lib/Transform/MaximalStaticExpansion.cpp

//===- MaximalStaticExpansion.cpp -----------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This pass fully expand the memory accesses of a Scop to get rid of
// dependencies.
//
//===----------------------------------------------------------------------===//

#include "polly/DependenceInfo.h"
#include "polly/LinkAllPasses.h"
#include "polly/ScopInfo.h"
#include "polly/ScopPass.h"
#include "polly/Support/ISLTools.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "isl/isl-noexceptions.h"
#include "isl/union_map.h"
#include <cassert>
#include <limits>
#include <string>
#include <vector>

using namespace llvm;
using namespace polly;

#define DEBUG_TYPE "polly-mse"

namespace {

class MaximalStaticExpander : public ScopPass {
public:
  static char ID;

  explicit MaximalStaticExpander() : ScopPass(ID) {}

  ~MaximalStaticExpander() override = default;

  /// Expand the accesses of the SCoP.
  ///
  /// @param S The SCoP that must be expanded.
  bool runOnScop(Scop &S) override;

  /// Print the SCoP.
  ///
  /// @param OS The stream where to print.
  /// @param S The SCop that must be printed.
  void printScop(raw_ostream &OS, Scop &S) const override;

  /// Register all analyses and transformations required.
  void getAnalysisUsage(AnalysisUsage &AU) const override;

private:
  /// OptimizationRemarkEmitter object for displaying diagnostic remarks.
  OptimizationRemarkEmitter *ORE;

  /// Emit remark
  void emitRemark(StringRef Msg, Instruction *Inst);

  /// Return true if the SAI in parameter is expandable.
  ///
  /// @param SAI the SAI that need to be checked.
  /// @param Writes A set that will contains all the write accesses.
  /// @param Reads A set that will contains all the read accesses.
  /// @param S The SCop in which the SAI is in.
  /// @param Dependences The RAW dependences of the SCop.
  bool isExpandable(const ScopArrayInfo *SAI,
                    SmallPtrSetImpl<MemoryAccess *> &Writes,
                    SmallPtrSetImpl<MemoryAccess *> &Reads, Scop &S,
                    const isl::union_map &Dependences);

  /// Expand the MemoryAccess according to its domain.
  ///
  /// @param S The SCop in which the memory access appears in.
  /// @param MA The memory access that need to be expanded.
  ScopArrayInfo *expandAccess(Scop &S, MemoryAccess *MA);

  /// Filter the dependences to have only one related to current memory access.
  ///
  /// @param S The SCop in which the memory access appears in.
  /// @param MapDependences The dependences to filter.
  /// @param MA The memory access that need to be expanded.
  isl::union_map filterDependences(Scop &S,
                                   const isl::union_map &MapDependences,
                                   MemoryAccess *MA);

  /// Expand the MemoryAccess according to Dependences and already expanded
  /// MemoryAccesses.
  ///
  /// @param The SCop in which the memory access appears in.
  /// @param The memory access that need to be expanded.
  /// @param Dependences The RAW dependences of the SCop.
  /// @param ExpandedSAI The expanded SAI created during write expansion.
  /// @param Reverse if true, the Dependences union_map is reversed before
  /// intersection.
  void mapAccess(Scop &S, SmallPtrSetImpl<MemoryAccess *> &Accesses,
                 const isl::union_map &Dependences, ScopArrayInfo *ExpandedSAI,
                 bool Reverse);

  /// Expand PHI memory accesses.
  ///
  /// @param The SCop in which the memory access appears in.
  /// @param The ScopArrayInfo representing the PHI accesses to expand.
  /// @param Dependences The RAW dependences of the SCop.
  void expandPhi(Scop &S, const ScopArrayInfo *SAI,
                 const isl::union_map &Dependences);
};
} // namespace

#ifndef NDEBUG
/// Whether a dimension of a set is bounded (lower and upper) by a constant,
/// i.e. there are two constants Min and Max, such that every value x of the
/// chosen dimensions is Min <= x <= Max.
static bool isDimBoundedByConstant(isl::set Set, unsigned dim) {
  auto ParamDims = Set.dim(isl::dim::param);
  Set = Set.project_out(isl::dim::param, 0, ParamDims);
  Set = Set.project_out(isl::dim::set, 0, dim);
  auto SetDims = Set.dim(isl::dim::set);
  Set = Set.project_out(isl::dim::set, 1, SetDims - 1);
  return bool(Set.is_bounded());
}
#endif

char MaximalStaticExpander::ID = 0;

isl::union_map MaximalStaticExpander::filterDependences(
    Scop &S, const isl::union_map &Dependences, MemoryAccess *MA) {
  auto SAI = MA->getLatestScopArrayInfo();

  auto AccessDomainSet = MA->getAccessRelation().domain();
  auto AccessDomainId = AccessDomainSet.get_tuple_id();

  isl::union_map MapDependences = isl::union_map::empty(S.getParamSpace());

  for (isl::map Map : Dependences.get_map_list()) {
    // Filter out Statement to Statement dependences.
    if (!Map.can_curry())
      continue;

