diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -173,8 +173,10 @@
   void addStore(int64_t OffsetFromFirst, StoreInst *SI) {
     int64_t StoreSize = DL.getTypeStoreSize(SI->getOperand(0)->getType());
 
-    addRange(OffsetFromFirst, StoreSize,
-             SI->getPointerOperand(), SI->getAlignment(), SI);
+    unsigned Align = SI->getAlignment();
+    if (!Align)
+      Align = DL.getABITypeAlignment(SI->getOperand(0)->getType());
+    addRange(OffsetFromFirst, StoreSize, SI->getPointerOperand(), Align, SI);
   }
 
   void addMemSet(int64_t OffsetFromFirst, MemSetInst *MSI) {
@@ -387,13 +389,8 @@
     // Get the starting pointer of the block.
     StartPtr = Range.StartPtr;
 
-    // Determine alignment
-    const Align Alignment = DL.getValueOrABITypeAlignment(
-        MaybeAlign(Range.Alignment),
-        cast<PointerType>(StartPtr->getType())->getElementType());
-
     AMemSet = Builder.CreateMemSet(StartPtr, ByteVal, Range.End - Range.Start,
-                                   Alignment);
+                                   MaybeAlign(Range.Alignment));
     LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
                                                    : Range.TheStores) dbgs()
                                               << *SI << '\n';
diff --git a/llvm/test/Transforms/MemCpyOpt/form-memset.ll b/llvm/test/Transforms/MemCpyOpt/form-memset.ll
--- a/llvm/test/Transforms/MemCpyOpt/form-memset.ll
+++ b/llvm/test/Transforms/MemCpyOpt/form-memset.ll
@@ -217,7 +217,7 @@
   tail call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 12, i1 false)
   ret void
 ; CHECK-LABEL: @test6(
-; CHECK: call void @llvm.memset.p0i8.i64(i8* align 4 %2, i8 0, i64 24, i1 false)
+; CHECK: call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 24, i1 false)
 }
 
 ; More aggressive heuristic
@@ -280,7 +280,7 @@
   ret void
 ; CHECK-LABEL: @test10(
 ; CHECK-NOT: memset
-; CHECK: call void @llvm.memset.p0i8.i64(i8* align 1 %P, i8 0, i64 42, i1 false)
+; CHECK: call void @llvm.memset.p0i8.i64(i8* %P, i8 0, i64 42, i1 false)
 ; CHECK-NOT: memset
 ; CHECK: ret void
 }
@@ -299,3 +299,16 @@
 ; CHECK-NOT: store
 ; CHECK: call void @llvm.memset.p0i8.i64(i8* align 4 %1, i8 1, i64 23, i1 false)
 }
+
+; Alignment should be preserved when there is a store with default align
+define void @test12(i32* nocapture %P) nounwind ssp {
+entry:
+  store i32 0, i32* %P
+  %add.ptr = getelementptr inbounds i32, i32* %P, i64 1
+  %0 = bitcast i32* %add.ptr to i8*
+  tail call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 11, i1 false)
+  ret void
+; CHECK-LABEL: @test12(
+; CHECK-NOT: store
+; CHECK: call void @llvm.memset.p0i8.i64(i8* align 4 %1, i8 0, i64 15, i1 false)
+}