[clang] [llvm] [Arch64][SVE] Lower svrev_* to llvm.vector.reverse and fold svrev(svrev(x)) -> x (PR #116422)

[Sander de Smalen via cfe-commits]

================
@@ -21962,6 +21962,35 @@ SDValue tryLowerPartialReductionToWideAdd(SDNode *N,
   return DAG.getNode(TopOpcode, DL, AccVT, BottomNode, ExtOp);
 }
 
+static SDValue foldRevInvolution(SDNode *N) {
+  SDValue InnerRev = N->getOperand(1);
+  if (!InnerRev.hasOneUse())
+    return SDValue();
+
+  unsigned OuterIId = getIntrinsicID(N);
+  unsigned InnerIId = getIntrinsicID(InnerRev.getNode());
+  if (OuterIId != InnerIId)
+    return SDValue();
+
+  switch (OuterIId) {
+  case Intrinsic::aarch64_sve_revb:
+  case Intrinsic::aarch64_sve_revd:
+  case Intrinsic::aarch64_sve_revh:
+  case Intrinsic::aarch64_sve_revw:
+    if (N->getOperand(2) != InnerRev.getOperand(2) ||
+        N->getOperand(3) != InnerRev.getOperand(3))
+      return SDValue();
+    [[fallthrough]];
+  case Intrinsic::aarch64_sve_rev:
+  case Intrinsic::aarch64_sve_rev_b16:
+  case Intrinsic::aarch64_sve_rev_b32:
+  case Intrinsic::aarch64_sve_rev_b64:
+    return InnerRev.getOperand(1);
----------------
sdesmalen-arm wrote:

I realise that I steered you to doing this in `performIntrinsicCombine`, but given my comment above, perhaps it's worth doing the optimization for `rev.b16/b32/b64` in `AArch64TTIimpl::instCombineIntrinsic` instead. That way the canonicalised input into the code-generator is already optimised.

https://github.com/llvm/llvm-project/pull/116422