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Remove SCEVCache and FindConstantPointers from complete loop unrollin…
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…g heuristic.

Summary:
Using some SCEV functionality helped to entirely remove SCEVCache class and FindConstantPointers SCEV visitor.
Also, this makes the code more universal - I'll take advandate of it in next patches where I start handling additional types of instructions.

Test Plan: Tests would be submitted in subsequent patches.

Reviewers: atrick, chandlerc

Reviewed By: atrick, chandlerc

Subscribers: atrick, llvm-commits

Differential Revision: http://reviews.llvm.org/D10205

llvm-svn: 239282
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Michael Zolotukhin committed Jun 8, 2015
1 parent f811472 commit a60bdb5
Showing 1 changed file with 89 additions and 212 deletions.
301 changes: 89 additions & 212 deletions llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -250,187 +250,6 @@ Pass *llvm::createSimpleLoopUnrollPass() {
return llvm::createLoopUnrollPass(-1, -1, 0, 0);
}

namespace {
/// \brief SCEV expressions visitor used for finding expressions that would
/// become constants if the loop L is unrolled.
struct FindConstantPointers {
/// \brief Shows whether the expression is ConstAddress+Constant or not.
bool IndexIsConstant;

/// \brief Used for filtering out SCEV expressions with two or more AddRec
/// subexpressions.
///
/// Used to filter out complicated SCEV expressions, having several AddRec
/// sub-expressions. We don't handle them, because unrolling one loop
/// would help to replace only one of these inductions with a constant, and
/// consequently, the expression would remain non-constant.
bool HaveSeenAR;

/// \brief If the SCEV expression becomes ConstAddress+Constant, this value
/// holds ConstAddress. Otherwise, it's nullptr.
Value *BaseAddress;

/// \brief The loop, which we try to completely unroll.
const Loop *L;

ScalarEvolution &SE;

FindConstantPointers(const Loop *L, ScalarEvolution &SE)
: IndexIsConstant(true), HaveSeenAR(false), BaseAddress(nullptr),
L(L), SE(SE) {}

/// Examine the given expression S and figure out, if it can be a part of an
/// expression, that could become a constant after the loop is unrolled.
/// The routine sets IndexIsConstant and HaveSeenAR according to the analysis
/// results.
/// \returns true if we need to examine subexpressions, and false otherwise.
bool follow(const SCEV *S) {
if (const SCEVUnknown *SC = dyn_cast<SCEVUnknown>(S)) {
// We've reached the leaf node of SCEV, it's most probably just a
// variable.
// If it's the only one SCEV-subexpression, then it might be a base
// address of an index expression.
// If we've already recorded base address, then just give up on this SCEV
// - it's too complicated.
if (BaseAddress) {
IndexIsConstant = false;
return false;
}
BaseAddress = SC->getValue();
return false;
}
if (isa<SCEVConstant>(S))
return false;
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// If the current SCEV expression is AddRec, and its loop isn't the loop
// we are about to unroll, then we won't get a constant address after
// unrolling, and thus, won't be able to eliminate the load.
if (AR->getLoop() != L) {
IndexIsConstant = false;
return false;
}
// We don't handle multiple AddRecs here, so give up in this case.
if (HaveSeenAR) {
IndexIsConstant = false;
return false;
}
HaveSeenAR = true;
}

// Continue traversal.
return true;
}
bool isDone() const { return !IndexIsConstant; }
};
} // End anonymous namespace.

namespace {
/// \brief A cache of SCEV results used to optimize repeated queries to SCEV on
/// the same set of instructions.
///
/// The primary cost this saves is the cost of checking the validity of a SCEV
/// every time it is looked up. However, in some cases we can provide a reduced
/// and especially useful model for an instruction based upon SCEV that is
/// non-trivial to compute but more useful to clients.
class SCEVCache {
public:
/// \brief Struct to represent a GEP whose start and step are known fixed
/// offsets from a base address due to SCEV's analysis.
struct GEPDescriptor {
Value *BaseAddr = nullptr;
unsigned Start = 0;
unsigned Step = 0;
};

Optional<GEPDescriptor> getGEPDescriptor(GetElementPtrInst *GEP);

SCEVCache(const Loop &L, ScalarEvolution &SE) : L(L), SE(SE) {}

private:
const Loop &L;
ScalarEvolution &SE;

SmallDenseMap<GetElementPtrInst *, GEPDescriptor> GEPDescriptors;
};
} // End anonymous namespace.

