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1123cc74df
isDependenceDistanceOfOne asserts that the store and the load access through the same type. This function is also used by removeDependencesFromMultipleStores so we need to make sure we filter out mismatching types before reaching this point. Now we do this when the initial candidates are gathered. This is a refinement of the fix made in r262267. Fixes PR27048. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264313 91177308-0d34-0410-b5e6-96231b3b80d8
592 lines
22 KiB
C++
592 lines
22 KiB
C++
//===- LoopLoadElimination.cpp - Loop Load Elimination Pass ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implement a loop-aware load elimination pass.
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//
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// It uses LoopAccessAnalysis to identify loop-carried dependences with a
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// distance of one between stores and loads. These form the candidates for the
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// transformation. The source value of each store then propagated to the user
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// of the corresponding load. This makes the load dead.
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//
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// The pass can also version the loop and add memchecks in order to prove that
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// may-aliasing stores can't change the value in memory before it's read by the
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// load.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/LoopAccessAnalysis.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ScalarEvolutionExpander.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/LoopVersioning.h"
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#include <forward_list>
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#define LLE_OPTION "loop-load-elim"
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#define DEBUG_TYPE LLE_OPTION
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using namespace llvm;
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static cl::opt<unsigned> CheckPerElim(
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"runtime-check-per-loop-load-elim", cl::Hidden,
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cl::desc("Max number of memchecks allowed per eliminated load on average"),
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cl::init(1));
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static cl::opt<unsigned> LoadElimSCEVCheckThreshold(
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"loop-load-elimination-scev-check-threshold", cl::init(8), cl::Hidden,
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cl::desc("The maximum number of SCEV checks allowed for Loop "
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"Load Elimination"));
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STATISTIC(NumLoopLoadEliminted, "Number of loads eliminated by LLE");
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namespace {
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/// \brief Represent a store-to-forwarding candidate.
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struct StoreToLoadForwardingCandidate {
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LoadInst *Load;
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StoreInst *Store;
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StoreToLoadForwardingCandidate(LoadInst *Load, StoreInst *Store)
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: Load(Load), Store(Store) {}
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/// \brief Return true if the dependence from the store to the load has a
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/// distance of one. E.g. A[i+1] = A[i]
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bool isDependenceDistanceOfOne(PredicatedScalarEvolution &PSE,
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Loop *L) const {
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Value *LoadPtr = Load->getPointerOperand();
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Value *StorePtr = Store->getPointerOperand();
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Type *LoadPtrType = LoadPtr->getType();
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Type *LoadType = LoadPtrType->getPointerElementType();
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assert(LoadPtrType->getPointerAddressSpace() ==
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StorePtr->getType()->getPointerAddressSpace() &&
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LoadType == StorePtr->getType()->getPointerElementType() &&
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"Should be a known dependence");
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// Currently we only support accesses with unit stride. FIXME: we should be
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// able to handle non unit stirde as well as long as the stride is equal to
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// the dependence distance.
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if (isStridedPtr(PSE, LoadPtr, L) != 1 ||
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isStridedPtr(PSE, LoadPtr, L) != 1)
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return false;
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auto &DL = Load->getParent()->getModule()->getDataLayout();
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unsigned TypeByteSize = DL.getTypeAllocSize(const_cast<Type *>(LoadType));
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auto *LoadPtrSCEV = cast<SCEVAddRecExpr>(PSE.getSCEV(LoadPtr));
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auto *StorePtrSCEV = cast<SCEVAddRecExpr>(PSE.getSCEV(StorePtr));
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// We don't need to check non-wrapping here because forward/backward
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// dependence wouldn't be valid if these weren't monotonic accesses.
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auto *Dist = cast<SCEVConstant>(
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PSE.getSE()->getMinusSCEV(StorePtrSCEV, LoadPtrSCEV));
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const APInt &Val = Dist->getAPInt();
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return Val == TypeByteSize;
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}
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Value *getLoadPtr() const { return Load->getPointerOperand(); }
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#ifndef NDEBUG
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friend raw_ostream &operator<<(raw_ostream &OS,
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const StoreToLoadForwardingCandidate &Cand) {
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OS << *Cand.Store << " -->\n";
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OS.indent(2) << *Cand.Load << "\n";
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return OS;
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}
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#endif
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};
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/// \brief Check if the store dominates all latches, so as long as there is no
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/// intervening store this value will be loaded in the next iteration.
