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<title>GNU Compiler Collection (GCC) Internals: Disable Insn Alternatives</title>

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<a name="Disable-Insn-Alternatives"></a>
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<hr>
<a name="Disable-insn-alternatives-using-the-enabled-attribute"></a>
<h4 class="subsection">16.8.6 Disable insn alternatives using the <code>enabled</code> attribute</h4>
<a name="index-enabled"></a>

<p>The <code>enabled</code> insn attribute may be used to disable certain insn
alternatives for machine-specific reasons.  This is useful when adding
new instructions to an existing pattern which are only available for
certain cpu architecture levels as specified with the <code>-march=</code>
option.
</p>
<p>If an insn alternative is disabled, then it will never be used.  The
compiler treats the constraints for the disabled alternative as
unsatisfiable.
</p>
<p>In order to make use of the <code>enabled</code> attribute a back end has to add
in the machine description files:
</p>
<ol>
<li> A definition of the <code>enabled</code> insn attribute.  The attribute is
defined as usual using the <code>define_attr</code> command.  This
definition should be based on other insn attributes and/or target flags.
The <code>enabled</code> attribute is a numeric attribute and should evaluate to
<code>(const_int 1)</code> for an enabled alternative and to
<code>(const_int 0)</code> otherwise.
</li><li> A definition of another insn attribute used to describe for what
reason an insn alternative might be available or
not.  E.g. <code>cpu_facility</code> as in the example below.
</li><li> An assignment for the second attribute to each insn definition
combining instructions which are not all available under the same
circumstances.  (Note: It obviously only makes sense for definitions
with more than one alternative.  Otherwise the insn pattern should be
disabled or enabled using the insn condition.)
</li></ol>

<p>E.g. the following two patterns could easily be merged using the <code>enabled</code>
attribute:
</p>
<div class="smallexample">
<pre class="smallexample">
(define_insn &quot;*movdi_old&quot;
  [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d&quot;)
        (match_operand:DI 1 &quot;register_operand&quot; &quot; d&quot;))]
  &quot;!TARGET_NEW&quot;
  &quot;lgr %0,%1&quot;)

(define_insn &quot;*movdi_new&quot;
  [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d,f,d&quot;)
        (match_operand:DI 1 &quot;register_operand&quot; &quot; d,d,f&quot;))]
  &quot;TARGET_NEW&quot;
  &quot;@
   lgr  %0,%1
   ldgr %0,%1
   lgdr %0,%1&quot;)

</pre></div>

<p>to:
</p>
<div class="smallexample">
<pre class="smallexample">
(define_insn &quot;*movdi_combined&quot;
  [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d,f,d&quot;)
        (match_operand:DI 1 &quot;register_operand&quot; &quot; d,d,f&quot;))]
  &quot;&quot;
  &quot;@
   lgr  %0,%1
   ldgr %0,%1
   lgdr %0,%1&quot;
  [(set_attr &quot;cpu_facility&quot; &quot;*,new,new&quot;)])

</pre></div>

<p>with the <code>enabled</code> attribute defined like this:
</p>
<div class="smallexample">
<pre class="smallexample">
(define_attr &quot;cpu_facility&quot; &quot;standard,new&quot; (const_string &quot;standard&quot;))

(define_attr &quot;enabled&quot; &quot;&quot;
  (cond [(eq_attr &quot;cpu_facility&quot; &quot;standard&quot;) (const_int 1)
         (and (eq_attr &quot;cpu_facility&quot; &quot;new&quot;)
              (ne (symbol_ref &quot;TARGET_NEW&quot;) (const_int 0)))
         (const_int 1)]
        (const_int 0)))

</pre></div>


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