VPlanHelpers.h

//===- VPlanHelpers.h - VPlan-related auxiliary helpers -------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file
/// This file contains the declarations of different VPlan-related auxiliary
/// helpers.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_VECTORIZE_VPLANHELPERS_H
#define LLVM_TRANSFORMS_VECTORIZE_VPLANHELPERS_H

#include "VPlanAnalysis.h"
#include "VPlanDominatorTree.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/ModuleSlotTracker.h"
#include "llvm/Support/InstructionCost.h"

namespace llvm {

class AssumptionCache;
class BasicBlock;
class DominatorTree;
class InnerLoopVectorizer;
class IRBuilderBase;
class LoopInfo;
class SCEV;
class Type;
class VPBasicBlock;
class VPRegionBlock;
class VPlan;
class Value;

/// Returns a calculation for the total number of elements for a given \p VF.
/// For fixed width vectors this value is a constant, whereas for scalable
/// vectors it is an expression determined at runtime.
Value *getRuntimeVF(IRBuilderBase &B, Type *Ty, ElementCount VF);

/// Return a value for Step multiplied by VF.
Value *createStepForVF(IRBuilderBase &B, Type *Ty, ElementCount VF,
                       int64_t Step);

/// A helper function that returns how much we should divide the cost of a
/// predicated block by. Typically this is the reciprocal of the block
/// probability, i.e. if we return X we are assuming the predicated block will
/// execute once for every X iterations of the loop header so the block should
/// only contribute 1/X of its cost to the total cost calculation, but when
/// optimizing for code size it will just be 1 as code size costs don't depend
/// on execution probabilities.
///
/// TODO: We should use actual block probability here, if available. Currently,
///       we always assume predicated blocks have a 50% chance of executing.
inline unsigned
getPredBlockCostDivisor(TargetTransformInfo::TargetCostKind CostKind) {
  return CostKind == TTI::TCK_CodeSize ? 1 : 2;
}

/// A range of powers-of-2 vectorization factors with fixed start and
/// adjustable end. The range includes start and excludes end, e.g.,:
/// [1, 16) = {1, 2, 4, 8}
struct VFRange {
  // A power of 2.
  const ElementCount Start;

  // A power of 2. If End <= Start range is empty.
  ElementCount End;

  bool isEmpty() const {
    return End.getKnownMinValue() <= Start.getKnownMinValue();
  }

  VFRange(const ElementCount &Start, const ElementCount &End)
      : Start(Start), End(End) {
    assert(Start.isScalable() == End.isScalable() &&
           "Both Start and End should have the same scalable flag");
    assert(isPowerOf2_32(Start.getKnownMinValue()) &&
           "Expected Start to be a power of 2");
    assert(isPowerOf2_32(End.getKnownMinValue()) &&
           "Expected End to be a power of 2");
  }

  /// Iterator to iterate over vectorization factors in a VFRange.
  class iterator
      : public iterator_facade_base<iterator, std::forward_iterator_tag,
                                    ElementCount> {
    ElementCount VF;

  public:
    iterator(ElementCount VF) : VF(VF) {}

    bool operator==(const iterator &Other) const { return VF == Other.VF; }

    ElementCount operator*() const { return VF; }

    iterator &operator++() {
      VF *= 2;
      return *this;
    }
  };

  iterator begin() { return iterator(Start); }
  iterator end() {
    assert(isPowerOf2_32(End.getKnownMinValue()));
    return iterator(End);
  }
};

/// In what follows, the term "input IR" refers to code that is fed into the
/// vectorizer whereas the term "output IR" refers to code that is generated by
/// the vectorizer.

/// VPLane provides a way to access lanes in both fixed width and scalable
/// vectors, where for the latter the lane index sometimes needs calculating
/// as a runtime expression.
class VPLane {
public:
  /// Kind describes how to interpret Lane.
  enum class Kind : uint8_t {
    /// For First, Lane is the index into the first N elements of a
    /// fixed-vector <N x <ElTy>> or a scalable vector <vscale x N x <ElTy>>.
    First,
    /// For ScalableLast, Lane is the offset from the start of the last
    /// N-element subvector in a scalable vector <vscale x N x <ElTy>>. For
    /// example, a Lane of 0 corresponds to lane `(vscale - 1) * N`, a Lane of
    /// 1 corresponds to `((vscale - 1) * N) + 1`, etc.
    ScalableLast
  };

private:
  /// in [0..VF)
  unsigned Lane;

