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

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//===------ Simplify.cpp ----------------------------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Simplify a SCoP by removing unnecessary statements and accesses.
//
//===----------------------------------------------------------------------===//
#include "polly/Simplify.h"
#include "polly/ScopInfo.h"
#include "polly/ScopPass.h"
#include "polly/Support/GICHelper.h"
#include "polly/Support/ISLOStream.h"
#include "polly/Support/VirtualInstruction.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#define DEBUG_TYPE "polly-simplify"
using namespace llvm;
using namespace polly;
namespace {
STATISTIC(ScopsProcessed, "Number of SCoPs processed");
STATISTIC(ScopsModified, "Number of SCoPs simplified");
STATISTIC(PairUnequalAccRels, "Number of Load-Store pairs NOT removed because "
"of different access relations");
STATISTIC(InBetweenStore, "Number of Load-Store pairs NOT removed because "
"there is another store between them");
STATISTIC(TotalOverwritesRemoved, "Number of removed overwritten writes");
STATISTIC(TotalRedundantWritesRemoved,
"Number of writes of same value removed in any SCoP");
STATISTIC(TotalEmptyPartialAccessesRemoved,
"Number of empty partial accesses removed");
STATISTIC(TotalDeadAccessesRemoved, "Number of dead accesses removed");
STATISTIC(TotalDeadInstructionsRemoved,
"Number of unused instructions removed");
STATISTIC(TotalStmtsRemoved, "Number of statements removed in any SCoP");
static bool isImplicitRead(MemoryAccess *MA) {
return MA->isRead() && MA->isOriginalScalarKind();
}
static bool isExplicitAccess(MemoryAccess *MA) {
return MA->isOriginalArrayKind();
}
static bool isImplicitWrite(MemoryAccess *MA) {
return MA->isWrite() && MA->isOriginalScalarKind();
}
/// Return a vector that contains MemoryAccesses in the order in
/// which they are executed.
///
/// The order is:
/// - Implicit reads (BlockGenerator::generateScalarLoads)
/// - Explicit reads and writes (BlockGenerator::generateArrayLoad,
/// BlockGenerator::generateArrayStore)
/// - In block statements, the accesses are in order in which their
/// instructions are executed.
/// - In region statements, that order of execution is not predictable at
/// compile-time.
/// - Implicit writes (BlockGenerator::generateScalarStores)
/// The order in which implicit writes are executed relative to each other is
/// undefined.
static SmallVector<MemoryAccess *, 32> getAccessesInOrder(ScopStmt &Stmt) {
SmallVector<MemoryAccess *, 32> Accesses;
for (MemoryAccess *MemAcc : Stmt)
if (isImplicitRead(MemAcc))
Accesses.push_back(MemAcc);
for (MemoryAccess *MemAcc : Stmt)
if (isExplicitAccess(MemAcc))
Accesses.push_back(MemAcc);
for (MemoryAccess *MemAcc : Stmt)
if (isImplicitWrite(MemAcc))
Accesses.push_back(MemAcc);
return Accesses;
}
class Simplify : public ScopPass {
private:
/// The last/current SCoP that is/has been processed.
Scop *S;
/// Number of writes that are overwritten anyway.
int OverwritesRemoved = 0;
/// Number of redundant writes removed from this SCoP.
int RedundantWritesRemoved = 0;
/// Number of writes with empty access domain removed.
int EmptyPartialAccessesRemoved = 0;
/// Number of unused accesses removed from this SCoP.
int DeadAccessesRemoved = 0;
/// Number of unused instructions removed from this SCoP.
int DeadInstructionsRemoved = 0;
/// Number of unnecessary statements removed from the SCoP.
int StmtsRemoved = 0;
/// Return whether at least one simplification has been applied.
bool isModified() const {
return OverwritesRemoved > 0 || RedundantWritesRemoved > 0 ||
EmptyPartialAccessesRemoved > 0 || DeadAccessesRemoved > 0 ||
DeadInstructionsRemoved > 0 || StmtsRemoved > 0;
}
MemoryAccess *getReadAccessForValue(ScopStmt *Stmt, llvm::Value *Val) {
if (!isa<Instruction>(Val))
return nullptr;
for (auto *MA : *Stmt) {
if (!MA->isRead())
continue;
if (MA->getAccessValue() != Val)
continue;
return MA;
}
return nullptr;
}
/// Return a write access that occurs between @p From and @p To.