    // Intersect with the relevant SAI.
    auto TmpMapDomainId =
        Map.get_space().domain().unwrap().range().get_tuple_id(isl::dim::set);

    ScopArrayInfo *UserSAI =
        static_cast<ScopArrayInfo *>(TmpMapDomainId.get_user());

    if (SAI != UserSAI)
      continue;

    // Get the correct S1[] -> S2[] dependence.
    auto NewMap = Map.factor_domain();
    auto NewMapDomainId = NewMap.domain().get_tuple_id();

    if (AccessDomainId.get() != NewMapDomainId.get())
      continue;

    // Add the corresponding map to MapDependences.
    MapDependences = MapDependences.add_map(NewMap);
  }

  return MapDependences;
}

bool MaximalStaticExpander::isExpandable(
    const ScopArrayInfo *SAI, SmallPtrSetImpl<MemoryAccess *> &Writes,
    SmallPtrSetImpl<MemoryAccess *> &Reads, Scop &S,
    const isl::union_map &Dependences) {
  if (SAI->isValueKind()) {
    Writes.insert(S.getValueDef(SAI));
    for (auto MA : S.getValueUses(SAI))
      Reads.insert(MA);
    return true;
  } else if (SAI->isPHIKind()) {
    auto Read = S.getPHIRead(SAI);

    auto StmtDomain = isl::union_set(Read->getStatement()->getDomain());

    auto Writes = S.getPHIIncomings(SAI);

    // Get the domain where all the writes are writing to.
    auto WriteDomain = isl::union_set::empty(S.getParamSpace());

    for (auto Write : Writes) {
      auto MapDeps = filterDependences(S, Dependences, Write);
      for (isl::map Map : MapDeps.get_map_list())
        WriteDomain = WriteDomain.add_set(Map.range());
    }

    // For now, read from original scalar is not possible.
    if (!StmtDomain.is_equal(WriteDomain)) {
      emitRemark(SAI->getName() + " read from its original value.",
                 Read->getAccessInstruction());
      return false;
    }

    return true;
  } else if (SAI->isExitPHIKind()) {
    // For now, we are not able to expand ExitPhi.
    emitRemark(SAI->getName() + " is a ExitPhi node.",
               S.getEnteringBlock()->getFirstNonPHI());
    return false;
  }

  int NumberWrites = 0;
  for (ScopStmt &Stmt : S) {
    auto StmtReads = isl::union_map::empty(S.getParamSpace());
    auto StmtWrites = isl::union_map::empty(S.getParamSpace());

    for (MemoryAccess *MA : Stmt) {
      // Check if the current MemoryAccess involved the current SAI.
      if (SAI != MA->getLatestScopArrayInfo())
        continue;

      // For now, we are not able to expand array where read come after write
      // (to the same location) in a same statement.
      auto AccRel = isl::union_map(MA->getAccessRelation());
      if (MA->isRead()) {
        // Reject load after store to same location.
        if (!StmtWrites.is_disjoint(AccRel)) {
          emitRemark(SAI->getName() + " has read after write to the same "
                                      "element in same statement. The "
                                      "dependences found during analysis may "
                                      "be wrong because Polly is not able to "
                                      "handle such case for now.",
                     MA->getAccessInstruction());
          return false;
        }

        StmtReads = StmtReads.unite(AccRel);
      } else {
        StmtWrites = StmtWrites.unite(AccRel);
      }

      // For now, we are not able to expand MayWrite.
      if (MA->isMayWrite()) {
        emitRemark(SAI->getName() + " has a maywrite access.",
                   MA->getAccessInstruction());
        return false;
      }

      // For now, we are not able to expand SAI with more than one write.
      if (MA->isMustWrite()) {
        Writes.insert(MA);
        NumberWrites++;
        if (NumberWrites > 1) {
          emitRemark(SAI->getName() + " has more than 1 write access.",
                     MA->getAccessInstruction());
          return false;
        }
      }

