[VectorCombine] New folding pattern for extract/binop/shuffle chains (#145232)

Resolves #144654
Part of #143088

This adds a new `foldShuffleChainsToReduce` for horizontal reduction of
patterns like:

```llvm
define i16 @test_reduce_v8i16(<8 x i16> %a0) local_unnamed_addr #0 {
  %1 = shufflevector <8 x i16> %a0, <8 x i16> poison, <8 x i32> <i32 4, i32 5, i32 6, i32 7, i32 poison, i32 poison, i32 poison, i32 poison>
  %2 = tail call <8 x i16> @llvm.umin.v8i16(<8 x i16> %a0, <8 x i16> %1)
  %3 = shufflevector <8 x i16> %2, <8 x i16> poison, <8 x i32> <i32 2, i32 3, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
  %4 = tail call <8 x i16> @llvm.umin.v8i16(<8 x i16> %2, <8 x i16> %3)
  %5 = shufflevector <8 x i16> %4, <8 x i16> poison, <8 x i32> <i32 1, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
  %6 = tail call <8 x i16> @llvm.umin.v8i16(<8 x i16> %4, <8 x i16> %5)
  %7 = extractelement <8 x i16> %6, i64 0
  ret i16 %7
}
```
...which can be reduced to a llvm.vector.reduce.umin.v8i16(%a0)
intrinsic call.

Similar transformation for other ops when costs permit to do so.

Author: Rajveer100

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -371,6 +371,9 @@ LLVM_ABI bool canSinkOrHoistInst(Instruction &I, AAResults *AA,
 /// Returns the llvm.vector.reduce intrinsic that corresponds to the recurrence
 /// kind.
 LLVM_ABI constexpr Intrinsic::ID getReductionIntrinsicID(RecurKind RK);
+/// Returns the llvm.vector.reduce min/max intrinsic that corresponds to the
+/// intrinsic op.
+LLVM_ABI Intrinsic::ID getMinMaxReductionIntrinsicID(Intrinsic::ID IID);
 
 /// Returns the arithmetic instruction opcode used when expanding a reduction.
 LLVM_ABI unsigned getArithmeticReductionInstruction(Intrinsic::ID RdxID);

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -956,6 +956,21 @@ constexpr Intrinsic::ID llvm::getReductionIntrinsicID(RecurKind RK) {
   }
 }
 
+Intrinsic::ID llvm::getMinMaxReductionIntrinsicID(Intrinsic::ID IID) {
+  switch (IID) {
+  default:
+    llvm_unreachable("Unexpected intrinsic id");
+  case Intrinsic::umin:
+    return Intrinsic::vector_reduce_umin;
+  case Intrinsic::umax:
+    return Intrinsic::vector_reduce_umax;
+  case Intrinsic::smin:
+    return Intrinsic::vector_reduce_smin;
+  case Intrinsic::smax:
+    return Intrinsic::vector_reduce_smax;
+  }
+}
+
 // This is the inverse to getReductionForBinop
 unsigned llvm::getArithmeticReductionInstruction(Intrinsic::ID RdxID) {
   switch (RdxID) {

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -135,6 +135,7 @@ class VectorCombine {
   bool foldShuffleOfIntrinsics(Instruction &I);
   bool foldShuffleToIdentity(Instruction &I);
   bool foldShuffleFromReductions(Instruction &I);
+  bool foldShuffleChainsToReduce(Instruction &I);
   bool foldCastFromReductions(Instruction &I);
   bool foldSelectShuffle(Instruction &I, bool FromReduction = false);
   bool foldInterleaveIntrinsics(Instruction &I);
@@ -3136,6 +3137,267 @@ bool VectorCombine::foldShuffleFromReductions(Instruction &I) {
   return MadeChanges;
 }
 
