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
@@ -3734,9 +3734,13 @@
                     << *BB);
 
   // If there are any extra values that couldn't be folded into the switch
-  // then we evaluate them with an explicit branch first.  Split the block
+  // then we evaluate them with an explicit branch first. Split the block
   // right before the condbr to handle it.
   if (ExtraCase) {
+    // MSAN don't like undefs as branch condition which can be introduced
+    // with "explicit branch".
+    if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
+      return false;
     BasicBlock *NewBB =
         BB->splitBasicBlock(BI->getIterator(), "switch.early.test");
     // Remove the uncond branch added to the old block.
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_msan.ll b/llvm/test/Transforms/SimplifyCFG/switch_msan.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/switch_msan.ll
@@ -0,0 +1,75 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+
+declare i8 @next_char();
+
+define void @test_no_msan() {
+; CHECK-LABEL: define void @test_no_msan()
+
+entry:
+  br label %while.body
+
+while.body:
+  br label %while.body.i
+
+while.body.i:
+; CHECK-LABEL: while.body.i:
+  %maybe_undef = phi i1 [ undef, %while.body ], [ %next_maybe_undef, %while.body.i ]
+  %c = call fastcc signext i8 @next_char()
+  %c_10 = icmp eq i8 %c, 10
+  %c_13 = icmp eq i8 %c, 13
+  %c_10_or_13 = or i1 %c_10, %c_13
+  %next_maybe_undef = or i1 %maybe_undef, %c_10_or_13
+  %c_not_10_or_13 = xor i1 %c_10_or_13, true
+  br i1 %c_not_10_or_13, label %while.body.i, label %while.body.i.break
+  ; CHECK: br i1 %c_not_10_or_13, label %while.body.i, label %while.body.i.break
+
+while.body.i.break:
+; CHECK-LABEL: while.body.i.break:
+  ; NEXT_MAYBE_UNDEF is never undef if here
+  br i1 %next_maybe_undef, label %while.body, label %return
+  ; CHECK: br i1 %maybe_undef, label %while.body, label %switch.early.test
+
+; CHECK-LABEL: switch.early.test:
+; CHECK: switch i8 %c, label %return [
+; CHECK:   i8 13, label %while.body
+; CHECK:   i8 10, label %while.body
+; CHECK: ]
+
+return:
+  ret void
+}
+
+
+define void @test_msan() sanitize_memory {
+; CHECK-LABEL: define void @test_msan()
+
+entry:
+  br label %while.body
+
+while.body:
+  br label %while.body.i
+
+while.body.i:
+; CHECK-LABEL: while.body.i:
+  %maybe_undef = phi i1 [ undef, %while.body ], [ %next_maybe_undef, %while.body.i ]
+  %c = call fastcc signext i8 @next_char()
+  %c_10 = icmp eq i8 %c, 10
+  %c_13 = icmp eq i8 %c, 13
+  %c_10_or_13 = or i1 %c_10, %c_13
+  %next_maybe_undef = or i1 %maybe_undef, %c_10_or_13
+  %c_not_10_or_13 = xor i1 %c_10_or_13, true
+  br i1 %c_not_10_or_13, label %while.body.i, label %while.body.i.break
+  ; CHECK: br i1 %c_not_10_or_13, label %while.body.i, label %while.body.i.break
+
+while.body.i.break:
+; CHECK-LABEL: while.body.i.break:
+  ; NEXT_MAYBE_UNDEF is never undef if here
+  br i1 %next_maybe_undef, label %while.body, label %return
+  ; CHECK: br i1 %next_maybe_undef, label %while.body, label %return
+
+; CHECK-NOT: switch.early.test:
+
+return:
+  ret void
+}