
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<div class="header">
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<p>
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Next: <a href="Predicates.html#Predicates" accesskey="n" rel="next">Predicates</a>, Previous: <a href="Output-Template.html#Output-Template" accesskey="p" rel="prev">Output Template</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="C-Statements-for-Assembler-Output"></a>
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<h3 class="section">17.6 C Statements for Assembler Output</h3>
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<a name="index-output-statements"></a>
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<a name="index-C-statements-for-assembler-output"></a>
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<a name="index-generating-assembler-output"></a>
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<p>Often a single fixed template string cannot produce correct and efficient
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assembler code for all the cases that are recognized by a single
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instruction pattern. For example, the opcodes may depend on the kinds of
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operands; or some unfortunate combinations of operands may require extra
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machine instructions.
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</p>
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<p>If the output control string starts with a ‘<samp>@</samp>’, then it is actually
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a series of templates, each on a separate line. (Blank lines and
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leading spaces and tabs are ignored.) The templates correspond to the
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pattern’s constraint alternatives (see <a href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>). For example,
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if a target machine has a two-address add instruction ‘<samp>addr</samp>’ to add
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into a register and another ‘<samp>addm</samp>’ to add a register to memory, you
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might write this pattern:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn "addsi3"
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[(set (match_operand:SI 0 "general_operand" "=r,m")
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(plus:SI (match_operand:SI 1 "general_operand" "0,0")
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(match_operand:SI 2 "general_operand" "g,r")))]
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""
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"@
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addr %2,%0
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addm %2,%0")
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</pre></div>
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<a name="index-_002a-in-template"></a>
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<a name="index-asterisk-in-template"></a>
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<p>If the output control string starts with a ‘<samp>*</samp>’, then it is not an
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output template but rather a piece of C program that should compute a
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template. It should execute a <code>return</code> statement to return the
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template-string you want. Most such templates use C string literals, which
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require doublequote characters to delimit them. To include these
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doublequote characters in the string, prefix each one with ‘<samp>\</samp>’.
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</p>
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<p>If the output control string is written as a brace block instead of a
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double-quoted string, it is automatically assumed to be C code. In that
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case, it is not necessary to put in a leading asterisk, or to escape the
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doublequotes surrounding C string literals.
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</p>
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<p>The operands may be found in the array <code>operands</code>, whose C data type
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is <code>rtx []</code>.
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</p>
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<p>It is very common to select different ways of generating assembler code
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based on whether an immediate operand is within a certain range. Be
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careful when doing this, because the result of <code>INTVAL</code> is an
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integer on the host machine. If the host machine has more bits in an
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<code>int</code> than the target machine has in the mode in which the constant
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will be used, then some of the bits you get from <code>INTVAL</code> will be
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superfluous. For proper results, you must carefully disregard the
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values of those bits.
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</p>
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<a name="index-output_005fasm_005finsn"></a>
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<p>It is possible to output an assembler instruction and then go on to output
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or compute more of them, using the subroutine <code>output_asm_insn</code>. This
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receives two arguments: a template-string and a vector of operands. The
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vector may be <code>operands</code>, or it may be another array of <code>rtx</code>
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that you declare locally and initialize yourself.
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</p>
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<a name="index-which_005falternative"></a>
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<p>When an insn pattern has multiple alternatives in its constraints, often
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the appearance of the assembler code is determined mostly by which alternative
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was matched. When this is so, the C code can test the variable
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<code>which_alternative</code>, which is the ordinal number of the alternative
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that was actually satisfied (0 for the first, 1 for the second alternative,
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etc.).
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</p>
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<p>For example, suppose there are two opcodes for storing zero, ‘<samp>clrreg</samp>’
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for registers and ‘<samp>clrmem</samp>’ for memory locations. Here is how
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a pattern could use <code>which_alternative</code> to choose between them:
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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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[(set (match_operand:SI 0 "general_operand" "=r,m")
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(const_int 0))]
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""
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{
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return (which_alternative == 0
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? "clrreg %0" : "clrmem %0");
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})
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</pre></div>
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<p>The example above, where the assembler code to generate was
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<em>solely</em> determined by the alternative, could also have been specified
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as follows, having the output control string start with a ‘<samp>@</samp>’:
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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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[(set (match_operand:SI 0 "general_operand" "=r,m")
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(const_int 0))]
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""
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"@
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clrreg %0
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clrmem %0")
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</pre></div>
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<p>If you just need a little bit of C code in one (or a few) alternatives,
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you can use ‘<samp>*</samp>’ inside of a ‘<samp>@</samp>’ multi-alternative template:
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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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[(set (match_operand:SI 0 "general_operand" "=r,<,m")
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(const_int 0))]
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""
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"@
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clrreg %0
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* return stack_mem_p (operands[0]) ? \"push 0\" : \"clrmem %0\";
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clrmem %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="Predicates.html#Predicates" accesskey="n" rel="next">Predicates</a>, Previous: <a href="Output-Template.html#Output-Template" accesskey="p" rel="prev">Output Template</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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