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bool doesStoreDominatesAllLatches(BasicBlock *StoreBlock, Loop *L,
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DominatorTree *DT) {
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SmallVector<BasicBlock *, 8> Latches;
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L->getLoopLatches(Latches);
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return std::all_of(Latches.begin(), Latches.end(),
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[&](const BasicBlock *Latch) {
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return DT->dominates(StoreBlock, Latch);
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});
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}
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/// \brief The per-loop class that does most of the work.
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class LoadEliminationForLoop {
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public:
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LoadEliminationForLoop(Loop *L, LoopInfo *LI, const LoopAccessInfo &LAI,
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DominatorTree *DT)
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: L(L), LI(LI), LAI(LAI), DT(DT), PSE(LAI.PSE) {}
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/// \brief Look through the loop-carried and loop-independent dependences in
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/// this loop and find store->load dependences.
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///
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/// Note that no candidate is returned if LAA has failed to analyze the loop
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/// (e.g. if it's not bottom-tested, contains volatile memops, etc.)
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std::forward_list<StoreToLoadForwardingCandidate>
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findStoreToLoadDependences(const LoopAccessInfo &LAI) {
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std::forward_list<StoreToLoadForwardingCandidate> Candidates;
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const auto *Deps = LAI.getDepChecker().getDependences();
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if (!Deps)
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return Candidates;
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// Find store->load dependences (consequently true dep). Both lexically
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// forward and backward dependences qualify. Disqualify loads that have
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// other unknown dependences.
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SmallSet<Instruction *, 4> LoadsWithUnknownDepedence;
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for (const auto &Dep : *Deps) {
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Instruction *Source = Dep.getSource(LAI);
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Instruction *Destination = Dep.getDestination(LAI);
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if (Dep.Type == MemoryDepChecker::Dependence::Unknown) {
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if (isa<LoadInst>(Source))
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LoadsWithUnknownDepedence.insert(Source);
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if (isa<LoadInst>(Destination))
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LoadsWithUnknownDepedence.insert(Destination);
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continue;
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}
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if (Dep.isBackward())
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// Note that the designations source and destination follow the program
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// order, i.e. source is always first. (The direction is given by the
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// DepType.)
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std::swap(Source, Destination);
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else
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assert(Dep.isForward() && "Needs to be a forward dependence");
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auto *Store = dyn_cast<StoreInst>(Source);
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if (!Store)
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continue;
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auto *Load = dyn_cast<LoadInst>(Destination);
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if (!Load)
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continue;
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// Only progagate the value if they are of the same type.
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if (Store->getPointerOperand()->getType() !=
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Load->getPointerOperand()->getType())
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continue;
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Candidates.emplace_front(Load, Store);
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}
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if (!LoadsWithUnknownDepedence.empty())
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Candidates.remove_if([&](const StoreToLoadForwardingCandidate &C) {
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return LoadsWithUnknownDepedence.count(C.Load);
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});
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return Candidates;
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}
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/// \brief Return the index of the instruction according to program order.
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unsigned getInstrIndex(Instruction *Inst) {
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auto I = InstOrder.find(Inst);
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assert(I != InstOrder.end() && "No index for instruction");
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return I->second;
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}
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/// \brief If a load has multiple candidates associated (i.e. different
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/// stores), it means that it could be forwarding from multiple stores
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/// depending on control flow. Remove these candidates.
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///
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/// Here, we rely on LAA to include the relevant loop-independent dependences.
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/// LAA is known to omit these in the very simple case when the read and the
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/// write within an alias set always takes place using the *same* pointer.
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///
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/// However, we know that this is not the case here, i.e. we can rely on LAA
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/// to provide us with loop-independent dependences for the cases we're
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/// interested. Consider the case for example where a loop-independent
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/// dependece S1->S2 invalidates the forwarding S3->S2.
