http://llvm.org/bugs/show_bug.cgi?id=15596
Bug ID: 15596
Summary: LoopSimplify inserts backedge block in wrong spot
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
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.
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