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<h5 class="subsubsection">16.23.1.2 Substitution in Mode Iterators</h5>
<p><a name="index-define_005fmode_005fattr-3839"></a>
If an <samp><span class="file">.md</span></samp> file construct uses mode iterators, each version of the
construct will often need slightly different strings or modes. For
example:
<ul>
<li>When a <code>define_expand</code> defines several <code>add</code><var>m</var><code>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>When a <code>define_insn</code> defines several instruction patterns,
each instruction will often use a different assembler mnemonic.
<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).
</ul>
<p>GCC supports such variations through a system of “mode attributes”.
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:
<pre class="smallexample"> (define_mode_attr <var>name</var> [(<var>mode1</var> "<var>value1</var>") ... (<var>moden</var> "<var>valuen</var>")])
</pre>
<p>where <var>name</var> is the name of the attribute and <var>valuei</var>
is the value associated with <var>modei</var>.
<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><</code><var>iterator</var><code>:</code><var>attr</var><code>></code>, where <var>attr</var> is the name of a
mode attribute. If the attribute is defined for <var>mode</var>, the whole
<code><...></code> sequence will be replaced by the appropriate attribute
value.
<p>For example, suppose an <samp><span class="file">.md</span></samp> file has:
<pre class="smallexample"> (define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
(define_mode_attr load [(SI "lw") (DI "ld")])
</pre>
<p>If one of the patterns that uses <code>:P</code> contains the string
<code>"<P:load>\t%0,%1"</code>, the <code>SI</code> version of that pattern
will use <code>"lw\t%0,%1"</code> and the <code>DI</code> version will use
<code>"ld\t%0,%1"</code>.
<p>Here is an example of using an attribute for a mode:
<pre class="smallexample"> (define_mode_iterator LONG [SI DI])
(define_mode_attr SHORT [(SI "HI") (DI "SI")])
(define_insn ...
(sign_extend:LONG (match_operand:<LONG:SHORT> ...)) ...)
</pre>
<p>The <var>iterator</var><code>:</code> prefix may be omitted, in which case the
substitution will be attempted for every iterator expansion.
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