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<H2><A NAME="SECTION00915000000000000000"></A><A NAME="4072"></A>
<BR>
8.1.5 Loop Reversing
</H2>

<P>
This optimization is done to reduce the overhead of checking loop
boundaries for every iteration. Some simple loops can be reversed
and implemented using a ``decrement and jump if not zero'' instruction.
SDCC checks for the following criterion to determine if a loop is
reversible (note: more sophisticated compilers use data-dependency
analysis to make this determination, SDCC uses a more simple minded
analysis).

<UL>
<LI>The 'for' loop is of the form 
<BR>
<BR><TT>for(&lt;symbol&gt; = &lt;expression&gt;; &lt;sym&gt; [&lt; | &lt;=] &lt;expression&gt;;
[&lt;sym&gt;++ | &lt;sym&gt; += 1])</TT>&nbsp;
<BR><TT>&nbsp;&nbsp;&nbsp;&nbsp;&lt;for body&gt;</TT>
</LI>
<LI>The &lt;for body&gt; does not contain ``continue'' or 'break''.
</LI>
<LI>All goto's are contained within the loop.
</LI>
<LI>No function calls within the loop.
</LI>
<LI>The loop control variable &lt;sym&gt; is not assigned any value within the
loop
</LI>
<LI>The loop control variable does NOT participate in any arithmetic operation
within the loop.
</LI>
<LI>There are NO switch statements in the loop.
</LI>
</UL>

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<ADDRESS>

2011-03-20
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