diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6598,9 +6598,17 @@
           // destination from conditional branches.
           if (BI->isUnconditional())
             Builder.CreateUnreachable();
-          else
+          else {
+            // Preserve guarding condition in assume, because it might not be
+            // inferrable from any dominating condition.
+            Value *Cond = BI->getCondition();
+            if (BI->getSuccessor(0) == BB)
+              Builder.CreateAssumption(Builder.CreateNot(Cond));
+            else
+              Builder.CreateAssumption(Cond);
             Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
                                                        : BI->getSuccessor(0));
+          }
           BI->eraseFromParent();
           if (DTU)
             DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});
diff --git a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
--- a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
+++ b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
@@ -14,8 +14,12 @@
 ; CHECK-NEXT:    unreachable
 ; CHECK:       bb21:
 ; CHECK-NEXT:    [[I:%.*]] = icmp eq %0* [[ARG17:%.*]], null
+; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[I]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
 ; CHECK-NEXT:    call void @hoge()
-; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0:%.*]], %0* [[ARG19:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[ARG16:%.*]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP1]])
+; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0]], %0* [[ARG19:%.*]], i64 0, i32 0
 ; CHECK-NEXT:    [[I28:%.*]] = load %1*, %1** [[I27]], align 8
 ; CHECK-NEXT:    call void @pluto.1(%1* [[I28]])
 ; CHECK-NEXT:    call void @pluto()