      // Check if it is possible to expand this read.
      if (MA->isRead()) {
        // Get the domain of the current ScopStmt.
        auto StmtDomain = Stmt.getDomain();

        // Get the domain of the future Read access.
        auto ReadDomainSet = MA->getAccessRelation().domain();
        auto ReadDomain = isl::union_set(ReadDomainSet);

        // Get the dependences relevant for this MA
        auto MapDependences = filterDependences(S, Dependences.reverse(), MA);
        unsigned NumberElementMap = isl_union_map_n_map(MapDependences.get());

        if (NumberElementMap == 0) {
          emitRemark("The expansion of " + SAI->getName() +
                         " would lead to a read from the original array.",
                     MA->getAccessInstruction());
          return false;
        }

        auto DepsDomain = MapDependences.domain();

        // If there are multiple maps in the Deps, we cannot handle this case
        // for now.
        if (NumberElementMap != 1) {
          emitRemark(SAI->getName() +
                         " has too many dependences to be handle for now.",
                     MA->getAccessInstruction());
          return false;
        }

        auto DepsDomainSet = isl::set(DepsDomain);

        // For now, read from the original array is not possible.
        if (!StmtDomain.is_subset(DepsDomainSet)) {
          emitRemark("The expansion of " + SAI->getName() +
                         " would lead to a read from the original array.",
                     MA->getAccessInstruction());
          return false;
        }

        Reads.insert(MA);
      }
    }
  }

  // No need to expand SAI with no write.
  if (NumberWrites == 0) {
    emitRemark(SAI->getName() + " has 0 write access.",
               S.getEnteringBlock()->getFirstNonPHI());
    return false;
  }

  return true;
}

void MaximalStaticExpander::mapAccess(Scop &S,
                                      SmallPtrSetImpl<MemoryAccess *> &Accesses,
                                      const isl::union_map &Dependences,
                                      ScopArrayInfo *ExpandedSAI,
                                      bool Reverse) {
  for (auto MA : Accesses) {
    // Get the current AM.
    auto CurrentAccessMap = MA->getAccessRelation();

    // Get RAW dependences for the current WA.
    auto DomainSet = MA->getAccessRelation().domain();
    auto Domain = isl::union_set(DomainSet);

    // Get the dependences relevant for this MA.
    isl::union_map MapDependences =
        filterDependences(S, Reverse ? Dependences.reverse() : Dependences, MA);

    // If no dependences, no need to modify anything.
    if (MapDependences.is_empty())
      return;

    assert(isl_union_map_n_map(MapDependences.get()) == 1 &&
           "There are more than one RAW dependencies in the union map.");
    auto NewAccessMap = isl::map::from_union_map(MapDependences);

    auto Id = ExpandedSAI->getBasePtrId();

    // Replace the out tuple id with the one of the access array.
    NewAccessMap = NewAccessMap.set_tuple_id(isl::dim::out, Id);

    // Set the new access relation.
    MA->setNewAccessRelation(NewAccessMap);
  }
}

ScopArrayInfo *MaximalStaticExpander::expandAccess(Scop &S, MemoryAccess *MA) {
  // Get the current AM.
  auto CurrentAccessMap = MA->getAccessRelation();

  unsigned in_dimensions = CurrentAccessMap.dim(isl::dim::in);

  // Get domain from the current AM.
  auto Domain = CurrentAccessMap.domain();

  // Create a new AM from the domain.
  auto NewAccessMap = isl::map::from_domain(Domain);

  // Add dimensions to the new AM according to the current in_dim.
  NewAccessMap = NewAccessMap.add_dims(isl::dim::out, in_dimensions);

  // Create the string representing the name of the new SAI.
  // One new SAI for each statement so that each write go to a different memory
  // cell.
  auto CurrentStmtDomain = MA->getStatement()->getDomain();
  auto CurrentStmtName = CurrentStmtDomain.get_tuple_name();
  auto CurrentOutId = CurrentAccessMap.get_tuple_id(isl::dim::out);
  std::string CurrentOutIdString =
      MA->getScopArrayInfo()->getName() + "_" + CurrentStmtName + "_expanded";

  // Set the tuple id for the out dimension.
  NewAccessMap = NewAccessMap.set_tuple_id(isl::dim::out, CurrentOutId);

