[llvm] [AArch64] Utilize `XAR` for certain vector rotates (PR #137629)

[Rajveer Singh Bharadwaj via llvm-commits]

https://github.com/Rajveer100 updated https://github.com/llvm/llvm-project/pull/137629

>From 0fa71c5d28f3c8dc793fe724e347114c5bf84827 Mon Sep 17 00:00:00 2001
From: Rajveer <rajveer.developer at icloud.com>
Date: Mon, 28 Apr 2025 18:42:32 +0530
Subject: [PATCH] [AArch64] Utilize `XAR` for certain vector rotates

Resolves #137162

For cases when there isn't any `XOR` in the transformation,
replace with a zero register.
---
 .../Target/AArch64/AArch64ISelDAGToDAG.cpp    | 35 ++++++++++++++-----
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp b/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
index 40944e3d43d6b..d035230d2235a 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
@@ -13,6 +13,7 @@
 #include "AArch64MachineFunctionInfo.h"
 #include "AArch64TargetMachine.h"
 #include "MCTargetDesc/AArch64AddressingModes.h"
+#include "MCTargetDesc/AArch64MCTargetDesc.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/CodeGen/ISDOpcodes.h"
 #include "llvm/CodeGen/SelectionDAGISel.h"
@@ -4558,9 +4559,16 @@ bool AArch64DAGToDAGISel::trySelectXAR(SDNode *N) {
         !TLI->isAllActivePredicate(*CurDAG, N1.getOperand(0)))
       return false;
 
-    SDValue XOR = N0.getOperand(1);
-    if (XOR.getOpcode() != ISD::XOR || XOR != N1.getOperand(1))
-      return false;
+    SDValue R1, R2;
+    if (N0.getOperand(1).getOpcode() != ISD::XOR) {
+      if (N0.getOperand(1) != N1.getOperand(1))
+        return false;
+      SDLoc DL(N1->getOperand(0));
+      SDValue Zero =
+          CurDAG->getRegister(AArch64::XZR, N1->getOperand(0).getValueType());
+      R1 = N1->getOperand(0);
+      R2 = Zero;
+    }
 
     APInt ShlAmt, ShrAmt;
     if (!ISD::isConstantSplatVector(N0.getOperand(2).getNode(), ShlAmt) ||
@@ -4574,7 +4582,7 @@ bool AArch64DAGToDAGISel::trySelectXAR(SDNode *N) {
     SDValue Imm =
         CurDAG->getTargetConstant(ShrAmt.getZExtValue(), DL, MVT::i32);
 
-    SDValue Ops[] = {XOR.getOperand(0), XOR.getOperand(1), Imm};
+    SDValue Ops[] = {R1, R2, Imm};
     if (auto Opc = SelectOpcodeFromVT<SelectTypeKind::Int>(
             VT, {AArch64::XAR_ZZZI_B, AArch64::XAR_ZZZI_H, AArch64::XAR_ZZZI_S,
                  AArch64::XAR_ZZZI_D})) {
@@ -4591,13 +4599,22 @@ bool AArch64DAGToDAGISel::trySelectXAR(SDNode *N) {
       N1->getOpcode() != AArch64ISD::VLSHR)
     return false;
 
-  if (N0->getOperand(0) != N1->getOperand(0) ||
-      N1->getOperand(0)->getOpcode() != ISD::XOR)
+
+  if (N0->getOperand(0) != N1->getOperand(0))
     return false;
 
-  SDValue XOR = N0.getOperand(0);
-  SDValue R1 = XOR.getOperand(0);
-  SDValue R2 = XOR.getOperand(1);
+  SDValue R1, R2;
+  if (N1->getOperand(0)->getOpcode() != ISD::XOR) {
+    SDLoc DL(N1->getOperand(0));
+    SDValue Zero =
+        CurDAG->getRegister(AArch64::XZR, N1->getOperand(0).getValueType());
+    R1 = N1->getOperand(0);
+    R2 = Zero;
+  } else {
+    SDValue XOR = N0.getOperand(0);
+    R1 = XOR.getOperand(0);
+    R2 = XOR.getOperand(1);
+  }
 
   unsigned HsAmt = N0.getConstantOperandVal(1);
   unsigned ShAmt = N1.getConstantOperandVal(1);