[llvm] r239282 - Remove SCEVCache and FindConstantPointers from complete loop unrolling heuristic.
Michael Zolotukhin
mzolotukhin at apple.com
Sun Jun 7 20:28:07 PDT 2015
Author: mzolotukhin
Date: Sun Jun 7 22:28:06 2015
New Revision: 239282
URL: http://llvm.org/viewvc/llvm-project?rev=239282&view=rev
Log:
Remove SCEVCache and FindConstantPointers from complete loop unrolling 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
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp?rev=239282&r1=239281&r2=239282&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp Sun Jun 7 22:28:06 2015
@@ -251,187 +251,6 @@ Pass *llvm::createSimpleLoopUnrollPass()
}
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
// loads might be replaced with concrete constant values and that could trigger
@@ -451,17 +270,31 @@ 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
@@ -469,18 +302,71 @@ private:
/// 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.
@@ -497,6 +383,7 @@ private:
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))
@@ -508,24 +395,21 @@ private:
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())
@@ -536,13 +420,10 @@ private:
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.
@@ -599,10 +480,6 @@ analyzeLoopUnrollCost(const Loop *L, uns
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;
@@ -619,7 +496,7 @@ analyzeLoopUnrollCost(const Loop *L, uns
// 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());
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