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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/DependenceInfo.h"
#include "polly/LinkAllPasses.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::ZeroOrMore, cl::init(-1), cl::cat(PollyCategory));
class DeadCodeElim : public ScopPass {
public:
static char ID;
explicit DeadCodeElim() : ScopPass(ID) {}
/// Remove dead iterations from the schedule of @p S.
bool runOnScop(Scop &S) override;
/// Register all analyses and transformation required.
void getAnalysisUsage(AnalysisUsage &AU) const override;
private:
/// 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);
};
} // namespace
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 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.
bool DeadCodeElim::eliminateDeadCode(Scop &S, int PreciseSteps) {
DependenceInfo &DI = getAnalysis<DependenceInfo>();
const Dependences &D = DI.getDependences(Dependences::AL_Statement);
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();
bool Changed = S.restrictDomains(Live);
// FIXME: We can probably avoid the recomputation of all dependences by
// updating them explicitly.
if (Changed)
DI.recomputeDependences(Dependences::AL_Statement);
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(ScopInfoRegionPass)
INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
false, false)
|