///
/// In region statements the order is ignored because we cannot predict it.
///
/// @param Stmt Statement of both writes.
/// @param From Start looking after this access.
/// @param To Stop looking at this access, with the access itself.
/// @param Targets Look for an access that may wrote to one of these elements.
///
/// @return A write access between @p From and @p To that writes to at least
/// one element in @p Targets.
MemoryAccess *hasWriteBetween(ScopStmt *Stmt, MemoryAccess *From,
MemoryAccess *To, isl::map Targets) {
auto TargetsSpace = Targets.get_space();
bool Started = Stmt->isRegionStmt();
auto Accesses = getAccessesInOrder(*Stmt);
for (auto *Acc : Accesses) {
if (Acc->isLatestScalarKind())
continue;
if (Stmt->isBlockStmt() && From == Acc) {
assert(!Started);
Started = true;
continue;
}
if (Stmt->isBlockStmt() && To == Acc) {
assert(Started);
return nullptr;
}
if (!Started)
continue;
if (!Acc->isWrite())
continue;
isl::map AccRel = Acc->getAccessRelation();
auto AccRelSpace = AccRel.get_space();
// Spaces being different means that they access different arrays.
if (!TargetsSpace.has_equal_tuples(AccRelSpace))
continue;
AccRel = AccRel.intersect_domain(give(Acc->getStatement()->getDomain()));
AccRel = AccRel.intersect_params(give(S->getContext()));
auto CommonElt = Targets.intersect(AccRel);
if (!CommonElt.is_empty())
return Acc;
}
assert(Stmt->isRegionStmt() &&
"To must be encountered in block statements");
return nullptr;
}
/// Remove writes that are overwritten unconditionally later in the same
/// statement.
///
/// There must be no read of the same value between the write (that is to be
/// removed) and the overwrite.
void removeOverwrites() {
for (auto &Stmt : *S) {
auto Domain = give(Stmt.getDomain());
isl::union_map WillBeOverwritten =
isl::union_map::empty(give(S->getParamSpace()));
SmallVector<MemoryAccess *, 32> Accesses(getAccessesInOrder(Stmt));
// Iterate in reverse order, so the overwrite comes before the write that
// is to be removed.
for (auto *MA : reverse(Accesses)) {
// In region statements, the explicit accesses can be in blocks that are
// can be executed in any order. We therefore process only the implicit
// writes and stop after that.
if (Stmt.isRegionStmt() && isExplicitAccess(MA))
break;
auto AccRel = MA->getAccessRelation();
AccRel = AccRel.intersect_domain(Domain);
AccRel = AccRel.intersect_params(give(S->getContext()));
// If a value is read in-between, do not consider it as overwritten.
if (MA->isRead()) {
WillBeOverwritten = WillBeOverwritten.subtract(AccRel);
continue;
}
// If all of a write's elements are overwritten, remove it.
isl::union_map AccRelUnion = AccRel;
if (AccRelUnion.is_subset(WillBeOverwritten)) {
DEBUG(dbgs() << "Removing " << MA
<< " which will be overwritten anyway\n");
Stmt.removeSingleMemoryAccess(MA);
OverwritesRemoved++;
TotalOverwritesRemoved++;
}
// Unconditional writes overwrite other values.
if (MA->isMustWrite())
WillBeOverwritten = WillBeOverwritten.add_map(AccRel);
}
}
}
/// Remove writes that just write the same value already stored in the
/// element.
void removeRedundantWrites() {
// Delay actual removal to not invalidate iterators.