  /// Indicates how the Lane should be interpreted, as described above.
  Kind LaneKind = Kind::First;

public:
  VPLane(unsigned Lane) : Lane(Lane) {}
  VPLane(unsigned Lane, Kind LaneKind) : Lane(Lane), LaneKind(LaneKind) {}

  static VPLane getFirstLane() { return VPLane(0, VPLane::Kind::First); }

  static VPLane getLaneFromEnd(const ElementCount &VF, unsigned Offset) {
    assert(Offset > 0 && Offset <= VF.getKnownMinValue() &&
           "trying to extract with invalid offset");
    unsigned LaneOffset = VF.getKnownMinValue() - Offset;
    Kind LaneKind;
    if (VF.isScalable())
      // In this case 'LaneOffset' refers to the offset from the start of the
      // last subvector with VF.getKnownMinValue() elements.
      LaneKind = VPLane::Kind::ScalableLast;
    else
      LaneKind = VPLane::Kind::First;
    return VPLane(LaneOffset, LaneKind);
  }

  static VPLane getLastLaneForVF(const ElementCount &VF) {
    return getLaneFromEnd(VF, 1);
  }

  /// Returns a compile-time known value for the lane index and asserts if the
  /// lane can only be calculated at runtime.
  unsigned getKnownLane() const {
    assert(LaneKind == Kind::First &&
           "can only get known lane from the beginning");
    return Lane;
  }

  /// Returns an expression describing the lane index that can be used at
  /// runtime.
  Value *getAsRuntimeExpr(IRBuilderBase &Builder, const ElementCount &VF) const;

  /// Returns the Kind of lane offset.
  Kind getKind() const { return LaneKind; }

  /// Returns true if this is the first lane of the whole vector.
  bool isFirstLane() const { return Lane == 0 && LaneKind == Kind::First; }

  /// Maps the lane to a cache index based on \p VF.
  unsigned mapToCacheIndex(const ElementCount &VF) const {
    switch (LaneKind) {
    case VPLane::Kind::ScalableLast:
      assert(VF.isScalable() && Lane < VF.getKnownMinValue() &&
             "ScalableLast can only be used with scalable VFs");
      return VF.getKnownMinValue() + Lane;
    default:
      assert(Lane < VF.getKnownMinValue() &&
             "Cannot extract lane larger than VF");
      return Lane;
    }
  }
};

/// VPTransformState holds information passed down when "executing" a VPlan,
/// needed for generating the output IR.
struct VPTransformState {
  VPTransformState(const TargetTransformInfo *TTI, ElementCount VF,
                   LoopInfo *LI, DominatorTree *DT, AssumptionCache *AC,
                   IRBuilderBase &Builder, VPlan *Plan, Loop *CurrentParentLoop,
                   Type *CanonicalIVTy);
  /// Target Transform Info.
  const TargetTransformInfo *TTI;

  /// The chosen Vectorization Factor of the loop being vectorized.
  ElementCount VF;

  /// Hold the index to generate specific scalar instructions. Null indicates
  /// that all instances are to be generated, using either scalar or vector
  /// instructions.
  std::optional<VPLane> Lane;

  struct DataState {
    // Each value from the original loop, when vectorized, is represented by a
    // vector value in the map.
    DenseMap<const VPValue *, Value *> VPV2Vector;

    DenseMap<const VPValue *, SmallVector<Value *, 4>> VPV2Scalars;
  } Data;

  /// Get the generated vector Value for a given VPValue \p Def if \p IsScalar
  /// is false, otherwise return the generated scalar. \See set.
  Value *get(const VPValue *Def, bool IsScalar = false);

  /// Get the generated Value for a given VPValue and given Part and Lane.
  Value *get(const VPValue *Def, const VPLane &Lane);

  bool hasVectorValue(const VPValue *Def) {
    return Data.VPV2Vector.contains(Def);
  }

  bool hasScalarValue(const VPValue *Def, VPLane Lane) {
    auto I = Data.VPV2Scalars.find(Def);
    if (I == Data.VPV2Scalars.end())
      return false;
    unsigned CacheIdx = Lane.mapToCacheIndex(VF);
    return CacheIdx < I->second.size() && I->second[CacheIdx];
  }

