[BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInformation

Summary:
We are incorrectly updating the LI when loop-simplify generates
dedicated exit blocks for a loop. The issue is that there's an implicit
assumption that the Preds passed into UpdateAnalysisInformation are
reachable. However, this is not true and breaks LI by incorrectly
updating the header of a loop.

One such case is when we generate dedicated exits when the exit block is
a landing pad (through SplitLandingPadPredecessors). There maybe other
cases as well, since we do not guarantee that Preds passed in are
reachable basic blocks.

The added test case shows how loop-simplify breaks LI for the outer loop (and DT in turn)
after we try to generate the LoopSimplifyForm.

Reviewers: davide, chandlerc, sanjoy

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41519

llvm-svn: 321653

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 5f04f14518d..8f59913e14b 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -323,6 +323,12 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
   bool IsLoopEntry = !!L;
   bool SplitMakesNewLoopHeader = false;
   for (BasicBlock *Pred : Preds) {
+    // Preds that are not reachable from entry should not be used to identify if
+    // OldBB is a loop entry or if SplitMakesNewLoopHeader. Unreachable blocks
+    // are not within any loops, so we incorrectly mark SplitMakesNewLoopHeader
+    // as true and make the NewBB the header of some loop. This breaks LI.
+    if (!DT->isReachableFromEntry(Pred))
+      continue;
     // If we need to preserve LCSSA, determine if any of the preds is a loop
     // exit.
     if (PreserveLCSSA)