+/// For a given chain of patterns of the following form:
+///
+/// ```
+///   %1 = shufflevector <n x ty1> %0, <n x ty1> poison <n x ty2> mask
+///
+///   %2 = tail call <n x ty1> llvm.<umin/umax/smin/smax>(<n x ty1> %0, <n x
+///   ty1> %1)
+///     OR
+///   %2 = add/mul/or/and/xor <n x ty1> %0, %1
+///
+///   %3 = shufflevector <n x ty1> %2, <n x ty1> poison <n x ty2> mask
+///   ...
+///   ...
+///   %(i - 1) = tail call <n x ty1> llvm.<umin/umax/smin/smax>(<n x ty1> %(i -
+///   3), <n x ty1> %(i - 2)
+///     OR
+///   %(i - 1) = add/mul/or/and/xor <n x ty1> %(i - 3), %(i - 2)
+///
+///   %(i) = extractelement <n x ty1> %(i - 1), 0
+/// ```
+///
+/// Where:
+///    `mask` follows a partition pattern:
+///
+/// Ex:
+///    [n = 8, p = poison]
+///
+///    4 5 6 7 | p p p p
+///    2 3 | p p p p p p
+///    1 | p p p p p p p
+///
+///    For powers of 2, there's a consistent pattern, but for other cases
+///    the parity of the current half value at each step decides the
+///    next partition half (see `ExpectedParityMask` for more logical details
+///    in generalising this).
+///
+/// Ex:
+///    [n = 6]
+///
+///    3 4 5 | p p p
+///    1 2 | p p p p
+///    1 | p p p p p
+bool VectorCombine::foldShuffleChainsToReduce(Instruction &I) {
+  // Going bottom-up for the pattern.
+  std::queue<Value *> InstWorklist;
+  InstructionCost OrigCost = 0;
+
+  // Common instruction operation after each shuffle op.
+  std::optional<unsigned int> CommonCallOp = std::nullopt;
+  std::optional<Instruction::BinaryOps> CommonBinOp = std::nullopt;
+
+  bool IsFirstCallOrBinInst = true;
+  bool ShouldBeCallOrBinInst = true;
+
+  // This stores the last used instructions for shuffle/common op.
+  //
+  // PrevVecV[0] / PrevVecV[1] store the last two simultaneous
+  // instructions from either shuffle/common op.
+  SmallVector<Value *, 2> PrevVecV(2, nullptr);
+
+  Value *VecOpEE;
+  if (!match(&I, m_ExtractElt(m_Value(VecOpEE), m_Zero())))
+    return false;
+
+  auto *FVT = dyn_cast<FixedVectorType>(VecOpEE->getType());
+  if (!FVT)
+    return false;
+
+  int64_t VecSize = FVT->getNumElements();
+  if (VecSize < 2)
+    return false;
+
+  // Number of levels would be ~log2(n), considering we always partition
+  // by half for this fold pattern.
+  unsigned int NumLevels = Log2_64_Ceil(VecSize), VisitedCnt = 0;
+  int64_t ShuffleMaskHalf = 1, ExpectedParityMask = 0;
+
+  // This is how we generalise for all element sizes.
+  // At each step, if vector size is odd, we need non-poison
+  // values to cover the dominant half so we don't miss out on any element.
+  //
+  // This mask will help us retrieve this as we go from bottom to top:
+  //
+  // Mask Set -> N = N * 2 - 1
+  // Mask Unset -> N = N * 2
+  for (int Cur = VecSize, Mask = NumLevels - 1; Cur > 1;
+       Cur = (Cur + 1) / 2, --Mask) {
+    if (Cur & 1)
+      ExpectedParityMask |= (1ll << Mask);
+  }
+
+  InstWorklist.push(VecOpEE);
+
+  while (!InstWorklist.empty()) {
+    Value *CI = InstWorklist.front();
+    InstWorklist.pop();
+
+    if (auto *II = dyn_cast<IntrinsicInst>(CI)) {
+      if (!ShouldBeCallOrBinInst)
+        return false;
+
+      if (!IsFirstCallOrBinInst &&
+          any_of(PrevVecV, [](Value *VecV) { return VecV == nullptr; }))
+        return false;
+
+      // For the first found call/bin op, the vector has to come from the
+      // extract element op.
+      if (II != (IsFirstCallOrBinInst ? VecOpEE : PrevVecV[0]))
+        return false;
+      IsFirstCallOrBinInst = false;
+
+      if (!CommonCallOp)
+        CommonCallOp = II->getIntrinsicID();
+      if (II->getIntrinsicID() != *CommonCallOp)
+        return false;
+
+      switch (II->getIntrinsicID()) {
+      case Intrinsic::umin:
+      case Intrinsic::umax:
+      case Intrinsic::smin:
+      case Intrinsic::smax: {
+        auto *Op0 = II->getOperand(0);
+        auto *Op1 = II->getOperand(1);
+        PrevVecV[0] = Op0;
+        PrevVecV[1] = Op1;
+        break;
+      }
+      default:
+        return false;
+      }
+      ShouldBeCallOrBinInst ^= 1;
+
+      IntrinsicCostAttributes ICA(
+          *CommonCallOp, II->getType(),
+          {PrevVecV[0]->getType(), PrevVecV[1]->getType()});
+      OrigCost += TTI.getIntrinsicInstrCost(ICA, CostKind);
+
+      // We may need a swap here since it can be (a, b) or (b, a)
