DeadCodeElimination.cpp

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//===- DeadCodeElimination.cpp - Eliminate dead iteration  ----------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// 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/DeadCodeElimination.h"
#include "polly/DependenceInfo.h"
#include "polly/Options.h"
#include "polly/ScopInfo.h"
#include "llvm/Support/CommandLine.h"
#include "isl/isl-noexceptions.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::init(-1), cl::cat(PollyCategory));
 
/// 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.
///
/// 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.
static isl::union_set getLiveOut(Scop &S) {
  isl::union_map Schedule = S.getSchedule();
  isl::union_map MustWrites = S.getMustWrites();
  isl::union_map WriteIterations = MustWrites.reverse();
  isl::union_map WriteTimes = WriteIterations.apply_range(Schedule);
 
  isl::union_map LastWriteTimes = WriteTimes.lexmax();
  isl::union_map LastWriteIterations =
      LastWriteTimes.apply_range(Schedule.reverse());
 
  isl::union_set Live = LastWriteIterations.range();
  isl::union_map MayWrites = S.getMayWrites();
  Live = Live.unite(MayWrites.domain());
  return Live.coalesce();
}
 
/// 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.
static bool runDeadCodeElimination(Scop &S, int PreciseSteps,
                                   const Dependences &D) {
  if (!D.hasValidDependences())
    return false;
 
  isl::union_set Live = getLiveOut(S);
  isl::union_map Dep =
      D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED);
  Dep = Dep.reverse();
 
  if (PreciseSteps == -1)
    Live = Live.affine_hull();
 
  isl::union_set OriginalDomain = S.getDomains();
  int Steps = 0;
  while (true) {
    Steps++;
 
    isl::union_set Extra = Live.apply(Dep);
 
    if (Extra.is_subset(Live))
      break;
 
    Live = Live.unite(Extra);
 
    if (Steps > PreciseSteps) {
      Steps = 0;
      Live = Live.affine_hull();
    }
 
    Live = Live.intersect(OriginalDomain);
  }
 
  Live = Live.coalesce();
 
  return S.restrictDomains(Live);
}
 
} // namespace
 
bool polly::runDeadCodeElim(Scop &S, DependenceAnalysis::Result &DA) {
  const Dependences &Deps = DA.getDependences(Dependences::AL_Statement);
 
  bool Changed = runDeadCodeElimination(S, DCEPreciseSteps, Deps);
 
  // FIXME: We can probably avoid the recomputation of all dependences by
  // updating them explicitly.
  if (Changed)
    DA.recomputeDependences(Dependences::AL_Statement);
 
  return Changed;
}