SmallVector<MemoryAccess *, 8> StoresToRemove;
for (auto &Stmt : *S) {
for (auto *WA : Stmt) {
if (!WA->isMustWrite())
continue;
if (!WA->isLatestArrayKind())
continue;
if (!isa<StoreInst>(WA->getAccessInstruction()) && !WA->isPHIKind())
continue;
llvm::Value *ReadingValue = WA->tryGetValueStored();
if (!ReadingValue)
continue;
auto RA = getReadAccessForValue(&Stmt, ReadingValue);
if (!RA)
continue;
if (!RA->isLatestArrayKind())
continue;
auto WARel = WA->getLatestAccessRelation();
WARel = WARel.intersect_domain(give(WA->getStatement()->getDomain()));
WARel = WARel.intersect_params(give(S->getContext()));
auto RARel = RA->getLatestAccessRelation();
RARel = RARel.intersect_domain(give(RA->getStatement()->getDomain()));
RARel = RARel.intersect_params(give(S->getContext()));
if (!RARel.is_equal(WARel)) {
PairUnequalAccRels++;
DEBUG(dbgs() << "Not cleaning up " << WA
<< " because of unequal access relations:\n");
DEBUG(dbgs() << " RA: " << RARel << "\n");
DEBUG(dbgs() << " WA: " << WARel << "\n");
continue;
}
if (auto *Conflicting = hasWriteBetween(&Stmt, RA, WA, WARel)) {
(void)Conflicting;
InBetweenStore++;
DEBUG(dbgs() << "Not cleaning up " << WA
<< " because there is another store to the same element "
"between\n");
DEBUG(Conflicting->print(dbgs()));
continue;
}
StoresToRemove.push_back(WA);
}
}
for (auto *WA : StoresToRemove) {
auto Stmt = WA->getStatement();
auto AccRel = WA->getAccessRelation();
auto AccVal = WA->getAccessValue();
DEBUG(dbgs() << "Cleanup of " << WA << ":\n");
DEBUG(dbgs() << " Scalar: " << *AccVal << "\n");
DEBUG(dbgs() << " AccRel: " << AccRel << "\n");
(void)AccVal;
(void)AccRel;
Stmt->removeSingleMemoryAccess(WA);
RedundantWritesRemoved++;
TotalRedundantWritesRemoved++;
}
}
/// Remove statements without side effects.
void removeUnnecessaryStmts() {
auto NumStmtsBefore = S->getSize();
S->simplifySCoP(true);
assert(NumStmtsBefore >= S->getSize());
StmtsRemoved = NumStmtsBefore - S->getSize();
DEBUG(dbgs() << "Removed " << StmtsRemoved << " (of " << NumStmtsBefore
<< ") statements\n");
TotalStmtsRemoved += StmtsRemoved;
}
/// Remove accesses that have an empty domain.
void removeEmptyPartialAccesses() {
for (ScopStmt &Stmt : *S) {
// Defer the actual removal to not invalidate iterators.
SmallVector<MemoryAccess *, 8> DeferredRemove;
for (MemoryAccess *MA : Stmt) {
if (!MA->isWrite())
continue;
isl::map AccRel = MA->getAccessRelation();
if (!AccRel.is_empty().is_true())
continue;
DEBUG(dbgs() << "Removing " << MA
<< " because it's a partial access that never occurs\n");
DeferredRemove.push_back(MA);
}
for (MemoryAccess *MA : DeferredRemove) {
Stmt.removeSingleMemoryAccess(MA);
EmptyPartialAccessesRemoved++;
TotalEmptyPartialAccessesRemoved++;
}
}
}
/// Mark all reachable instructions and access, and sweep those that are not
/// reachable.
void markAndSweep(LoopInfo *LI) {
DenseSet<MemoryAccess *> UsedMA;
DenseSet<VirtualInstruction> UsedInsts;
// Get all reachable instructions and accesses.
markReachable(S, LI, UsedInsts, UsedMA);
// Remove all non-reachable accesses.
// We need get all MemoryAccesses first, in order to not invalidate the
// iterators when removing them.