+      // and accordingly change as we go up.
+      if (!isa<ShuffleVectorInst>(PrevVecV[1]))
+        std::swap(PrevVecV[0], PrevVecV[1]);
+      InstWorklist.push(PrevVecV[1]);
+      InstWorklist.push(PrevVecV[0]);
+    } else if (auto *BinOp = dyn_cast<BinaryOperator>(CI)) {
+      // Similar logic for bin ops.
+
+      if (!ShouldBeCallOrBinInst)
+        return false;
+
+      if (!IsFirstCallOrBinInst &&
+          any_of(PrevVecV, [](Value *VecV) { return VecV == nullptr; }))
+        return false;
+
+      if (BinOp != (IsFirstCallOrBinInst ? VecOpEE : PrevVecV[0]))
+        return false;
+      IsFirstCallOrBinInst = false;
+
+      if (!CommonBinOp)
+        CommonBinOp = BinOp->getOpcode();
+
+      if (BinOp->getOpcode() != *CommonBinOp)
+        return false;
+
+      switch (*CommonBinOp) {
+      case BinaryOperator::Add:
+      case BinaryOperator::Mul:
+      case BinaryOperator::Or:
+      case BinaryOperator::And:
+      case BinaryOperator::Xor: {
+        auto *Op0 = BinOp->getOperand(0);
+        auto *Op1 = BinOp->getOperand(1);
+        PrevVecV[0] = Op0;
+        PrevVecV[1] = Op1;
+        break;
+      }
+      default:
+        return false;
+      }
+      ShouldBeCallOrBinInst ^= 1;
+
+      OrigCost +=
+          TTI.getArithmeticInstrCost(*CommonBinOp, BinOp->getType(), CostKind);
+
+      if (!isa<ShuffleVectorInst>(PrevVecV[1]))
+        std::swap(PrevVecV[0], PrevVecV[1]);
+      InstWorklist.push(PrevVecV[1]);
+      InstWorklist.push(PrevVecV[0]);
+    } else if (auto *SVInst = dyn_cast<ShuffleVectorInst>(CI)) {
+      // We shouldn't have any null values in the previous vectors,
+      // is so, there was a mismatch in pattern.
+      if (ShouldBeCallOrBinInst ||
+          any_of(PrevVecV, [](Value *VecV) { return VecV == nullptr; }))
+        return false;
+
+      if (SVInst != PrevVecV[1])
+        return false;
+
+      ArrayRef<int> CurMask;
+      if (!match(SVInst, m_Shuffle(m_Specific(PrevVecV[0]), m_Poison(),
+                                   m_Mask(CurMask))))
+        return false;
+
+      // Subtract the parity mask when checking the condition.
+      for (int Mask = 0, MaskSize = CurMask.size(); Mask != MaskSize; ++Mask) {
+        if (Mask < ShuffleMaskHalf &&
+            CurMask[Mask] != ShuffleMaskHalf + Mask - (ExpectedParityMask & 1))
+          return false;
+        if (Mask >= ShuffleMaskHalf && CurMask[Mask] != -1)
+          return false;
+      }
+
+      // Update mask values.
+      ShuffleMaskHalf *= 2;
+      ShuffleMaskHalf -= (ExpectedParityMask & 1);
+      ExpectedParityMask >>= 1;
+
+      OrigCost += TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc,
+                                     SVInst->getType(), SVInst->getType(),
+                                     CurMask, CostKind);
+
+      VisitedCnt += 1;
+      if (!ExpectedParityMask && VisitedCnt == NumLevels)
+        break;
+
+      ShouldBeCallOrBinInst ^= 1;
+    } else {
+      return false;
+    }
+  }
+
+  // Pattern should end with a shuffle op.
+  if (ShouldBeCallOrBinInst)
+    return false;
+
+  assert(VecSize != -1 && "Expected Match for Vector Size");
+
+  Value *FinalVecV = PrevVecV[0];
+  if (!FinalVecV)
+    return false;
+
+  auto *FinalVecVTy = cast<FixedVectorType>(FinalVecV->getType());
+
+  Intrinsic::ID ReducedOp =
+      (CommonCallOp ? getMinMaxReductionIntrinsicID(*CommonCallOp)
+                    : getReductionForBinop(*CommonBinOp));
+  if (!ReducedOp)
+    return false;
+
+  IntrinsicCostAttributes ICA(ReducedOp, FinalVecVTy, {FinalVecV});
+  InstructionCost NewCost = TTI.getIntrinsicInstrCost(ICA, CostKind);
+
+  if (NewCost >= OrigCost)
+    return false;
+
+  auto *ReducedResult =
+      Builder.CreateIntrinsic(ReducedOp, {FinalVecV->getType()}, {FinalVecV});
+  replaceValue(I, *ReducedResult);
+
+  return true;
+}
+
 /// Determine if its more efficient to fold:
 ///   reduce(trunc(x)) -> trunc(reduce(x)).
 ///   reduce(sext(x))  -> sext(reduce(x)).
@@ -4223,6 +4485,9 @@ bool VectorCombine::run() {
         if (foldCastFromReductions(I))
           return true;
         break;
+      case Instruction::ExtractElement:
+        if (foldShuffleChainsToReduce(I))
+          return true;
       case Instruction::ICmp:
       case Instruction::FCmp:
         if (foldExtractExtract(I))