Index: lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- lib/Transforms/Vectorize/LoopVectorize.cpp
+++ lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5049,13 +5049,13 @@
   bool Result = true;
   
   bool DoExtraAnalysis = ORE->allowExtraAnalysis(DEBUG_TYPE);
-  if (DoExtraAnalysis)
   // We must have a loop in canonical form. Loops with indirectbr in them cannot
   // be canonicalized.
   if (!TheLoop->getLoopPreheader()) {
+    DEBUG(dbgs() << "LV: Loop doesn't have a legal pre-header.\n");
     ORE->emit(createMissedAnalysis("CFGNotUnderstood")
               << "loop control flow is not understood by vectorizer");
-  if (DoExtraAnalysis)
+    if (DoExtraAnalysis)
       Result = false;
     else
       return false;
Index: test/Transforms/LoopVectorize/legal_preheader_check.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopVectorize/legal_preheader_check.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -loop-vectorize -debug -S -o /dev/null 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+; D40973
+; Make sure LV legal bails out when the loop doesn't have a legal pre-header.
+
+; CHECK: LV: Loop doesn't have a legal pre-header.
+
+define void @inc(i32 %n, i8* %P) {
+  %1 = icmp sgt i32 %n, 0
+  br i1 %1, label %BB1, label %BB2
+
+BB1:
+  indirectbr i8* %P, [label %.lr.ph]
+
+BB2:
+  br label %.lr.ph
+
+.lr.ph:
+  %indvars.iv = phi i32 [ %indvars.iv.next, %.lr.ph ], [ 0, %BB1 ], [ 0, %BB2 ]
+  %indvars.iv.next = add i32 %indvars.iv, 1
+  %exitcond = icmp eq i32 %indvars.iv.next, %n
+  br i1 %exitcond, label %._crit_edge, label %.lr.ph
+
+._crit_edge:
+  ret void
+}