Index: llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp
+++ llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp
@@ -1288,6 +1288,7 @@
     // Note: Correctness depends on this being initialized to 0, which densemap
     // does
     unsigned long LowerBound;
+    const BasicBlock *LowerBoundBlock;
     // This is where the last walk for this memory location ended.
     unsigned long LastKill;
     bool LastKillValid;
@@ -1333,7 +1334,6 @@
       VersionStack.pop_back();
     ++PopEpoch;
   }
-
   for (MemoryAccess &MA : *Accesses) {
     auto *MU = dyn_cast<MemoryUse>(&MA);
     if (!MU) {
@@ -1355,13 +1355,24 @@
     if (LocInfo.PopEpoch != PopEpoch) {
       LocInfo.PopEpoch = PopEpoch;
       LocInfo.StackEpoch = StackEpoch;
-      // If the lower bound was in the info we popped, we have to reset it.
-      if (LocInfo.LowerBound >= VersionStack.size()) {
+      // If the lower bound was in something that no longer dominates us, we
+      // have to reset it.
+      // We can't simply track stack size, because the stack may have had
+      // pushes/pops in the meantime.
+      // XXX: This is non-optimal, but only is slower cases with heavily
+      // branching dominator trees.  To get the optimal number of queries would
+      // be to make lowerbound and lastkill a per-loc stack, and pop it until
+      // the top of that stack dominates us.  This does not seem worth it ATM.
+      // A much cheaper optimization would be to always explore the deepest
+      // branch of the dominator tree first. This will guarantee this resets on
+      // the smallest set of blocks.
+      if (LocInfo.LowerBoundBlock && LocInfo.LowerBoundBlock != BB &&
+          !DT->dominates(LocInfo.LowerBoundBlock, BB)){
         // Reset the lower bound of things to check.
         // TODO: Some day we should be able to reset to last kill, rather than
         // 0.
-
         LocInfo.LowerBound = 0;
+        LocInfo.LowerBoundBlock = VersionStack[0]->getBlock();
         LocInfo.LastKillValid = false;
       }
     } else if (LocInfo.StackEpoch != StackEpoch) {
@@ -1437,6 +1448,7 @@
       MU->setDefiningAccess(VersionStack[LocInfo.LastKill]);
     }
     LocInfo.LowerBound = VersionStack.size() - 1;
+    LocInfo.LowerBoundBlock = BB;
   }
 }
 
Index: llvm/trunk/test/Transforms/Util/MemorySSA/pr28880.ll
===================================================================
--- llvm/trunk/test/Transforms/Util/MemorySSA/pr28880.ll
+++ llvm/trunk/test/Transforms/Util/MemorySSA/pr28880.ll
@@ -0,0 +1,51 @@
+; RUN: opt -basicaa -print-memoryssa -verify-memoryssa -analyze < %s 2>&1 | FileCheck %s
+; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>,verify<memoryssa>' -disable-output < %s 2>&1 | FileCheck %s
+
+; This testcase is reduced from SingleSource/Benchmarks/Misc/fbench.c
+; It is testing to make sure that the MemorySSA use optimizer
+; comes up with right answers when dealing with multiple MemoryLocations
+; over different blocks. See PR28880 for more details.
+@global = external hidden unnamed_addr global double, align 8
+@global.1 = external hidden unnamed_addr global double, align 8
+
+; Function Attrs: nounwind ssp uwtable
+define hidden fastcc void @hoge() unnamed_addr #0 {
+bb:
+  br i1 undef, label %bb1, label %bb2
+
+bb1:                                              ; preds = %bb
+; These accesses should not conflict.
+; CHECK:  1 = MemoryDef(liveOnEntry)
+; 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT:   store double undef, double* @global, align 8
+  store double undef, double* @global, align 8
+; CHECK:  MemoryUse(liveOnEntry)
+; MemoryUse(liveOnEntry)
+; CHECK-NEXT:   %tmp = load double, double* @global.1, align 8
+  %tmp = load double, double* @global.1, align 8
+  unreachable
+
+bb2:                                              ; preds = %bb
+  br label %bb3
+
+bb3:                                              ; preds = %bb2
+  br i1 undef, label %bb4, label %bb6
+
+bb4:                                              ; preds = %bb3
+; These accesses should conflict.
+; CHECK:  2 = MemoryDef(liveOnEntry)
+; 2 = MemoryDef(liveOnEntry)
+; CHECK-NEXT:   store double 0.000000e+00, double* @global.1, align 8
+  store double 0.000000e+00, double* @global.1, align 8
+; CHECK:  MemoryUse(2)
+; MemoryUse(2)
+; CHECK-NEXT:   %tmp5 = load double, double* @global.1, align 8
+  %tmp5 = load double, double* @global.1, align 8
+  unreachable
+
+bb6:                                              ; preds = %bb3
+  unreachable
+}
+
+attributes #0 = { nounwind ssp uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+