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

[Benjamin Maxwell via llvm-commits]

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@@ -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);
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MacDue wrote:

Also, is this correct for the predicated reverse? For example, if the predicate was `pfalse`, then `rev(rev())` would return the pass through. 

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