[llvm] [AArch64] Fold BUILD_VECTORs splats into users by using SVE immediates (PR #165559)

[Benjamin Maxwell via llvm-commits]

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@@ -15501,6 +15501,72 @@ SDValue AArch64TargetLowering::LowerFixedLengthBuildVectorToSVE(
   return convertFromScalableVector(DAG, VT, Vec);
 }
 
+static std::optional<int64_t> getSplatConstant(SDValue V,
+                                               ConstantSDNode *&Const) {
+  if (auto *BV = dyn_cast<BuildVectorSDNode>(V))
+    if ((Const = dyn_cast_if_present<ConstantSDNode>(BV->getSplatValue())))
+      return Const->getZExtValue();
+  return std::nullopt;
+}
+
+static bool isSVESplatImmForOp(unsigned Opcode, MVT VT, int64_t SplatImm) {
+  // TODO: Support more than integer binops.
+  switch (Opcode) {
+  case ISD::SUB:
+  case ISD::ADD:
+    return isUInt<8>(SplatImm) || (VT.getFixedSizeInBits() > 8 &&
+                                   isUInt<16>(SplatImm) && SplatImm % 256 == 0);
+  case ISD::XOR:
+  case ISD::OR:
+  case ISD::AND:
+    return AArch64_AM::isLogicalImmediate(SplatImm, 64);
+  case ISD::MUL:
+    return isInt<8>(SplatImm);
+  default:
+    return false;
+  }
+}
+
+static SDValue tryFoldSplatIntoUsersWithSVE(SDValue Op, SelectionDAG &DAG) {
+  auto &Subtarget = DAG.getSubtarget<AArch64Subtarget>();
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MacDue wrote:

I'm not sure if this is necessarily always preferable, or if there's some target property this should be dependent on?

https://github.com/llvm/llvm-project/pull/165559