/// \brief Get a simplified descriptor for a GEP instruction.
///
/// Where possible, this produces a simplified descriptor for a GEP instruction
/// using SCEV analysis of the containing loop. If this isn't possible, it
/// returns an empty optional.
///
/// The model is a base address, an initial offset, and a per-iteration step.
/// This fits very common patterns of GEPs inside loops and is something we can
/// use to simulate the behavior of a particular iteration of a loop.
///
/// This is a cached interface. The first call may do non-trivial work to
/// compute the result, but all subsequent calls will return a fast answer
/// based on a cached result. This includes caching negative results.
Optional<SCEVCache::GEPDescriptor>
SCEVCache::getGEPDescriptor(GetElementPtrInst *GEP) {
decltype(GEPDescriptors)::iterator It;
bool Inserted;

std::tie(It, Inserted) = GEPDescriptors.insert({GEP, {}});

if (!Inserted) {
if (!It->second.BaseAddr)
return None;

return It->second;
}

// We've inserted a new record into the cache, so compute the GEP descriptor
// if possible.
Value *V = cast<Value>(GEP);
if (!SE.isSCEVable(V->getType()))
return None;
const SCEV *S = SE.getSCEV(V);

// FIXME: It'd be nice if the worklist and set used by the
// SCEVTraversal could be re-used between loop iterations, but the
// interface doesn't support that. There is no way to clear the visited
// sets between uses.
FindConstantPointers Visitor(&L, SE);
SCEVTraversal<FindConstantPointers> T(Visitor);

// Try to find (BaseAddress+Step+Offset) tuple.
// If succeeded, save it to the cache - it might help in folding
// loads.
T.visitAll(S);
if (!Visitor.IndexIsConstant || !Visitor.BaseAddress)
return None;

const SCEV *BaseAddrSE = SE.getSCEV(Visitor.BaseAddress);
if (BaseAddrSE->getType() != S->getType())
return None;
const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE);
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE);

if (!AR)
return None;

const SCEVConstant *StepSE =
dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE));
const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart());
if (!StepSE || !StartSE)
return None;

// Check and skip caching if doing so would require lots of bits to
// avoid overflow.
APInt Start = StartSE->getValue()->getValue();
APInt Step = StepSE->getValue()->getValue();
if (Start.getActiveBits() > 32 || Step.getActiveBits() > 32)
return None;

// We found a cacheable SCEV model for the GEP.
It->second.BaseAddr = Visitor.BaseAddress;
It->second.Start = Start.getLimitedValue();
It->second.Step = Step.getLimitedValue();
return It->second;
}

namespace {
// This class is used to get an estimate of the optimization effects that we
// could get from complete loop unrolling. It comes from the fact that some
Expand All @@ -451,36 +270,103 @@ namespace {
class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
typedef InstVisitor<UnrolledInstAnalyzer, bool> Base;
friend class InstVisitor<UnrolledInstAnalyzer, bool>;
struct SimplifiedAddress {
Value *Base = nullptr;
ConstantInt *Offset = nullptr;
};

public:
UnrolledInstAnalyzer(unsigned Iteration,
DenseMap<Value *, Constant *> &SimplifiedValues,
SCEVCache &SC)
: Iteration(Iteration), SimplifiedValues(SimplifiedValues), SC(SC) {}
const Loop *L, ScalarEvolution &SE)
: Iteration(Iteration), SimplifiedValues(SimplifiedValues), L(L), SE(SE) {
IterationNumber = SE.getConstant(APInt(64, Iteration));
}

// Allow access to the initial visit method.
using Base::visit;

private:
/// \brief A cache of pointer bases and constant-folded offsets corresponding
/// to GEP (or derived from GEP) instructions.
///
/// In order to find the base pointer one needs to perform non-trivial
/// traversal of the corresponding SCEV expression, so it's good to have the
/// results saved.
DenseMap<Value *, SimplifiedAddress> SimplifiedAddresses;

/// \brief Number of currently simulated iteration.
///
/// If an expression is ConstAddress+Constant, then the Constant is
/// Start + Iteration*Step, where Start and Step could be obtained from
/// SCEVGEPCache.
unsigned Iteration;

// While we walk the loop instructions, we we build up and maintain a mapping
// of simplified values specific to this iteration. The idea is to propagate
// any special information we have about loads that can be replaced with
// constants after complete unrolling, and account for likely simplifications
// post-unrolling.
/// \brief SCEV expression corresponding to number of currently simulated
/// iteration.
const SCEV *IterationNumber;

/// \brief A Value->Constant map for keeping values that we managed to
/// constant-fold on the given iteration.
///
/// While we walk the loop instructions, we build up and maintain a mapping
/// of simplified values specific to this iteration. The idea is to propagate
/// any special information we have about loads that can be replaced with
/// constants after complete unrolling, and account for likely simplifications
/// post-unrolling.
DenseMap<Value *, Constant *> &SimplifiedValues;

