Index: llvm/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/lib/Transforms/Utils/Local.cpp
+++ llvm/lib/Transforms/Utils/Local.cpp
@@ -181,6 +181,12 @@
       return true;
     }
 
+    if (isa<UndefValue>(BI->getCondition())) {
+      // Branch on undef is undefined behavior.
+      changeToUnreachable(BI, /*PreserveLCSSA*/ false, DTU);
+      return true;
+    }
+
     return false;
   }
 
Index: llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
+++ llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
@@ -6,15 +6,7 @@
 define void @br_undef_simple() {
 ; CHECK-LABEL: @br_undef_simple(
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 undef, label [[IF:%.*]], label [[ELSE:%.*]]
-; CHECK:       common.ret:
-; CHECK-NEXT:    ret void
-; CHECK:       if:
-; CHECK-NEXT:    call void @foo(i32 1)
-; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
-; CHECK:       else:
-; CHECK-NEXT:    call void @foo(i32 2)
-; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK-NEXT:    unreachable
 ;
   call void @foo(i32 0)
   br i1 undef, label %if, label %else
@@ -31,15 +23,7 @@
 define void @br_poison_simple() {
 ; CHECK-LABEL: @br_poison_simple(
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 poison, label [[IF:%.*]], label [[ELSE:%.*]]
-; CHECK:       common.ret:
-; CHECK-NEXT:    ret void
-; CHECK:       if:
-; CHECK-NEXT:    call void @foo(i32 1)
-; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
-; CHECK:       else:
-; CHECK-NEXT:    call void @foo(i32 2)
-; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK-NEXT:    unreachable
 ;
   call void @foo(i32 0)
   br i1 poison, label %if, label %else
@@ -62,16 +46,14 @@
 ; CHECK-NEXT:    ]
 ; CHECK:       br:
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 poison, label [[IF]], label [[ELSE]]
+; CHECK-NEXT:    unreachable
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void
 ; CHECK:       if:
-; CHECK-NEXT:    [[PHI1:%.*]] = phi i32 [ 1, [[BR]] ], [ 2, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    call void @foo(i32 [[PHI1]])
+; CHECK-NEXT:    call void @foo(i32 2)
 ; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
 ; CHECK:       else:
-; CHECK-NEXT:    [[PHI2:%.*]] = phi i32 [ 3, [[BR]] ], [ 4, [[ENTRY]] ]
-; CHECK-NEXT:    call void @foo(i32 [[PHI2]])
+; CHECK-NEXT:    call void @foo(i32 4)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
 ;
 entry: