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99 lines
3.9 KiB
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99 lines
3.9 KiB
HTML
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<title>Example - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Patterns.html#Patterns" title="Patterns">
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<!--
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Copyright (C) 1988-2015 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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<div class="node">
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<a name="Example"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="RTL-Template.html#RTL-Template">RTL Template</a>,
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Previous: <a rel="previous" accesskey="p" href="Patterns.html#Patterns">Patterns</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.3 Example of <code>define_insn</code></h3>
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<p><a name="index-g_t_0040code_007bdefine_005finsn_007d-example-3249"></a>
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Here is an example of an instruction pattern, taken from the machine
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description for the 68000/68020.
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<pre class="smallexample"> (define_insn "tstsi"
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[(set (cc0)
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(match_operand:SI 0 "general_operand" "rm"))]
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""
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"*
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{
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if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
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return \"tstl %0\";
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return \"cmpl #0,%0\";
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}")
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</pre>
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<p class="noindent">This can also be written using braced strings:
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<pre class="smallexample"> (define_insn "tstsi"
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[(set (cc0)
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(match_operand:SI 0 "general_operand" "rm"))]
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""
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{
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if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
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return "tstl %0";
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return "cmpl #0,%0";
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})
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</pre>
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<p>This describes an instruction which sets the condition codes based on the
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value of a general operand. It has no condition, so any insn with an RTL
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description of the form shown may be matched to this pattern. The name
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‘<samp><span class="samp">tstsi</span></samp>’ means “test a <code>SImode</code> value” and tells the RTL
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generation pass that, when it is necessary to test such a value, an insn
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to do so can be constructed using this pattern.
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<p>The output control string is a piece of C code which chooses which
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output template to return based on the kind of operand and the specific
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type of CPU for which code is being generated.
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<p>‘<samp><span class="samp">"rm"</span></samp>’ is an operand constraint. Its meaning is explained below.
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</body></html>
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