LCSSA should be performed on the outermost affected loop while unrolling loop.

During loop-unroll, loop exits from the current loop may end up in in different
outer loop. This requires to re-form LCSSA recursively for one level down from
the outer most loop where loop exits are landed during unroll. This fixes PR18861.

Differential Revision: http://reviews.llvm.org/D2976

llvm-svn: 209796

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index d953e307310..16975b9e637 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -487,6 +487,15 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
     if (OuterL) {
       ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>();
       simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ nullptr, SE);
+
+      // LCSSA must be performed on the outermost affected loop. The unrolled
+      // loop's last loop latch is guaranteed to be in the outermost loop after
+      // deleteLoopFromQueue updates LoopInfo.
+      Loop *LatchLoop = LI->getLoopFor(Latches.back());
+      if (!OuterL->contains(LatchLoop))
+        while (OuterL->getParentLoop() != LatchLoop)
+          OuterL = OuterL->getParentLoop();
+
       formLCSSARecursively(*OuterL, *DT, SE);
     }
   }