[llvm] [RISCV] Fold vp.reverse(vp.load(ADDR, MASK)) -> vp.strided.load(ADDR, -1, MASK). (PR #123115)

[Philip Reames via llvm-commits]

================
@@ -16229,6 +16229,69 @@ static SDValue performBITREVERSECombine(SDNode *N, SelectionDAG &DAG,
   return DAG.getNode(RISCVISD::BREV8, DL, VT, Src.getOperand(0));
 }
 
+static SDValue performVP_REVERSECombine(SDNode *N, SelectionDAG &DAG,
+                                        const RISCVSubtarget &Subtarget) {
+  // Fold:
+  //    vp.reverse(vp.load(ADDR, MASK)) -> vp.strided.load(ADDR, -1, MASK)
+
+  // Check if its first operand is a vp.load.
+  auto *VPLoad = dyn_cast<VPLoadSDNode>(N->getOperand(0));
+  if (!VPLoad)
+    return SDValue();
+
+  EVT LoadVT = VPLoad->getValueType(0);
+  // We do not have a strided_load version for masks, and the evl of vp.reverse
+  // and vp.load should always be the same.
+  if (!LoadVT.getVectorElementType().isByteSized() ||
+      N->getOperand(2) != VPLoad->getVectorLength() ||
+      !N->getOperand(0).hasOneUse())
+    return SDValue();
+
+  // Check if the mask of outer vp.reverse are all 1's.
+  if (!isOneOrOneSplat(N->getOperand(1)))
+    return SDValue();
+
+  SDValue LoadMask = VPLoad->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(LoadMask)) {
+    // Check if the mask of vp.reverse in vp.load are all 1's and
----------------
preames wrote:

I don't really follow the value of this bit, but it's tested, and appears correct, so non-blocking.

https://github.com/llvm/llvm-project/pull/123115