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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LoopSimplify inserts backedge block in wrong spot"
href="http://llvm.org/bugs/show_bug.cgi?id=15596">15596</a>
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<th>Summary</th>
<td>LoopSimplify inserts backedge block in wrong spot
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>tim.besard@elis.ugent.be
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>When a loop contains multiple backedges, LoopSimplify inserts a new backedge in
order to ensure that all loops have exactly one backedge
(LoopSimplify::InsertUniqueBackedgeBlock). When inserting the newly created
back-edge, the comments detail that the block is to be inserted right after the
last backedge block. This is implemented by inserting before
"BackedgeBlocks.back()+1".
However, BackedgeBlocks is filled by a loop which iterates over the
predecessors of the loop header. As it seems, pred_iterator does not iterate
the predecessors in order of occurrence in the program, but (as far as I see)
the other way around: it starts with the last predecessor, and ends up with the
first one.
This means that "BackedgeBlocks.back()+1" actually points to right after the
first backedge block, rather than the last one.
I don't know whether pred_iterator is supposed to iterate the predecessors from
last predecessor to the first one, but if it is I think it would suffice to
change "InsertPos = BackedgeBlocks.back()" to "BackedgeBlocks.front()" in order
to get the documented behaviour.</pre>
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</p>
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