diff --git a/lib/Transforms/Scalar/DeadStoreElimination.cpp b/lib/Transforms/Scalar/DeadStoreElimination.cpp
index f473cf5ca54..9f36a386917 100644
--- a/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -361,8 +361,10 @@ static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
   //
   //        |--earlier--|
   //    |-----  later  ------|
+  //
+  // We have to be careful here as *Off is signed while *.Size is unsigned.
   if (EarlierOff >= LaterOff &&
-      EarlierOff + Earlier.Size <= LaterOff + Later.Size)
+      uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
     return true;
 
   // Otherwise, they don't completely overlap.
diff --git a/test/Transforms/DeadStoreElimination/2011-03-25-DSEMiscompile.ll b/test/Transforms/DeadStoreElimination/2011-03-25-DSEMiscompile.ll
index e268dd54140..079eec43bf8 100644
--- a/test/Transforms/DeadStoreElimination/2011-03-25-DSEMiscompile.ll
+++ b/test/Transforms/DeadStoreElimination/2011-03-25-DSEMiscompile.ll
@@ -1,5 +1,6 @@
 ; RUN: opt < %s -basicaa -dse -S | FileCheck %s
 ; PR9561
+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-f80:128:128-n8:16:32"
 target triple = "i386-apple-darwin9.8"
 
 @A = external global [0 x i32]