  /// Set the generated vector Value for a given VPValue, if \p
  /// IsScalar is false. If \p IsScalar is true, set the scalar in lane 0.
  void set(const VPValue *Def, Value *V, bool IsScalar = false) {
    if (IsScalar) {
      set(Def, V, VPLane(0));
      return;
    }
    assert((VF.isScalar() || isVectorizedTy(V->getType())) &&
           "scalar values must be stored as (0, 0)");
    Data.VPV2Vector[Def] = V;
  }

  /// Reset an existing vector value for \p Def and a given \p Part.
  void reset(const VPValue *Def, Value *V) {
    assert(Data.VPV2Vector.contains(Def) && "need to overwrite existing value");
    Data.VPV2Vector[Def] = V;
  }

  /// Set the generated scalar \p V for \p Def and the given \p Lane.
  void set(const VPValue *Def, Value *V, const VPLane &Lane) {
    auto &Scalars = Data.VPV2Scalars[Def];
    unsigned CacheIdx = Lane.mapToCacheIndex(VF);
    if (Scalars.size() <= CacheIdx)
      Scalars.resize(CacheIdx + 1);
    assert(!Scalars[CacheIdx] && "should overwrite existing value");
    Scalars[CacheIdx] = V;
  }

  /// Reset an existing scalar value for \p Def and a given \p Lane.
  void reset(const VPValue *Def, Value *V, const VPLane &Lane) {
    auto Iter = Data.VPV2Scalars.find(Def);
    assert(Iter != Data.VPV2Scalars.end() &&
           "need to overwrite existing value");
    unsigned CacheIdx = Lane.mapToCacheIndex(VF);
    assert(CacheIdx < Iter->second.size() &&
           "need to overwrite existing value");
    Iter->second[CacheIdx] = V;
  }

  /// Set the debug location in the builder using the debug location \p DL.
  void setDebugLocFrom(DebugLoc DL);

  /// Insert the scalar value of \p Def at \p Lane into \p Lane of \p WideValue
  /// and return the resulting value.
  Value *packScalarIntoVectorizedValue(const VPValue *Def, Value *WideValue,
                                       const VPLane &Lane);

  /// Hold state information used when constructing the CFG of the output IR,
  /// traversing the VPBasicBlocks and generating corresponding IR BasicBlocks.
  struct CFGState {
    /// The previous VPBasicBlock visited. Initially set to null.
    VPBasicBlock *PrevVPBB = nullptr;

    /// The previous IR BasicBlock created or used. Initially set to the new
    /// header BasicBlock.
    BasicBlock *PrevBB = nullptr;

    /// The last IR BasicBlock in the output IR. Set to the exit block of the
    /// vector loop.
    BasicBlock *ExitBB = nullptr;

    /// A mapping of each VPBasicBlock to the corresponding BasicBlock. In case
    /// of replication, maps the BasicBlock of the last replica created.
    SmallDenseMap<const VPBasicBlock *, BasicBlock *> VPBB2IRBB;

    /// Updater for the DominatorTree.
    DomTreeUpdater DTU;

    CFGState(DominatorTree *DT)
        : DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy) {}
  } CFG;

  /// Hold a pointer to LoopInfo to register new basic blocks in the loop.
  LoopInfo *LI;

  /// Hold a pointer to AssumptionCache to register new assumptions after
  /// replicating assume calls.
  AssumptionCache *AC;

  /// Hold a reference to the IRBuilder used to generate output IR code.
  IRBuilderBase &Builder;

  /// Pointer to the VPlan code is generated for.
  VPlan *Plan;

  /// The parent loop object for the current scope, or nullptr.
  Loop *CurrentParentLoop = nullptr;

  /// VPlan-based type analysis.
  VPTypeAnalysis TypeAnalysis;

  /// VPlan-based dominator tree.
  VPDominatorTree VPDT;
};

/// Struct to hold various analysis needed for cost computations.
struct VPCostContext {
  const TargetTransformInfo &TTI;
  const TargetLibraryInfo &TLI;
  VPTypeAnalysis Types;
  LLVMContext &LLVMCtx;
  LoopVectorizationCostModel &CM;
  SmallPtrSet<Instruction *, 8> SkipCostComputation;
  TargetTransformInfo::TargetCostKind CostKind;

  VPCostContext(const TargetTransformInfo &TTI, const TargetLibraryInfo &TLI,
                Type *CanIVTy, LoopVectorizationCostModel &CM,
                TargetTransformInfo::TargetCostKind CostKind)
      : TTI(TTI), TLI(TLI), Types(CanIVTy), LLVMCtx(CanIVTy->getContext()),
        CM(CM), CostKind(CostKind) {}