// We use a cache to wrap all our SCEV queries.
SCEVCache &SC;
const Loop *L;
ScalarEvolution &SE;

/// \brief Try to simplify instruction \param I using its SCEV expression.
///
/// The idea is that some AddRec expressions become constants, which then
/// could trigger folding of other instructions. However, that only happens
/// for expressions whose start value is also constant, which isn't always the
/// case. In another common and important case the start value is just some
/// address (i.e. SCEVUnknown) - in this case we compute the offset and save
/// it along with the base address instead.
bool simplifyInstWithSCEV(Instruction *I) {
if (!SE.isSCEVable(I->getType()))
return false;

const SCEV *S = SE.getSCEV(I);
if (auto *SC = dyn_cast<SCEVConstant>(S)) {
SimplifiedValues[I] = SC->getValue();
return true;
}

auto *AR = dyn_cast<SCEVAddRecExpr>(S);
if (!AR)
return false;

const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
// Check if the AddRec expression becomes a constant.
if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
SimplifiedValues[I] = SC->getValue();
return true;
}

// Check if the offset from the base address becomes a constant.
auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
if (!Base)
return false;
auto *Offset =
dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
if (!Offset)
return false;
SimplifiedAddress Address;
Address.Base = Base->getValue();
Address.Offset = Offset->getValue();
SimplifiedAddresses[I] = Address;
return true;
}

/// Base case for the instruction visitor.
bool visitInstruction(Instruction &I) { return false; };
bool visitInstruction(Instruction &I) {
return simplifyInstWithSCEV(&I);
}

/// TODO: Add visitors for other instruction types, e.g. ZExt, SExt.

Expand All @@ -497,6 +383,7 @@ class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
if (!isa<Constant>(RHS))
if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
RHS = SimpleRHS;

Value *SimpleV = nullptr;
const DataLayout &DL = I.getModule()->getDataLayout();
if (auto FI = dyn_cast<FPMathOperator>(&I))
Expand All @@ -508,24 +395,21 @@ class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
if (Constant *C = dyn_cast_or_null<Constant>(SimpleV))
SimplifiedValues[&I] = C;

return SimpleV;
if (SimpleV)
return true;
return Base::visitBinaryOperator(I);
}

/// Try to fold load I.
bool visitLoad(LoadInst &I) {
Value *AddrOp = I.getPointerOperand();
if (!isa<Constant>(AddrOp))
if (Constant *SimplifiedAddrOp = SimplifiedValues.lookup(AddrOp))
AddrOp = SimplifiedAddrOp;

auto *GEP = dyn_cast<GetElementPtrInst>(AddrOp);
if (!GEP)
return false;
auto OptionalGEPDesc = SC.getGEPDescriptor(GEP);
if (!OptionalGEPDesc)
auto AddressIt = SimplifiedAddresses.find(AddrOp);
if (AddressIt == SimplifiedAddresses.end())
return false;
ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;

auto GV = dyn_cast<GlobalVariable>(OptionalGEPDesc->BaseAddr);
auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
// We're only interested in loads that can be completely folded to a
// constant.
if (!GV || !GV->hasInitializer())
Expand All @@ -536,13 +420,10 @@ class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
if (!CDS)
return false;

// This calculation should never overflow because we bound Iteration quite
// low and both the start and step are 32-bit integers. We use signed
// integers so that UBSan will catch if a bug sneaks into the code.
int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
int64_t Index = ((int64_t)OptionalGEPDesc->Start +
(int64_t)OptionalGEPDesc->Step * (int64_t)Iteration) /
ElemSize;
assert(SimplifiedAddrOp->getValue().getActiveBits() < 64 &&
"Unexpectedly large index value.");
int64_t Index = SimplifiedAddrOp->getSExtValue() / ElemSize;
if (Index >= CDS->getNumElements()) {
// FIXME: For now we conservatively ignore out of bound accesses, but
// we're allowed to perform the optimization in this case.
Expand Down Expand Up @@ -599,10 +480,6 @@ analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
SmallSetVector<BasicBlock *, 16> BBWorklist;
DenseMap<Value *, Constant *> SimplifiedValues;

// Use a cache to access SCEV expressions so that we don't pay the cost on
// each iteration. This cache is lazily self-populating.
SCEVCache SC(*L, SE);

// The estimated cost of the unrolled form of the loop. We try to estimate
// this by simplifying as much as we can while computing the estimate.
unsigned UnrolledCost = 0;
Expand All @@ -619,7 +496,7 @@ analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
// we literally have to go through all loop's iterations.
for (unsigned Iteration = 0; Iteration < TripCount; ++Iteration) {
SimplifiedValues.clear();
UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, SC);
UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, L, SE);

BBWorklist.clear();
BBWorklist.insert(L->getHeader());
Expand Down

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