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<a name="Temporaries"></a>
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<a name="Temporaries-1"></a>
<h3 class="section">11.4 Temporaries</h3>
<a name="index-Temporaries"></a>

<p>When gimplification encounters a subexpression that is too
complex, it creates a new temporary variable to hold the value of
the subexpression, and adds a new statement to initialize it
before the current statement. These special temporaries are known
as &lsquo;<samp>expression temporaries</samp>&rsquo;, and are allocated using
<code>get_formal_tmp_var</code>.  The compiler tries to always evaluate
identical expressions into the same temporary, to simplify
elimination of redundant calculations.
</p>
<p>We can only use expression temporaries when we know that it will
not be reevaluated before its value is used, and that it will not
be otherwise modified<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>. Other temporaries can be allocated
using <code>get_initialized_tmp_var</code> or <code>create_tmp_var</code>.
</p>
<p>Currently, an expression like <code>a = b + 5</code> is not reduced any
further.  We tried converting it to something like
</p><div class="smallexample">
<pre class="smallexample">T1 = b + 5;
a = T1;
</pre></div>
<p>but this bloated the representation for minimal benefit.  However, a
variable which must live in memory cannot appear in an expression; its
value is explicitly loaded into a temporary first.  Similarly, storing
the value of an expression to a memory variable goes through a
temporary.
</p>
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<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
<p>These restrictions are derived
from those in Morgan 4.8.</p>
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