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
diff --git a/polly/lib/Analysis/ScopDetection.cpp b/polly/lib/Analysis/ScopDetection.cpp
index d672130..7d2dcbe 100644
--- a/polly/lib/Analysis/ScopDetection.cpp
+++ b/polly/lib/Analysis/ScopDetection.cpp
@@ -309,6 +309,35 @@
// 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))