SmallVector<MemoryAccess *, 64> AllMAs;
for (ScopStmt &Stmt : *S)
AllMAs.append(Stmt.begin(), Stmt.end());
for (MemoryAccess *MA : AllMAs) {
if (UsedMA.count(MA))
continue;
DEBUG(dbgs() << "Removing " << MA << " because its value is not used\n");
ScopStmt *Stmt = MA->getStatement();
Stmt->removeSingleMemoryAccess(MA);
DeadAccessesRemoved++;
TotalDeadAccessesRemoved++;
}
// Remove all non-reachable instructions.
for (ScopStmt &Stmt : *S) {
if (!Stmt.isBlockStmt())
continue;
SmallVector<Instruction *, 32> AllInsts(Stmt.insts_begin(),
Stmt.insts_end());
SmallVector<Instruction *, 32> RemainInsts;
for (Instruction *Inst : AllInsts) {
auto It = UsedInsts.find({&Stmt, Inst});
if (It == UsedInsts.end()) {
DEBUG(dbgs() << "Removing "; Inst->print(dbgs());
dbgs() << " because it is not used\n");
DeadInstructionsRemoved++;
TotalDeadInstructionsRemoved++;
continue;
}
RemainInsts.push_back(Inst);
// If instructions appear multiple times, keep only the first.
UsedInsts.erase(It);
}
// Set the new instruction list to be only those we did not remove.
Stmt.setInstructions(RemainInsts);
}
}
/// Print simplification statistics to @p OS.
void printStatistics(llvm::raw_ostream &OS, int Indent = 0) const {
OS.indent(Indent) << "Statistics {\n";
OS.indent(Indent + 4) << "Overwrites removed: " << OverwritesRemoved
<< '\n';
OS.indent(Indent + 4) << "Redundant writes removed: "
<< RedundantWritesRemoved << "\n";
OS.indent(Indent + 4) << "Accesses with empty domains removed: "
<< EmptyPartialAccessesRemoved << "\n";
OS.indent(Indent + 4) << "Dead accesses removed: " << DeadAccessesRemoved
<< '\n';
OS.indent(Indent + 4) << "Dead instructions removed: "
<< DeadInstructionsRemoved << '\n';
OS.indent(Indent + 4) << "Stmts removed: " << StmtsRemoved << "\n";
OS.indent(Indent) << "}\n";
}
/// Print the current state of all MemoryAccesses to @p OS.
void printAccesses(llvm::raw_ostream &OS, int Indent = 0) const {
OS.indent(Indent) << "After accesses {\n";
for (auto &Stmt : *S) {
OS.indent(Indent + 4) << Stmt.getBaseName() << "\n";
for (auto *MA : Stmt)
MA->print(OS);
}
OS.indent(Indent) << "}\n";
}
public:
static char ID;
explicit Simplify() : ScopPass(ID) {}
virtual void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequiredTransitive<ScopInfoRegionPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.setPreservesAll();
}
virtual bool runOnScop(Scop &S) override {
// Reset statistics of last processed SCoP.
releaseMemory();
assert(!isModified());
// Prepare processing of this SCoP.
this->S = &S;
ScopsProcessed++;
DEBUG(dbgs() << "Removing partial writes that never happen...\n");
removeEmptyPartialAccesses();
DEBUG(dbgs() << "Removing overwrites...\n");
removeOverwrites();
DEBUG(dbgs() << "Removing redundant writes...\n");
removeRedundantWrites();
DEBUG(dbgs() << "Cleanup unused accesses...\n");
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
markAndSweep(LI);
DEBUG(dbgs() << "Removing statements without side effects...\n");
removeUnnecessaryStmts();
if (isModified())
ScopsModified++;
DEBUG(dbgs() << "\nFinal Scop:\n");
DEBUG(dbgs() << S);
return false;
}
virtual void printScop(raw_ostream &OS, Scop &S) const override {
assert(&S == this->S &&
"Can only print analysis for the last processed SCoP");
printStatistics(OS);
if (!isModified()) {
OS << "SCoP could not be simplified\n";
return;
}
printAccesses(OS);
}
virtual void releaseMemory() override {
S = nullptr;
OverwritesRemoved = 0;
RedundantWritesRemoved = 0;
EmptyPartialAccessesRemoved = 0;
DeadAccessesRemoved = 0;
DeadInstructionsRemoved = 0;
StmtsRemoved = 0;
}
};
char Simplify::ID;
} // anonymous namespace
Pass *polly::createSimplifyPass() { return new Simplify(); }
INITIALIZE_PASS_BEGIN(Simplify, "polly-simplify", "Polly - Simplify", false,
false)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(Simplify, "polly-simplify", "Polly - Simplify", false,
false)