diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index 79f80f37eea..874db9ff152 100644
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -1537,6 +1537,15 @@ InnerLoopVectorizer::createEmptyLoop(LoopVectorizationLegality *Legal) {
   const SCEV *ExitCount = SE->getBackedgeTakenCount(OrigLoop);
   assert(ExitCount != SE->getCouldNotCompute() && "Invalid loop count");
 
+  // The exit count might have the type of i64 while the phi is i32. This can
+  // happen if we have an induction variable that is sign extended before the
+  // compare. The only way that we get a backedge taken count is that the
+  // induction variable was signed and as such will not overflow. In such a case
+  // truncation is legal.
+  if (ExitCount->getType()->getPrimitiveSizeInBits() >
+      IdxTy->getPrimitiveSizeInBits())
+    ExitCount = SE->getTruncateOrNoop(ExitCount, IdxTy);
+
   ExitCount = SE->getNoopOrZeroExtend(ExitCount, IdxTy);
   // Get the total trip count from the count by adding 1.
   ExitCount = SE->getAddExpr(ExitCount,
diff --git a/test/Transforms/LoopVectorize/X86/tripcount.ll b/test/Transforms/LoopVectorize/X86/tripcount.ll
new file mode 100644
index 00000000000..6b38bacf888
--- /dev/null
+++ b/test/Transforms/LoopVectorize/X86/tripcount.ll
@@ -0,0 +1,39 @@
+; RUN: opt -S -loop-vectorize -force-vector-width=2 -force-vector-unroll=1 -mcpu=prescott < %s | FileCheck %s
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32-S128"
+target triple = "i386-unknown-freebsd11.0"
+
+@big = external global [0 x i32]
+
+; PR18049
+; We need to truncate the exit count to i32. This is legal because the
+; arithmetic is signed (%inc is nsw).
+
+; CHECK-LABEL: tripcount
+; CHECK: trunc i64 %count to i32
+
+define void @tripcount(i64 %count) {
+entry:
+  %cmp6 = icmp sgt i64 %count, 0
+  br i1 %cmp6, label %for.body.preheader, label %for.end
+
+for.body.preheader:
+  br label %for.body
+
+for.body:
+  %i.07 = phi i32 [ %inc, %for.body ], [ 0, %for.body.preheader ]
+  %arrayidx = getelementptr inbounds [0 x i32]* @big, i32 0, i32 %i.07
+  %0 = load i32* %arrayidx, align 4
+  %neg = xor i32 %0, -1
+  store i32 %neg, i32* %arrayidx, align 4
+  %inc = add nsw i32 %i.07, 1
+  %conv = sext i32 %inc to i64
+  %cmp = icmp slt i64 %conv, %count
+  br i1 %cmp, label %for.body, label %for.end.loopexit
+
+for.end.loopexit:
+  br label %for.end
+
+for.end:
+  ret void
+}