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Add comment on why loops/regions can overlap. NFC. 

The case is described in llvm.org/PR28071 which was fixed in the
previous commit.

llvm-svn: 273906
This commit is contained in:
Michael Kruse 2016-06-27 19:00:55 +00:00
parent 41f046a282
commit a1a303f31e
1 changed files with 29 additions and 0 deletions
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29
polly/lib/Analysis/ScopDetection.cpp
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@ -309,6 +309,35 @@ bool ScopDetection::addOverApproximatedRegion(Region *AR,
// Reject if the surrounding loop does not entirely contain the nonaffine
// subregion.
// This can happen because a region can contain BBs that have no path to the
// exit block (Infinite loops, UnreachableInst), but such blocks are never
// part of a loop.
//
// _______________
// | Loop Header | <-----------.
// --------------- |
// | |
// _______________ ______________
// | RegionEntry |-----> | RegionExit |----->
// --------------- --------------
// |
// _______________
// | EndlessLoop | <--.
// --------------- |
// | |
// \------------/
//
// In the example above, the loop (LoopHeader,RegionEntry,RegionExit) is
// neither entirely contained in the region RegionEntry->RegionExit
// (containing RegionEntry,EndlessLoop) nor is the region entirely contained
// in the loop.
// The block EndlessLoop is contained is in the region because
// Region::contains tests whether it is not dominated by RegionExit. This is
// probably to not having to query the PostdominatorTree.
// Instead of an endless loop, a dead end can also be formed by
// UnreachableInst. This case is already caught by isErrorBlock(). We hence
// only have to test whether there is an endless loop not contained in the
// surrounding loop.
BasicBlock *BBEntry = AR->getEntry();
Loop *L = LI->getLoopFor(BBEntry);
while (L && AR->contains(L))