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///
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/// A[i] = ... (S1)
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/// ... = A[i] (S2)
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/// A[i+1] = ... (S3)
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///
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/// LAA will perform dependence analysis here because there are two
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/// *different* pointers involved in the same alias set (&A[i] and &A[i+1]).
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void removeDependencesFromMultipleStores(
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std::forward_list<StoreToLoadForwardingCandidate> &Candidates) {
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// If Store is nullptr it means that we have multiple stores forwarding to
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// this store.
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typedef DenseMap<LoadInst *, const StoreToLoadForwardingCandidate *>
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LoadToSingleCandT;
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LoadToSingleCandT LoadToSingleCand;
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for (const auto &Cand : Candidates) {
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bool NewElt;
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LoadToSingleCandT::iterator Iter;
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std::tie(Iter, NewElt) =
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LoadToSingleCand.insert(std::make_pair(Cand.Load, &Cand));
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if (!NewElt) {
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const StoreToLoadForwardingCandidate *&OtherCand = Iter->second;
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// Already multiple stores forward to this load.
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if (OtherCand == nullptr)
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continue;
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// Handle the very basic case when the two stores are in the same block
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// so deciding which one forwards is easy. The later one forwards as
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// long as they both have a dependence distance of one to the load.
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if (Cand.Store->getParent() == OtherCand->Store->getParent() &&
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Cand.isDependenceDistanceOfOne(PSE, L) &&
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OtherCand->isDependenceDistanceOfOne(PSE, L)) {
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// They are in the same block, the later one will forward to the load.
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if (getInstrIndex(OtherCand->Store) < getInstrIndex(Cand.Store))
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OtherCand = &Cand;
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} else
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OtherCand = nullptr;
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}
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}
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Candidates.remove_if([&](const StoreToLoadForwardingCandidate &Cand) {
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if (LoadToSingleCand[Cand.Load] != &Cand) {
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DEBUG(dbgs() << "Removing from candidates: \n" << Cand
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<< " The load may have multiple stores forwarding to "
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<< "it\n");
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return true;
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}
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return false;
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});
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}
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/// \brief Given two pointers operations by their RuntimePointerChecking
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/// indices, return true if they require an alias check.
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///
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/// We need a check if one is a pointer for a candidate load and the other is
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/// a pointer for a possibly intervening store.
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bool needsChecking(unsigned PtrIdx1, unsigned PtrIdx2,
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const SmallSet<Value *, 4> &PtrsWrittenOnFwdingPath,
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const std::set<Value *> &CandLoadPtrs) {
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Value *Ptr1 =
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LAI.getRuntimePointerChecking()->getPointerInfo(PtrIdx1).PointerValue;
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Value *Ptr2 =
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LAI.getRuntimePointerChecking()->getPointerInfo(PtrIdx2).PointerValue;
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return ((PtrsWrittenOnFwdingPath.count(Ptr1) && CandLoadPtrs.count(Ptr2)) ||
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(PtrsWrittenOnFwdingPath.count(Ptr2) && CandLoadPtrs.count(Ptr1)));
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}
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/// \brief Return pointers that are possibly written to on the path from a
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/// forwarding store to a load.
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///
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/// These pointers need to be alias-checked against the forwarding candidates.
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SmallSet<Value *, 4> findPointersWrittenOnForwardingPath(
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const SmallVectorImpl<StoreToLoadForwardingCandidate> &Candidates) {
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// From FirstStore to LastLoad neither of the elimination candidate loads
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// should overlap with any of the stores.
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//
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// E.g.:
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//
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// st1 C[i]
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// ld1 B[i] <-------,
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// ld0 A[i] <----, | * LastLoad
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// ... | |
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// st2 E[i] | |
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// st3 B[i+1] -- | -' * FirstStore
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// st0 A[i+1] ---'
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// st4 D[i]
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//
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// st0 forwards to ld0 if the accesses in st4 and st1 don't overlap with
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// ld0.
