Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -199,7 +199,7 @@
   return MI;
 }
 
-Instruction *InstCombiner::SimplifyMemSet(MemSetInst *MI) {
+Instruction *InstCombiner::SimplifyAnyMemSet(AnyMemSetInst *MI) {
   unsigned Alignment = getKnownAlignment(MI->getDest(), DL, MI, &AC, &DT);
   if (MI->getDestAlignment() < Alignment) {
     MI->setDestAlignment(Alignment);
@@ -232,6 +232,8 @@
     StoreInst *S = Builder.CreateStore(ConstantInt::get(ITy, Fill), Dest,
                                        MI->isVolatile());
     S->setAlignment(Alignment);
+    if (isa<AtomicMemSetInst>(MI))
+      S->setOrdering(AtomicOrdering::Unordered);
 
     // Set the size of the copy to 0, it will be deleted on the next iteration.
     MI->setLength(Constant::getNullValue(LenC->getType()));
@@ -1758,8 +1760,8 @@
     if (auto *MTI = dyn_cast<AnyMemTransferInst>(MI)) {
       if (Instruction *I = SimplifyAnyMemTransfer(MTI))
         return I;
-    } else if (MemSetInst *MSI = dyn_cast<MemSetInst>(MI)) {
-      if (Instruction *I = SimplifyMemSet(MSI))
+    } else if (auto *MSI = dyn_cast<AnyMemSetInst>(MI)) {
+      if (Instruction *I = SimplifyAnyMemSet(MSI))
         return I;
     }
 
Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineInternal.h
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineInternal.h
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -825,7 +825,7 @@
   bool SimplifyStoreAtEndOfBlock(StoreInst &SI);
 
   Instruction *SimplifyAnyMemTransfer(AnyMemTransferInst *MI);
-  Instruction *SimplifyMemSet(MemSetInst *MI);
+  Instruction *SimplifyAnyMemSet(AnyMemSetInst *MI);
 
   Value *EvaluateInDifferentType(Value *V, Type *Ty, bool isSigned);
 
Index: llvm/trunk/test/Transforms/InstCombine/element-atomic-memintrins.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/element-atomic-memintrins.ll
+++ llvm/trunk/test/Transforms/InstCombine/element-atomic-memintrins.ll
@@ -12,13 +12,15 @@
   ret void
 }
 
-; Placeholder test. This will chance once support for lowering atomic memsets is added to instcombine.
 define void @test_memset_to_store(i8* %dest) {
 ; CHECK-LABEL: @test_memset_to_store(
-; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 1 [[DEST:%.*]], i8 1, i32 1, i32 1)
-; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 1 [[DEST]], i8 1, i32 2, i32 1)
-; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 1 [[DEST]], i8 1, i32 4, i32 1)
-; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 1 [[DEST]], i8 1, i32 8, i32 1)
+; CHECK-NEXT:    store atomic i8 1, i8* [[DEST:%.*]] unordered, align 1
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DEST]] to i16*
+; CHECK-NEXT:    store atomic i16 257, i16* [[TMP1]] unordered, align 1
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i8* [[DEST]] to i32*
+; CHECK-NEXT:    store atomic i32 16843009, i32* [[TMP2]] unordered, align 1
+; CHECK-NEXT:    [[TMP3:%.*]] = bitcast i8* [[DEST]] to i64*
+; CHECK-NEXT:    store atomic i64 72340172838076673, i64* [[TMP3]] unordered, align 1
 ; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 1 [[DEST]], i8 1, i32 16, i32 1)
 ; CHECK-NEXT:    ret void
 ;
@@ -30,6 +32,60 @@
   ret void
 }
 
+define void @test_memset_to_store_2(i8* %dest) {
+; CHECK-LABEL: @test_memset_to_store_2(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DEST:%.*]] to i16*
+; CHECK-NEXT:    store atomic i16 257, i16* [[TMP1]] unordered, align 2
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i8* [[DEST]] to i32*
+; CHECK-NEXT:    store atomic i32 16843009, i32* [[TMP2]] unordered, align 2
+; CHECK-NEXT:    [[TMP3:%.*]] = bitcast i8* [[DEST]] to i64*
+; CHECK-NEXT:    store atomic i64 72340172838076673, i64* [[TMP3]] unordered, align 2
+; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 2 [[DEST]], i8 1, i32 16, i32 2)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 2 %dest, i8 1, i32 2, i32 2)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 2 %dest, i8 1, i32 4, i32 2)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 2 %dest, i8 1, i32 8, i32 2)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 2 %dest, i8 1, i32 16, i32 2)
+  ret void
+}
+
+define void @test_memset_to_store_4(i8* %dest) {
+; CHECK-LABEL: @test_memset_to_store_4(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DEST:%.*]] to i32*
+; CHECK-NEXT:    store atomic i32 16843009, i32* [[TMP1]] unordered, align 4
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i8* [[DEST]] to i64*
+; CHECK-NEXT:    store atomic i64 72340172838076673, i64* [[TMP2]] unordered, align 4
+; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 4 [[DEST]], i8 1, i32 16, i32 4)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 4 %dest, i8 1, i32 4, i32 4)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 4 %dest, i8 1, i32 8, i32 4)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 4 %dest, i8 1, i32 16, i32 4)
+  ret void
+}
+
+define void @test_memset_to_store_8(i8* %dest) {
+; CHECK-LABEL: @test_memset_to_store_8(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DEST:%.*]] to i64*
+; CHECK-NEXT:    store atomic i64 72340172838076673, i64* [[TMP1]] unordered, align 8
+; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 8 [[DEST]], i8 1, i32 16, i32 8)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 8 %dest, i8 1, i32 8, i32 8)
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 8 %dest, i8 1, i32 16, i32 8)
+  ret void
+}
+
+define void @test_memset_to_store_16(i8* %dest) {
+; CHECK-LABEL: @test_memset_to_store_16(
+; CHECK-NEXT:    call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 16 [[DEST:%.*]], i8 1, i32 16, i32 16)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* align 16 %dest, i8 1, i32 16, i32 16)
+  ret void
+}
+
 declare void @llvm.memset.element.unordered.atomic.p0i8.i32(i8* nocapture writeonly, i8, i32, i32) nounwind argmemonly