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//===- 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/Dependences.h"
#include "polly/LinkAllPasses.h"
#include "polly/ScopInfo.h"
#include "llvm/Support/CommandLine.h"
#include "isl/flow.h"
#include "isl/set.h"
#include "isl/map.h"
#include "isl/union_map.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) {}
virtual bool runOnScop(Scop &S);
void printScop(llvm::raw_ostream &OS) const;
void getAnalysisUsage(AnalysisUsage &AU) const;
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 first assume all must and may-writes are exposed
// and then subtract the set of statements that are definitely overwritten.
isl_union_set *DeadCodeElim::getLiveOut(Scop &S) {
isl_union_map *Kills = S.getMustWrites();
isl_union_map *Empty = isl_union_map_empty(S.getParamSpace());
isl_union_map *Covering;
isl_union_map *Writes = S.getWrites();
isl_union_map_compute_flow(Kills, Empty, isl_union_map_copy(Writes),
S.getSchedule(), NULL, &Covering, NULL, NULL);
isl_union_map *Exposed = Writes;
Exposed =
isl_union_map_subtract_domain(Exposed, isl_union_map_domain(Covering));
return isl_union_map_domain(Exposed);
}
/// 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) {
Dependences *D = &getAnalysis<Dependences>();
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);
return S.restrictDomains(isl_union_set_coalesce(Live));
}
bool DeadCodeElim::runOnScop(Scop &S) {
return eliminateDeadCode(S, DCEPreciseSteps);
}
void DeadCodeElim::printScop(raw_ostream &OS) const {}
void DeadCodeElim::getAnalysisUsage(AnalysisUsage &AU) const {
ScopPass::getAnalysisUsage(AU);
AU.addRequired<Dependences>();
}
Pass *polly::createDeadCodeElimPass() { return new DeadCodeElim(); }
INITIALIZE_PASS_BEGIN(DeadCodeElim, "polly-dce",
"Polly - Remove dead iterations", false, false)
INITIALIZE_PASS_DEPENDENCY(Dependences)
INITIALIZE_PASS_DEPENDENCY(ScopInfo)
INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
false, false)
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