llvm-project/polly/lib/Transform/DeadCodeElimination.cpp

//===- DeadCodeElimination.cpp - Eliminate dead iteration  ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The polyhedral dead code elimination pass analyses a SCoP to eliminate
// statement instances that can be proven dead.
// As a consequence, the code generated for this SCoP may execute a statement
// less often. This means, a statement may be executed only in certain loop
// iterations or it may not even be part of the generated code at all.
//
// This code:
//
//    for (i = 0; i < N; i++)
//        arr[i] = 0;
//    for (i = 0; i < N; i++)
//        arr[i] = 10;
//    for (i = 0; i < N; i++)
//        arr[i] = i;
//
// is e.g. simplified to:
//
//    for (i = 0; i < N; i++)
//        arr[i] = i;
//
// The idea and the algorithm used was first implemented by Sven Verdoolaege in
// the 'ppcg' tool.
//
//===----------------------------------------------------------------------===//

#include "polly/DependenceInfo.h"
#include "polly/LinkAllPasses.h"
#include "polly/ScopInfo.h"
#include "llvm/Support/CommandLine.h"
#include "isl/flow.h"
#include "isl/map.h"
#include "isl/set.h"
#include "isl/union_map.h"
#include "isl/union_set.h"

using namespace llvm;
using namespace polly;

namespace {

cl::opt<int> DCEPreciseSteps(
    "polly-dce-precise-steps",
    cl::desc("The number of precise steps between two approximating "
             "iterations. (A value of -1 schedules another approximation stage "
             "before the actual dead code elimination."),
    cl::ZeroOrMore, cl::init(-1));

class DeadCodeElim : public ScopPass {
public:
  static char ID;
  explicit DeadCodeElim() : ScopPass(ID) {}

  /// @brief Remove dead iterations from the schedule of @p S.
  bool runOnScop(Scop &S) override;

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

private:
  /// @brief Return the set of live iterations.
  ///
  /// The set of live iterations are all iterations that write to memory and for
  /// which we can not prove that there will be a later write that _must_
  /// overwrite the same memory location and is consequently the only one that
  /// is visible after the execution of the SCoP.
  ///
  isl_union_set *getLiveOut(Scop &S);
  bool eliminateDeadCode(Scop &S, int PreciseSteps);
};
}

char DeadCodeElim::ID = 0;

// To compute the live outs, we compute for the data-locations that are
// must-written to the last statement that touches these locations. On top of
// this we add all statements that perform may-write accesses.
//
// We could be more precise by removing may-write accesses for which we know
// that they are overwritten by a must-write after. However, at the moment the
// only may-writes we introduce access the full (unbounded) array, such that
// bounded write accesses can not overwrite all of the data-locations. As
// this means may-writes are in the current situation always live, there is
// no point in trying to remove them from the live-out set.
isl_union_set *DeadCodeElim::getLiveOut(Scop &S) {
  isl_union_map *Schedule = S.getSchedule();
  isl_union_map *WriteIterations = isl_union_map_reverse(S.getMustWrites());
  isl_union_map *WriteTimes =
      isl_union_map_apply_range(WriteIterations, isl_union_map_copy(Schedule));

  isl_union_map *LastWriteTimes = isl_union_map_lexmax(WriteTimes);
  isl_union_map *LastWriteIterations = isl_union_map_apply_range(
      LastWriteTimes, isl_union_map_reverse(Schedule));

  isl_union_set *Live = isl_union_map_range(LastWriteIterations);
  Live = isl_union_set_union(Live, isl_union_map_domain(S.getMayWrites()));
  return isl_union_set_coalesce(Live);
}

/// Performs polyhedral dead iteration elimination by:
/// o Assuming that the last write to each location is live.
/// o Following each RAW dependency from a live iteration backwards and adding
///   that iteration to the live set.
///
/// To ensure the set of live iterations does not get too complex we always
/// combine a certain number of precise steps with one approximating step that
/// simplifies the life set with an affine hull.
bool DeadCodeElim::eliminateDeadCode(Scop &S, int PreciseSteps) {
  DependenceInfo &DI = getAnalysis<DependenceInfo>();
  const Dependences &D = DI.getDependences();

  if (!D.hasValidDependences())
    return false;

  isl_union_set *Live = getLiveOut(S);
  isl_union_map *Dep =
      D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED);
  Dep = isl_union_map_reverse(Dep);

  if (PreciseSteps == -1)
    Live = isl_union_set_affine_hull(Live);

  isl_union_set *OriginalDomain = S.getDomains();
  int Steps = 0;
  while (true) {
    isl_union_set *Extra;
    Steps++;

    Extra =
        isl_union_set_apply(isl_union_set_copy(Live), isl_union_map_copy(Dep));

    if (isl_union_set_is_subset(Extra, Live)) {
      isl_union_set_free(Extra);
      break;
    }

    Live = isl_union_set_union(Live, Extra);

    if (Steps > PreciseSteps) {
      Steps = 0;
      Live = isl_union_set_affine_hull(Live);
    }

    Live = isl_union_set_intersect(Live, isl_union_set_copy(OriginalDomain));
  }
  isl_union_map_free(Dep);
  isl_union_set_free(OriginalDomain);

  bool Changed = S.restrictDomains(isl_union_set_coalesce(Live));

  // FIXME: We can probably avoid the recomputation of all dependences by
  // updating them explicitly.
  if (Changed)
    DI.recomputeDependences();
  return Changed;
}

bool DeadCodeElim::runOnScop(Scop &S) {
  return eliminateDeadCode(S, DCEPreciseSteps);
}

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

Pass *polly::createDeadCodeElimPass() { return new DeadCodeElim(); }

INITIALIZE_PASS_BEGIN(DeadCodeElim, "polly-dce",
                      "Polly - Remove dead iterations", false, false)
INITIALIZE_PASS_DEPENDENCY(DependenceInfo)
INITIALIZE_PASS_DEPENDENCY(ScopInfo)
INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
                    false, false)