Index: llvm/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/lib/Transforms/Utils/Local.cpp
+++ llvm/lib/Transforms/Utils/Local.cpp
@@ -2694,8 +2694,19 @@
                                         DominatorTree &DT,
                                         const BasicBlock *BB) {
   auto ProperlyDominates = [&DT](const BasicBlock *BB, const Use &U) {
-    auto *I = cast<Instruction>(U.getUser())->getParent();
-    return DT.properlyDominates(BB, I);
+    Instruction *UserInst = cast<Instruction>(U.getUser());
+    PHINode *PN = dyn_cast<PHINode>(UserInst);
+    // A phi use using a value from a block is dominated by the end of that
+    // block.  Note that the phi's parent block may not be.
+    if (PN && PN->getIncomingBlock(U) == BB)
+      return true;
+
+    const BasicBlock *UseBB;
+    if (PN)
+      UseBB = PN->getIncomingBlock(U);
+    else
+      UseBB = UserInst->getParent();
+    return DT.properlyDominates(BB, UseBB);
   };
   return ::replaceDominatedUsesWith(From, To, BB, ProperlyDominates);
 }
Index: llvm/test/Transforms/GVN/phi.ll
===================================================================
--- llvm/test/Transforms/GVN/phi.ll
+++ llvm/test/Transforms/GVN/phi.ll
@@ -39,7 +39,7 @@
 ; CHECK:       untaken:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ [[A]], [[TAKEN]] ], [ [[B:%.*]], [[UNTAKEN]] ]
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ 0, [[TAKEN]] ], [ [[B:%.*]], [[UNTAKEN]] ]
 ; CHECK-NEXT:    [[PHI2:%.*]] = phi i64 [ 0, [[TAKEN]] ], [ [[B]], [[UNTAKEN]] ]
 ; CHECK-NEXT:    [[RET:%.*]] = sub i64 [[PHI1]], [[PHI2]]
 ; CHECK-NEXT:    ret i64 [[RET]]
@@ -131,3 +131,80 @@
   %ret = sub i64 %phi4, %phi3
   ret i64 %ret
 }
+
+define i64 @test5(i1 %c, i64 %a) {
+; CHECK-LABEL: @test5(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
+; CHECK:       taken:
+; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       untaken:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    ret i64 0
+;
+  br i1 %c, label %taken, label %untaken
+taken:
+  %assumption = icmp eq i64 %a, 0
+  call void @llvm.assume(i1 %assumption)
+  br label %merge
+untaken:
+  br label %merge
+merge:
+  %phi = phi i64 [%a, %taken], [0, %untaken]
+  ret i64 %phi
+}
+
+define i64 @test6(i1 %c, i64 %a) {
+; CHECK-LABEL: @test6(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
+; CHECK:       taken:
+; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       untaken:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    ret i64 0
+;
+  br i1 %c, label %taken, label %untaken
+taken:
+  %assumption = icmp eq i64 %a, 0
+  call void @llvm.assume(i1 %assumption)
+  br label %next
+next:
+  br label %merge
+untaken:
+  br label %merge
+merge:
+  %phi = phi i64 [%a, %next], [0, %untaken]
+  ret i64 %phi
+}
+
+; negative test, phi use is NOT dominated by assume
+define i64 @test7(i1 %c, i64 %a) {
+; CHECK-LABEL: @test7(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[TAKEN:%.*]], label [[UNTAKEN:%.*]]
+; CHECK:       taken:
+; CHECK-NEXT:    [[ASSUMPTION:%.*]] = icmp eq i64 [[A:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ASSUMPTION]])
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       untaken:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    ret i64 [[A]]
+;
+  br i1 %c, label %taken, label %untaken
+taken:
+  %assumption = icmp eq i64 %a, 0
+  call void @llvm.assume(i1 %assumption)
+  br label %merge
+untaken:
+  br label %merge
+merge:
+  br label %next
+next:
+  %phi = phi i64 [%a, %merge]
+  ret i64 %phi
+}