  /// Return the cost for \p UI with \p VF using the legacy cost model as
  /// fallback until computing the cost of all recipes migrates to VPlan.
  InstructionCost getLegacyCost(Instruction *UI, ElementCount VF) const;

  /// Return true if the cost for \p UI shouldn't be computed, e.g. because it
  /// has already been pre-computed.
  bool skipCostComputation(Instruction *UI, bool IsVector) const;

  /// Returns the OperandInfo for \p V, if it is a live-in.
  TargetTransformInfo::OperandValueInfo getOperandInfo(VPValue *V) const;

  /// Return true if \p I is considered uniform-after-vectorization in the
  /// legacy cost model for \p VF. Only used to check for additional VPlan
  /// simplifications.
  bool isLegacyUniformAfterVectorization(Instruction *I, ElementCount VF) const;
};

/// This class can be used to assign names to VPValues. For VPValues without
/// underlying value, assign consecutive numbers and use those as names (wrapped
/// in vp<>). Otherwise, use the name from the underlying value (wrapped in
/// ir<>), appending a .V version number if there are multiple uses of the same
/// name. Allows querying names for VPValues for printing, similar to the
/// ModuleSlotTracker for IR values.
class VPSlotTracker {
  /// Keep track of versioned names assigned to VPValues with underlying IR
  /// values.
  DenseMap<const VPValue *, std::string> VPValue2Name;
  /// Keep track of the next number to use to version the base name.
  StringMap<unsigned> BaseName2Version;

  /// Number to assign to the next VPValue without underlying value.
  unsigned NextSlot = 0;

  /// Lazily created ModuleSlotTracker, used only when unnamed IR instructions
  /// require slot tracking.
  std::unique_ptr<ModuleSlotTracker> MST;

  void assignName(const VPValue *V);
  void assignNames(const VPlan &Plan);
  void assignNames(const VPBasicBlock *VPBB);
  std::string getName(const Value *V);

public:
  VPSlotTracker(const VPlan *Plan = nullptr) {
    if (Plan)
      assignNames(*Plan);
  }

  /// Returns the name assigned to \p V, if there is one, otherwise try to
  /// construct one from the underlying value, if there's one; else return
  /// <badref>.
  std::string getOrCreateName(const VPValue *V) const;
};

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// VPlanPrinter prints a given VPlan to a given output stream. The printing is
/// indented and follows the dot format.
class VPlanPrinter {
  raw_ostream &OS;
  const VPlan &Plan;
  unsigned Depth = 0;
  unsigned TabWidth = 2;
  std::string Indent;
  unsigned BID = 0;
  SmallDenseMap<const VPBlockBase *, unsigned> BlockID;

  VPSlotTracker SlotTracker;

  /// Handle indentation.
  void bumpIndent(int b) { Indent = std::string((Depth += b) * TabWidth, ' '); }

  /// Print a given \p Block of the Plan.
  void dumpBlock(const VPBlockBase *Block);

  /// Print the information related to the CFG edges going out of a given
  /// \p Block, followed by printing the successor blocks themselves.
  void dumpEdges(const VPBlockBase *Block);

  /// Print a given \p BasicBlock, including its VPRecipes, followed by printing
  /// its successor blocks.
  void dumpBasicBlock(const VPBasicBlock *BasicBlock);

  /// Print a given \p Region of the Plan.
  void dumpRegion(const VPRegionBlock *Region);

  unsigned getOrCreateBID(const VPBlockBase *Block) {
    return BlockID.count(Block) ? BlockID[Block] : BlockID[Block] = BID++;
  }

  Twine getOrCreateName(const VPBlockBase *Block);

  Twine getUID(const VPBlockBase *Block);

  /// Print the information related to a CFG edge between two VPBlockBases.
  void drawEdge(const VPBlockBase *From, const VPBlockBase *To, bool Hidden,
                const Twine &Label);

public:
  VPlanPrinter(raw_ostream &O, const VPlan &P)
      : OS(O), Plan(P), SlotTracker(&P) {}

  LLVM_DUMP_METHOD void dump();
};
#endif

} // end namespace llvm

#endif // LLVM_TRANSFORMS_VECTORIZE_VPLAN_H