  // Create the size vector.
  std::vector<unsigned> Sizes;
  for (unsigned i = 0; i < in_dimensions; i++) {
    assert(isDimBoundedByConstant(CurrentStmtDomain, i) &&
           "Domain boundary are not constant.");
    auto UpperBound = getConstant(CurrentStmtDomain.dim_max(i), true, false);
    assert(!UpperBound.is_null() && UpperBound.is_pos() &&
           !UpperBound.is_nan() &&
           "The upper bound is not a positive integer.");
    assert(UpperBound.le(isl::val(CurrentAccessMap.get_ctx(),
                                  std::numeric_limits<int>::max() - 1)) &&
           "The upper bound overflow a int.");
    Sizes.push_back(UpperBound.get_num_si() + 1);
  }

  // Get the ElementType of the current SAI.
  auto ElementType = MA->getLatestScopArrayInfo()->getElementType();

  // Create (or get if already existing) the new expanded SAI.
  auto ExpandedSAI =
      S.createScopArrayInfo(ElementType, CurrentOutIdString, Sizes);
  ExpandedSAI->setIsOnHeap(true);

  // Get the out Id of the expanded Array.
  auto NewOutId = ExpandedSAI->getBasePtrId();

  // Set the out id of the new AM to the new SAI id.
  NewAccessMap = NewAccessMap.set_tuple_id(isl::dim::out, NewOutId);

  // Add constraints to linked output with input id.
  auto SpaceMap = NewAccessMap.get_space();
  auto ConstraintBasicMap =
      isl::basic_map::equal(SpaceMap, SpaceMap.dim(isl::dim::in));
  NewAccessMap = isl::map(ConstraintBasicMap);

  // Set the new access relation map.
  MA->setNewAccessRelation(NewAccessMap);

  return ExpandedSAI;
}

void MaximalStaticExpander::expandPhi(Scop &S, const ScopArrayInfo *SAI,
                                      const isl::union_map &Dependences) {
  SmallPtrSet<MemoryAccess *, 4> Writes;
  for (auto MA : S.getPHIIncomings(SAI))
    Writes.insert(MA);
  auto Read = S.getPHIRead(SAI);
  auto ExpandedSAI = expandAccess(S, Read);

  mapAccess(S, Writes, Dependences, ExpandedSAI, false);
}

void MaximalStaticExpander::emitRemark(StringRef Msg, Instruction *Inst) {
  ORE->emit(OptimizationRemarkAnalysis(DEBUG_TYPE, "ExpansionRejection", Inst)
            << Msg);
}

bool MaximalStaticExpander::runOnScop(Scop &S) {
  // Get the ORE from OptimizationRemarkEmitterWrapperPass.
  ORE = &(getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE());

  // Get the RAW Dependences.
  auto &DI = getAnalysis<DependenceInfo>();
  auto &D = DI.getDependences(Dependences::AL_Reference);
  isl::union_map Dependences = D.getDependences(Dependences::TYPE_RAW);

  SmallVector<ScopArrayInfo *, 4> CurrentSAI(S.arrays().begin(),
                                             S.arrays().end());

  for (auto SAI : CurrentSAI) {
    SmallPtrSet<MemoryAccess *, 4> AllWrites;
    SmallPtrSet<MemoryAccess *, 4> AllReads;
    if (!isExpandable(SAI, AllWrites, AllReads, S, Dependences))
      continue;

    if (SAI->isValueKind() || SAI->isArrayKind()) {
      assert(AllWrites.size() == 1 || SAI->isValueKind());

      auto TheWrite = *(AllWrites.begin());
      ScopArrayInfo *ExpandedArray = expandAccess(S, TheWrite);

      mapAccess(S, AllReads, Dependences, ExpandedArray, true);
    } else if (SAI->isPHIKind()) {
      expandPhi(S, SAI, Dependences);
    }
  }

  return false;
}

void MaximalStaticExpander::printScop(raw_ostream &OS, Scop &S) const {
  S.print(OS, false);
}

void MaximalStaticExpander::getAnalysisUsage(AnalysisUsage &AU) const {
  ScopPass::getAnalysisUsage(AU);
  AU.addRequired<DependenceInfo>();
  AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
}

Pass *polly::createMaximalStaticExpansionPass() {
  return new MaximalStaticExpander();
}

INITIALIZE_PASS_BEGIN(MaximalStaticExpander, "polly-mse",
                      "Polly - Maximal static expansion of SCoP", false, false);
INITIALIZE_PASS_DEPENDENCY(DependenceInfo);
INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass);
INITIALIZE_PASS_END(MaximalStaticExpander, "polly-mse",
                    "Polly - Maximal static expansion of SCoP", false, false)