[llvm] [RISCV] Fold vp.store(vp.reverse(VAL), ADDR, MASK) -> vp.strided.store(VAL, NEW_ADDR, -1, MASK) (PR #123123)

[Luke Lau via llvm-commits]

================
@@ -16229,6 +16233,66 @@ static SDValue performBITREVERSECombine(SDNode *N, SelectionDAG &DAG,
   return DAG.getNode(RISCVISD::BREV8, DL, VT, Src.getOperand(0));
 }
 
+static SDValue performVP_STORECombine(SDNode *N, SelectionDAG &DAG,
+                                      const RISCVSubtarget &Subtarget) {
+  // Fold:
+  //    vp.store(vp.reverse(VAL), ADDR, MASK) -> vp.strided.store(VAL, NEW_ADDR,
+  //    -1, MASK)
+  auto *VPStore = cast<VPStoreSDNode>(N);
+
+  if (VPStore->getValue().getOpcode() != ISD::EXPERIMENTAL_VP_REVERSE)
+    return SDValue();
+
+  SDValue VPReverse = VPStore->getValue();
+  EVT ReverseVT = VPReverse->getValueType(0);
+
+  // We do not have a strided_store version for masks, and the evl of vp.reverse
+  // and vp.store should always be the same.
+  if (!ReverseVT.getVectorElementType().isByteSized() ||
+      VPStore->getVectorLength() != VPReverse.getOperand(2) ||
+      !VPReverse.hasOneUse())
+    return SDValue();
+
+  SDValue StoreMask = VPStore->getMask();
+  // If Mask is not all 1's, try to replace the mask if it's opcode
+  // is EXPERIMENTAL_VP_REVERSE and it's operand can be directly extracted.
+  if (!isOneOrOneSplat(StoreMask)) {
+    // Check if the mask of vp.reverse in vp.store are all 1's and
+    // the length of mask is same as evl.
----------------
lukel97 wrote:

Nit, maybe reword to something like "Allow a non-ones mask in vp.store if it's a vp.reverse"?

https://github.com/llvm/llvm-project/pull/123123