[RISCV] Add a combine to form masked.store from unit strided store

Add a DAG combine to form a masked.store from a masked_strided_store intrinsic
with stride equal to element size. This is the store analogy to PR #65674.

As seen in the tests, this does pickup a few cases that we'd previously missed
due to selection ordering.  We match strided stores early without going through
the recently added generic mscatter combines, and thus weren't recognizing the
unit strided store.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -14467,6 +14467,24 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
                                  ISD::UNINDEXED, ISD::NON_EXTLOAD);
       return SDValue();
     }
+    case Intrinsic::riscv_masked_strided_store: {
+      auto *Store = cast<MemIntrinsicSDNode>(N);
+      SDValue Value = N->getOperand(2);
+      SDValue Base = N->getOperand(3);
+      SDValue Stride = N->getOperand(4);
+      SDValue Mask = N->getOperand(5);
+
+      // If the stride is equal to the element size in bytes,  we can use
+      // a masked.store.
+      const unsigned ElementSize = Value.getValueType().getScalarStoreSize();
+      if (auto *StrideC = dyn_cast<ConstantSDNode>(Stride);
+          StrideC && StrideC->getZExtValue() == ElementSize)
+        return DAG.getMaskedStore(Store->getChain(), DL, Value, Base,
+                                  DAG.getUNDEF(XLenVT), Mask,
+                                  Store->getMemoryVT(), Store->getMemOperand(),
+                                  ISD::UNINDEXED, false);
+      return SDValue();
+    }
     case Intrinsic::riscv_vcpop:
     case Intrinsic::riscv_vcpop_mask:
     case Intrinsic::riscv_vfirst:

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-masked-scatter.ll

@@ -11296,9 +11296,8 @@ define void @mscatter_baseidx_v32i8(<32 x i8> %val, ptr %base, <32 x i8> %idxs,
 define void @mscatter_unit_stride(<8 x i16> %val, ptr %base) {
 ; CHECK-LABEL: mscatter_unit_stride:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    li a1, 2
 ; CHECK-NEXT:    vsetivli zero, 8, e16, m1, ta, ma
-; CHECK-NEXT:    vsse16.v v8, (a0), a1
+; CHECK-NEXT:    vse16.v v8, (a0)
 ; CHECK-NEXT:    ret
   %head = insertelement <8 x i1> poison, i1 true, i16 0
   %allones = shufflevector <8 x i1> %head, <8 x i1> poison, <8 x i32> zeroinitializer
@@ -11311,9 +11310,8 @@ define void @mscatter_unit_stride_with_offset(<8 x i16> %val, ptr %base) {
 ; CHECK-LABEL: mscatter_unit_stride_with_offset:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    addi a0, a0, 10
-; CHECK-NEXT:    li a1, 2
 ; CHECK-NEXT:    vsetivli zero, 8, e16, m1, ta, ma
-; CHECK-NEXT:    vsse16.v v8, (a0), a1
+; CHECK-NEXT:    vse16.v v8, (a0)
 ; CHECK-NEXT:    ret
   %head = insertelement <8 x i1> poison, i1 true, i16 0
   %allones = shufflevector <8 x i1> %head, <8 x i1> poison, <8 x i32> zeroinitializer

llvm/test/CodeGen/RISCV/rvv/mscatter-combine.ll

@@ -114,8 +114,7 @@ define void @stride_one_store(i64 %n, ptr %p) {
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    vsetvli a0, zero, e64, m1, ta, ma
 ; RV64-NEXT:    vmv.v.i v8, 0
-; RV64-NEXT:    li a0, 8
-; RV64-NEXT:    vsse64.v v8, (a1), a0
+; RV64-NEXT:    vs1r.v v8, (a1)
 ; RV64-NEXT:    ret
   %step = tail call <vscale x 1 x i64> @llvm.experimental.stepvector.nxv1i64()
   %gep = getelementptr inbounds i64, ptr %p, <vscale x 1 x i64> %step

llvm/test/CodeGen/RISCV/rvv/strided-load-store-intrinsics.ll

@@ -89,9 +89,8 @@ define void @strided_store_i8_nostride(ptr %p, <32 x i8> %v, <32 x i1> %m) {
 ; CHECK-LABEL: strided_store_i8_nostride:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li a1, 32
-; CHECK-NEXT:    li a2, 1
 ; CHECK-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; CHECK-NEXT:    vsse8.v v8, (a0), a2, v0.t
+; CHECK-NEXT:    vse8.v v8, (a0), v0.t
 ; CHECK-NEXT:    ret
   call void @llvm.riscv.masked.strided.store.v32i8.p0.i64(<32 x i8> %v, ptr %p, i64 1, <32 x i1> %m)
   ret void