151 lines
6.9 KiB
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151 lines
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HTML
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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: Substitutions</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Substitutions">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Substitutions">
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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="Mode-Iterators.html#Mode-Iterators" rel="up" title="Mode Iterators">
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<link href="Examples.html#Examples" rel="next" title="Examples">
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<link href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" rel="prev" title="Defining Mode Iterators">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Substitutions"></a>
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<div class="header">
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<p>
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Next: <a href="Examples.html#Examples" accesskey="n" rel="next">Examples</a>, Previous: <a href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" accesskey="p" rel="prev">Defining Mode Iterators</a>, Up: <a href="Mode-Iterators.html#Mode-Iterators" accesskey="u" rel="up">Mode Iterators</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="Substitution-in-Mode-Iterators"></a>
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<h4 class="subsubsection">16.23.1.2 Substitution in Mode Iterators</h4>
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<a name="index-define_005fmode_005fattr"></a>
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<p>If an <samp>.md</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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</p>
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<ul>
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<li> When a <code>define_expand</code> defines several <code>add<var>m</var>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><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><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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</li></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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</p>
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<div class="smallexample">
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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></div>
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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>
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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><<var>iterator</var>:<var>attr</var>></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>
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<p>For example, suppose an <samp>.md</samp> file has:
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</p>
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<div class="smallexample">
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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></div>
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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>
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<p>Here is an example of using an attribute for a mode:
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</p>
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<div class="smallexample">
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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></div>
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<p>The <code><var>iterator</var>:</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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</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="Examples.html#Examples" accesskey="n" rel="next">Examples</a>, Previous: <a href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" accesskey="p" rel="prev">Defining Mode Iterators</a>, Up: <a href="Mode-Iterators.html#Mode-Iterators" accesskey="u" rel="up">Mode Iterators</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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