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<a name="Substitutions"></a>
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<a name="Substitution-in-Mode-Iterators"></a>
<h4 class="subsubsection">17.23.1.2 Substitution in Mode Iterators</h4>
<a name="index-define_005fmode_005fattr"></a>

<p>If an <samp>.md</samp> file construct uses mode iterators, each version of the
construct will often need slightly different strings or modes.  For
example:
</p>
<ul>
<li> When a <code>define_expand</code> defines several <code>add<var>m</var>3</code> patterns
(see <a href="Standard-Names.html#Standard-Names">Standard Names</a>), each expander will need to use the
appropriate mode name for <var>m</var>.

</li><li> When a <code>define_insn</code> defines several instruction patterns,
each instruction will often use a different assembler mnemonic.

</li><li> When a <code>define_insn</code> requires operands with different modes,
using an iterator for one of the operand modes usually requires a specific
mode for the other operand(s).
</li></ul>

<p>GCC supports such variations through a system of &ldquo;mode attributes&rdquo;.
There are two standard attributes: <code>mode</code>, which is the name of
the mode in lower case, and <code>MODE</code>, which is the same thing in
upper case.  You can define other attributes using:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_attr <var>name</var> [(<var>mode1</var> &quot;<var>value1</var>&quot;) &hellip; (<var>moden</var> &quot;<var>valuen</var>&quot;)])
</pre></div>

<p>where <var>name</var> is the name of the attribute and <var>valuei</var>
is the value associated with <var>modei</var>.
</p>
<p>When GCC replaces some <var>:iterator</var> with <var>:mode</var>, it will scan
each string and mode in the pattern for sequences of the form
<code>&lt;<var>iterator</var>:<var>attr</var>&gt;</code>, where <var>attr</var> is the name of a
mode attribute.  If the attribute is defined for <var>mode</var>, the whole
<code>&lt;&hellip;&gt;</code> sequence will be replaced by the appropriate attribute
value.
</p>
<p>For example, suppose an <samp>.md</samp> file has:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_iterator P [(SI &quot;Pmode == SImode&quot;) (DI &quot;Pmode == DImode&quot;)])
(define_mode_attr load [(SI &quot;lw&quot;) (DI &quot;ld&quot;)])
</pre></div>

<p>If one of the patterns that uses <code>:P</code> contains the string
<code>&quot;&lt;P:load&gt;\t%0,%1&quot;</code>, the <code>SI</code> version of that pattern
will use <code>&quot;lw\t%0,%1&quot;</code> and the <code>DI</code> version will use
<code>&quot;ld\t%0,%1&quot;</code>.
</p>
<p>Here is an example of using an attribute for a mode:
</p>
<div class="smallexample">
<pre class="smallexample">(define_mode_iterator LONG [SI DI])
(define_mode_attr SHORT [(SI &quot;HI&quot;) (DI &quot;SI&quot;)])
(define_insn &hellip;
  (sign_extend:LONG (match_operand:&lt;LONG:SHORT&gt; &hellip;)) &hellip;)
</pre></div>

<p>The <code><var>iterator</var>:</code> prefix may be omitted, in which case the
substitution will be attempted for every iterator expansion.
</p>
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