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- <a name="Temporaries"></a>
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- <a name="Temporaries-1"></a>
- <h3 class="section">12.5 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 ‘<samp>expression temporaries</samp>’, 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.
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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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