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114 lines
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114 lines
5.1 KiB
HTML
4 years ago
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<html lang="en">
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<title>Substitutions - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Mode-Iterators.html#Mode-Iterators" title="Mode Iterators">
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<link rel="prev" href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" title="Defining Mode Iterators">
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<link rel="next" href="Examples.html#Examples" title="Examples">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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(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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</head>
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<body>
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<div class="node">
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<a name="Substitutions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Examples.html#Examples">Examples</a>,
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Previous: <a rel="previous" accesskey="p" href="Defining-Mode-Iterators.html#Defining-Mode-Iterators">Defining Mode Iterators</a>,
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Up: <a rel="up" accesskey="u" href="Mode-Iterators.html#Mode-Iterators">Mode Iterators</a>
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<hr>
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</div>
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<h5 class="subsubsection">16.23.1.2 Substitution in Mode Iterators</h5>
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<p><a name="index-define_005fmode_005fattr-3839"></a>
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If an <samp><span class="file">.md</span></samp> file construct uses mode iterators, each version of the
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construct will often need slightly different strings or modes. For
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example:
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<ul>
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<li>When a <code>define_expand</code> defines several <code>add</code><var>m</var><code>3</code> patterns
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(see <a href="Standard-Names.html#Standard-Names">Standard Names</a>), each expander will need to use the
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appropriate mode name for <var>m</var>.
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<li>When a <code>define_insn</code> defines several instruction patterns,
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each instruction will often use a different assembler mnemonic.
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<li>When a <code>define_insn</code> requires operands with different modes,
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using an iterator for one of the operand modes usually requires a specific
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mode for the other operand(s).
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</ul>
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<p>GCC supports such variations through a system of “mode attributes”.
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There are two standard attributes: <code>mode</code>, which is the name of
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the mode in lower case, and <code>MODE</code>, which is the same thing in
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upper case. You can define other attributes using:
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<pre class="smallexample"> (define_mode_attr <var>name</var> [(<var>mode1</var> "<var>value1</var>") ... (<var>moden</var> "<var>valuen</var>")])
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</pre>
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<p>where <var>name</var> is the name of the attribute and <var>valuei</var>
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is the value associated with <var>modei</var>.
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<p>When GCC replaces some <var>:iterator</var> with <var>:mode</var>, it will scan
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each string and mode in the pattern for sequences of the form
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<code><</code><var>iterator</var><code>:</code><var>attr</var><code>></code>, where <var>attr</var> is the name of a
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mode attribute. If the attribute is defined for <var>mode</var>, the whole
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<code><...></code> sequence will be replaced by the appropriate attribute
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value.
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<p>For example, suppose an <samp><span class="file">.md</span></samp> file has:
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<pre class="smallexample"> (define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
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(define_mode_attr load [(SI "lw") (DI "ld")])
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</pre>
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<p>If one of the patterns that uses <code>:P</code> contains the string
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<code>"<P:load>\t%0,%1"</code>, the <code>SI</code> version of that pattern
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will use <code>"lw\t%0,%1"</code> and the <code>DI</code> version will use
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<code>"ld\t%0,%1"</code>.
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<p>Here is an example of using an attribute for a mode:
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<pre class="smallexample"> (define_mode_iterator LONG [SI DI])
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(define_mode_attr SHORT [(SI "HI") (DI "SI")])
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(define_insn ...
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(sign_extend:LONG (match_operand:<LONG:SHORT> ...)) ...)
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</pre>
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<p>The <var>iterator</var><code>:</code> prefix may be omitted, in which case the
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substitution will be attempted for every iterator expansion.
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</body></html>
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