Index: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3729,12 +3729,17 @@
 
   BasicBlock *BB = BI->getParent();
 
+  // MSAN does not like undefs as branch condition which can be introduced
+  // with "explicit branch".
+  if (ExtraCase && BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
+    return false;
+
   LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
                     << " cases into SWITCH.  BB is:\n"
                     << *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) {
     BasicBlock *NewBB =
Index: llvm/trunk/test/Transforms/SimplifyCFG/switch_msan.ll
===================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/switch_msan.ll
+++ llvm/trunk/test/Transforms/SimplifyCFG/switch_msan.ll
@@ -0,0 +1,102 @@
+; 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: @test_no_msan(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
+; CHECK:       while.body:
+; CHECK-NEXT:    br label [[WHILE_BODY_I:%.*]]
+; CHECK:       while.body.i:
+; CHECK-NEXT:    [[MAYBE_UNDEF:%.*]] = phi i1 [ undef, [[WHILE_BODY]] ], [ [[NEXT_MAYBE_UNDEF:%.*]], [[WHILE_BODY_I]] ]
+; CHECK-NEXT:    [[C:%.*]] = call fastcc signext i8 @next_char()
+; CHECK-NEXT:    [[C_10:%.*]] = icmp eq i8 [[C]], 10
+; CHECK-NEXT:    [[C_13:%.*]] = icmp eq i8 [[C]], 13
+; CHECK-NEXT:    [[C_10_OR_13:%.*]] = or i1 [[C_10]], [[C_13]]
+; CHECK-NEXT:    [[NEXT_MAYBE_UNDEF]] = or i1 [[MAYBE_UNDEF]], [[C_10_OR_13]]
+; CHECK-NEXT:    [[C_NOT_10_OR_13:%.*]] = xor i1 [[C_10_OR_13]], true
+; CHECK-NEXT:    br i1 [[C_NOT_10_OR_13]], label [[WHILE_BODY_I]], label [[WHILE_BODY_I_BREAK:%.*]]
+; CHECK:       while.body.i.break:
+; CHECK-NEXT:    br i1 [[MAYBE_UNDEF]], label [[WHILE_BODY]], label [[SWITCH_EARLY_TEST:%.*]]
+; CHECK:       switch.early.test:
+; CHECK-NEXT:    switch i8 [[C]], label [[RETURN:%.*]] [
+; CHECK-NEXT:    i8 13, label [[WHILE_BODY]]
+; CHECK-NEXT:    i8 10, label [[WHILE_BODY]]
+; CHECK-NEXT:    ]
+; CHECK:       return:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  br label %while.body
+
+while.body:
+  br label %while.body.i
+
+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
+
+while.body.i.break:
+  ; NEXT_MAYBE_UNDEF is never undef if here
+  br i1 %next_maybe_undef, label %while.body, label %return
+
+
+return:
+  ret void
+}
+
+
+define void @test_msan() sanitize_memory {
+; CHECK-LABEL: @test_msan(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
+; CHECK:       while.body:
+; CHECK-NEXT:    br label [[WHILE_BODY_I:%.*]]
+; CHECK:       while.body.i:
+; CHECK-NEXT:    [[MAYBE_UNDEF:%.*]] = phi i1 [ undef, [[WHILE_BODY]] ], [ [[NEXT_MAYBE_UNDEF:%.*]], [[WHILE_BODY_I]] ]
+; CHECK-NEXT:    [[C:%.*]] = call fastcc signext i8 @next_char()
+; CHECK-NEXT:    [[C_10:%.*]] = icmp eq i8 [[C]], 10
+; CHECK-NEXT:    [[C_13:%.*]] = icmp eq i8 [[C]], 13
+; CHECK-NEXT:    [[C_10_OR_13:%.*]] = or i1 [[C_10]], [[C_13]]
+; CHECK-NEXT:    [[NEXT_MAYBE_UNDEF]] = or i1 [[MAYBE_UNDEF]], [[C_10_OR_13]]
+; CHECK-NEXT:    [[C_NOT_10_OR_13:%.*]] = xor i1 [[C_10_OR_13]], true
+; CHECK-NEXT:    br i1 [[C_NOT_10_OR_13]], label [[WHILE_BODY_I]], label [[WHILE_BODY_I_BREAK:%.*]]
+; CHECK:       while.body.i.break:
+; CHECK-NEXT:    br i1 [[NEXT_MAYBE_UNDEF]], label [[WHILE_BODY]], label [[RETURN:%.*]]
+; CHECK:       return:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  br label %while.body
+
+while.body:
+  br label %while.body.i
+
+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
+
+while.body.i.break:
+  ; NEXT_MAYBE_UNDEF is never undef if here
+  br i1 %next_maybe_undef, label %while.body, label %return
+
+
+return:
+  ret void
+}