Index: lib/Transforms/Scalar/SROA.cpp
===================================================================
--- lib/Transforms/Scalar/SROA.cpp
+++ lib/Transforms/Scalar/SROA.cpp
@@ -2547,7 +2547,9 @@
     bool EmitMemCpy
       = !VecTy && !IntTy && (BeginOffset > NewAllocaBeginOffset ||
                              EndOffset < NewAllocaEndOffset ||
-                             !NewAI.getAllocatedType()->isSingleValueType());
+                             NewAllocaEndOffset - NewAllocaBeginOffset <
+                               DL.getTypeStoreSize(NewAllocaTy) ||
+                             !NewAllocaTy->isSingleValueType());
 
     // If we're just going to emit a memcpy, the alloca hasn't changed, and the
     // size hasn't been shrunk based on analysis of the viable range, this is
Index: test/Transforms/SROA/vector-promotion.ll
===================================================================
--- test/Transforms/SROA/vector-promotion.ll
+++ test/Transforms/SROA/vector-promotion.ll
@@ -468,3 +468,38 @@
 ; CHECK: %[[insert:.*]] = or i32 %{{.*}}, %[[trunc]]
 ; CHECK: ret i32 %[[insert]]
 }
+
+%S7 = type { float, float, float }
+%U7 = type { <4 x float> }
+
+declare i32 @test7_helper(%S7*)
+
+define i32 @test7(%S7* %x) {
+; CHECK-LABEL: @test7(
+; Test case to verify that SROA does not rewrite a 12-byte memcpy into a 16-byte
+; vector store, hence accidentally putting gibberish onto the stack.
+entry:
+  ; Create a temporary variable %tmp1 and copy %x[0] into it
+  %tmp1 = alloca %S7, align 4
+  %0 = bitcast %S7* %tmp1 to i8*
+  %1 = bitcast %S7* %x to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %0, i8* %1, i32 12, i32 4, i1 false)
+
+  ; The following block does nothing; but appears to confuse SROA
+  %unused1 = bitcast %S7* %tmp1 to %U7*
+  %unused2 = getelementptr inbounds %U7* %unused1, i32 0, i32 0
+  %unused3 = load <4 x float>* %unused2, align 1
+
+  ; Create a second temporary and copy %tmp1 into it
+  %tmp2 = alloca %S7, align 4
+  %2 = bitcast %S7* %tmp2 to i8*
+  %3 = bitcast %S7* %tmp1 to i8*
+; CHECK: alloca
+; CHECK-NOT: store <4 x float>
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %2, i8* %3, i32 12, i32 4, i1 false)
+
+  %result = call i32 @test7_helper(%S7* %tmp2)
+  ret i32 %result
+; CHECK: ret i32 %result
+}
+