[DivergenceAnalysis] fix a bug in computing influence regions

Fixes PR25864



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256036 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 19 insertions, 7 deletions

diff --git a/lib/Analysis/DivergenceAnalysis.cpp b/lib/Analysis/DivergenceAnalysis.cpp
index 93a288858ba..5ae6d74130a 100644
--- a/lib/Analysis/DivergenceAnalysis.cpp
+++ b/lib/Analysis/DivergenceAnalysis.cpp
@@ -96,7 +96,7 @@ private:
   // A helper function that explores sync dependents of TI.
   void exploreSyncDependency(TerminatorInst *TI);
   // Computes the influence region from Start to End. This region includes all
-  // basic blocks on any path from Start to End.
+  // basic blocks on any simple path from Start to End.
   void computeInfluenceRegion(BasicBlock *Start, BasicBlock *End,
                               DenseSet<BasicBlock *> &InfluenceRegion);
   // Finds all users of I that are outside the influence region, and add these
@@ -198,21 +198,33 @@ void DivergencePropagator::findUsersOutsideInfluenceRegion(
   }
 }
 
+// A helper function for computeInfluenceRegion that adds successors of "ThisBB"
+// to the influence region.
+static void
+addSuccessorsToInfluenceRegion(BasicBlock *ThisBB, BasicBlock *End,
+                               DenseSet<BasicBlock *> &InfluenceRegion,
+                               std::vector<BasicBlock *> &InfluenceStack) {
+  for (BasicBlock *Succ : successors(ThisBB)) {
+    if (Succ != End && InfluenceRegion.insert(Succ).second)
+      InfluenceStack.push_back(Succ);
+  }
+}
+
 void DivergencePropagator::computeInfluenceRegion(
     BasicBlock *Start, BasicBlock *End,
     DenseSet<BasicBlock *> &InfluenceRegion) {
   assert(PDT.properlyDominates(End, Start) &&
          "End does not properly dominate Start");
+
+  // The influence region starts from the end of "Start" to the beginning of
+  // "End". Therefore, "Start" should not be in the region unless "Start" is in
+  // a loop that doesn't contain "End".
   std::vector<BasicBlock *> InfluenceStack;
-  InfluenceStack.push_back(Start);
-  InfluenceRegion.insert(Start);
+  addSuccessorsToInfluenceRegion(Start, End, InfluenceRegion, InfluenceStack);
   while (!InfluenceStack.empty()) {
     BasicBlock *BB = InfluenceStack.back();
     InfluenceStack.pop_back();
-    for (BasicBlock *Succ : successors(BB)) {
-      if (End != Succ && InfluenceRegion.insert(Succ).second)
-        InfluenceStack.push_back(Succ);
-    }
+    addSuccessorsToInfluenceRegion(BB, End, InfluenceRegion, InfluenceStack);
   }
 }