diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 8e0637a49b5..f3695929d16 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -97,38 +97,53 @@ static bool isEscapeSource(const Value *V) {
   return false;
 }
 
-/// isObjectSmallerThan - Return true if we can prove that the object specified
-/// by V is smaller than Size.
-static bool isObjectSmallerThan(const Value *V, uint64_t Size,
-                                const TargetData &TD) {
+/// getObjectSize - Return the size of the object specified by V, or
+/// UnknownSize if unknown.
+static uint64_t getObjectSize(const Value *V, const TargetData &TD) {
   const Type *AccessTy;
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
     if (!GV->hasDefinitiveInitializer())
-      return false;
+      return AliasAnalysis::UnknownSize;
     AccessTy = GV->getType()->getElementType();
   } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
     if (!AI->isArrayAllocation())
       AccessTy = AI->getType()->getElementType();
     else
-      return false;
+      return AliasAnalysis::UnknownSize;
   } else if (const CallInst* CI = extractMallocCall(V)) {
     if (!isArrayMalloc(V, &TD))
       // The size is the argument to the malloc call.
       if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
-        return (C->getZExtValue() < Size);
-    return false;
+        return C->getZExtValue();
+    return AliasAnalysis::UnknownSize;
   } else if (const Argument *A = dyn_cast<Argument>(V)) {
     if (A->hasByValAttr())
       AccessTy = cast<PointerType>(A->getType())->getElementType();
     else
-      return false;
+      return AliasAnalysis::UnknownSize;
   } else {
-    return false;
+    return AliasAnalysis::UnknownSize;
   }
   
   if (AccessTy->isSized())
-    return TD.getTypeAllocSize(AccessTy) < Size;
-  return false;
+    return TD.getTypeAllocSize(AccessTy);
+  return AliasAnalysis::UnknownSize;
+}
+
+/// isObjectSmallerThan - Return true if we can prove that the object specified
+/// by V is smaller than Size.
+static bool isObjectSmallerThan(const Value *V, uint64_t Size,
+                                const TargetData &TD) {
+  uint64_t ObjectSize = getObjectSize(V, TD);
+  return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
+}
+
+/// isObjectSize - Return true if we can prove that the object specified
+/// by V has size Size.
+static bool isObjectSize(const Value *V, uint64_t Size,
+                         const TargetData &TD) {
+  uint64_t ObjectSize = getObjectSize(V, TD);
+  return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
 }
 
 //===----------------------------------------------------------------------===//
@@ -1142,6 +1157,14 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
     if (Result != MayAlias) return Result;
   }
 
+  // If both pointers are pointing into the same object and one of them
+  // accesses is accessing the entire object, then the accesses must
+  // overlap in some way.
+  if (TD && O1 == O2)
+    if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) ||
+        (V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD)))
+      return PartialAlias;
+
   return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
                               Location(V2, V2Size, V2TBAAInfo));
 }
diff --git a/test/Analysis/BasicAA/full-store-partial-alias.ll b/test/Analysis/BasicAA/full-store-partial-alias.ll
new file mode 100644
index 00000000000..f92c689516e
--- /dev/null
+++ b/test/Analysis/BasicAA/full-store-partial-alias.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S -tbaa -basicaa -gvn < %s | grep {ret i32 %}
+; RUN: opt -S -tbaa -gvn < %s | grep {ret i32 0}
+; rdar://8875631
+
+; BasicAA should notice that the store stores to the entire %u object,
+; so the %tmp5 load is PartialAlias with the store and suppress TBAA.
+; Without BasicAA, TBAA should say that %tmp5 is NoAlias with the store.
+
+target datalayout = "e-p:64:64:64"
+
+%union.anon = type { double }
+
+@u = global %union.anon { double -2.500000e-01 }, align 8
+@endianness_test = global i64 1, align 8
+
+define i32 @signbit(double %x) nounwind {
+entry:
+  %u = alloca %union.anon, align 8
+  %tmp9 = getelementptr inbounds %union.anon* %u, i64 0, i32 0
+  store double %x, double* %tmp9, align 8, !tbaa !0
+  %tmp2 = load i32* bitcast (i64* @endianness_test to i32*), align 8, !tbaa !3
+  %idxprom = sext i32 %tmp2 to i64
+  %tmp4 = bitcast %union.anon* %u to [2 x i32]*
+  %arrayidx = getelementptr inbounds [2 x i32]* %tmp4, i64 0, i64 %idxprom
+  %tmp5 = load i32* %arrayidx, align 4, !tbaa !3
+  %tmp5.lobit = lshr i32 %tmp5, 31
+  ret i32 %tmp5.lobit
+}
+
+!0 = metadata !{metadata !"double", metadata !1}
+!1 = metadata !{metadata !"omnipotent char", metadata !2}
+!2 = metadata !{metadata !"Simple C/C++ TBAA", null}
+!3 = metadata !{metadata !"int", metadata !1}