[llvm] Handle VECREDUCE intrinsics in NVPTX backend (PR #136253)

[Princeton Ferro via llvm-commits]

================
@@ -1900,6 +1917,191 @@ static SDValue getPRMT(SDValue A, SDValue B, uint64_t Selector, SDLoc DL,
   return getPRMT(A, B, DAG.getConstant(Selector, DL, MVT::i32), DL, DAG, Mode);
 }
 
+/// A generic routine for constructing a tree reduction on a vector operand.
+/// This method groups elements bottom-up, progressively building each level.
+/// Unlike the shuffle reduction used in DAGTypeLegalizer and ExpandReductions,
+/// adjacent elements are combined first, leading to shorter live ranges. This
+/// approach makes the most sense if the shuffle reduction would use the same
+/// amount of registers.
+///
+/// The flags on the original reduction operation will be propagated to
+/// each scalar operation.
+static SDValue BuildTreeReduction(
+    const SmallVector<SDValue> &Elements, EVT EltTy,
+    ArrayRef<std::pair<unsigned /*NodeType*/, unsigned /*NumInputs*/>> Ops,
+    const SDLoc &DL, const SDNodeFlags Flags, SelectionDAG &DAG) {
+  // Build the reduction tree at each level, starting with all the elements.
+  SmallVector<SDValue> Level = Elements;
+
+  unsigned OpIdx = 0;
+  while (Level.size() > 1) {
+    // Try to reduce this level using the current operator.
+    const auto [DefaultScalarOp, DefaultGroupSize] = Ops[OpIdx];
+
+    // Build the next level by partially reducing all elements.
+    SmallVector<SDValue> ReducedLevel;
+    unsigned I = 0, E = Level.size();
+    for (; I + DefaultGroupSize <= E; I += DefaultGroupSize) {
+      // Reduce elements in groups of [DefaultGroupSize], as much as possible.
+      ReducedLevel.push_back(DAG.getNode(
+          DefaultScalarOp, DL, EltTy,
+          ArrayRef<SDValue>(Level).slice(I, DefaultGroupSize), Flags));
+    }
+
+    if (I < E) {
+      // Handle leftover elements.
+
+      if (ReducedLevel.empty()) {
+        // We didn't reduce anything at this level. We need to pick a smaller
+        // operator.
+        ++OpIdx;
+        assert(OpIdx < Ops.size() && "no smaller operators for reduction");
+        continue;
+      }
+
+      // We reduced some things but there's still more left, meaning the
+      // operator's number of inputs doesn't evenly divide this level size. Move
+      // these elements to the next level.
+      for (; I < E; ++I)
+        ReducedLevel.push_back(Level[I]);
+    }
+
+    // Process the next level.
+    Level = ReducedLevel;
+  }
+
+  return *Level.begin();
+}
+
+/// Lower reductions to either a sequence of operations or a tree if
+/// reassociations are allowed. This method will use larger operations like
+/// max3/min3 when the target supports them.
+SDValue NVPTXTargetLowering::LowerVECREDUCE(SDValue Op,
+                                            SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+  const SDNodeFlags Flags = Op->getFlags();
+  SDValue Vector = Op.getOperand(0);
+  SDValue Accumulator;
+
+  EVT EltTy = Vector.getValueType().getVectorElementType();
+  const bool CanUseMinMax3 = EltTy == MVT::f32 && STI.getSmVersion() >= 100 &&
+                             STI.getPTXVersion() >= 88;
+
+  // A list of SDNode opcodes with equivalent semantics, sorted descending by
+  // number of inputs they take.
+  SmallVector<std::pair<unsigned /*Op*/, unsigned /*NumIn*/>, 2> ScalarOps;
+
+  // Whether we can lower to scalar operations in an arbitrary order.
+  bool IsAssociative = allowUnsafeFPMath(DAG.getMachineFunction());
+
+  // Whether the data type and operation can be represented with fewer ops and
+  // registers in a shuffle reduction.
+  bool PrefersShuffle;
+
+  switch (Op->getOpcode()) {
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Prince781 wrote:

Done. 

https://github.com/llvm/llvm-project/pull/136253