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LoadInst *LastLoad =
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std::max_element(Candidates.begin(), Candidates.end(),
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[&](const StoreToLoadForwardingCandidate &A,
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const StoreToLoadForwardingCandidate &B) {
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return getInstrIndex(A.Load) < getInstrIndex(B.Load);
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})
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->Load;
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StoreInst *FirstStore =
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std::min_element(Candidates.begin(), Candidates.end(),
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[&](const StoreToLoadForwardingCandidate &A,
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const StoreToLoadForwardingCandidate &B) {
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return getInstrIndex(A.Store) <
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getInstrIndex(B.Store);
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})
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->Store;
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// We're looking for stores after the first forwarding store until the end
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// of the loop, then from the beginning of the loop until the last
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// forwarded-to load. Collect the pointer for the stores.
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SmallSet<Value *, 4> PtrsWrittenOnFwdingPath;
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auto InsertStorePtr = [&](Instruction *I) {
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if (auto *S = dyn_cast<StoreInst>(I))
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PtrsWrittenOnFwdingPath.insert(S->getPointerOperand());
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};
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const auto &MemInstrs = LAI.getDepChecker().getMemoryInstructions();
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std::for_each(MemInstrs.begin() + getInstrIndex(FirstStore) + 1,
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MemInstrs.end(), InsertStorePtr);
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std::for_each(MemInstrs.begin(), &MemInstrs[getInstrIndex(LastLoad)],
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InsertStorePtr);
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return PtrsWrittenOnFwdingPath;
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}
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/// \brief Determine the pointer alias checks to prove that there are no
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/// intervening stores.
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SmallVector<RuntimePointerChecking::PointerCheck, 4> collectMemchecks(
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const SmallVectorImpl<StoreToLoadForwardingCandidate> &Candidates) {
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SmallSet<Value *, 4> PtrsWrittenOnFwdingPath =
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findPointersWrittenOnForwardingPath(Candidates);
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// Collect the pointers of the candidate loads.
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// FIXME: SmallSet does not work with std::inserter.
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std::set<Value *> CandLoadPtrs;
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std::transform(Candidates.begin(), Candidates.end(),
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std::inserter(CandLoadPtrs, CandLoadPtrs.begin()),
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std::mem_fn(&StoreToLoadForwardingCandidate::getLoadPtr));
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const auto &AllChecks = LAI.getRuntimePointerChecking()->getChecks();
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SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks;
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std::copy_if(AllChecks.begin(), AllChecks.end(), std::back_inserter(Checks),
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[&](const RuntimePointerChecking::PointerCheck &Check) {
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for (auto PtrIdx1 : Check.first->Members)
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for (auto PtrIdx2 : Check.second->Members)
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if (needsChecking(PtrIdx1, PtrIdx2,
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PtrsWrittenOnFwdingPath, CandLoadPtrs))
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return true;
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return false;
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});
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DEBUG(dbgs() << "\nPointer Checks (count: " << Checks.size() << "):\n");
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DEBUG(LAI.getRuntimePointerChecking()->printChecks(dbgs(), Checks));
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return Checks;
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}
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/// \brief Perform the transformation for a candidate.
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void
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propagateStoredValueToLoadUsers(const StoreToLoadForwardingCandidate &Cand,
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SCEVExpander &SEE) {
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//
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// loop:
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// %x = load %gep_i
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// = ... %x
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// store %y, %gep_i_plus_1
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//
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// =>
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//
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// ph:
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// %x.initial = load %gep_0
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// loop:
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// %x.storeforward = phi [%x.initial, %ph] [%y, %loop]
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// %x = load %gep_i <---- now dead
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// = ... %x.storeforward
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// store %y, %gep_i_plus_1
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Value *Ptr = Cand.Load->getPointerOperand();
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auto *PtrSCEV = cast<SCEVAddRecExpr>(PSE.getSCEV(Ptr));
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auto *PH = L->getLoopPreheader();
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Value *InitialPtr = SEE.expandCodeFor(PtrSCEV->getStart(), Ptr->getType(),
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PH->getTerminator());
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Value *Initial =
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new LoadInst(InitialPtr, "load_initial", PH->getTerminator());
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PHINode *PHI = PHINode::Create(Initial->getType(), 2, "store_forwarded",
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&L->getHeader()->front());
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PHI->addIncoming(Initial, PH);
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PHI->addIncoming(Cand.Store->getOperand(0), L->getLoopLatch());
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Cand.Load->replaceAllUsesWith(PHI);
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}
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/// \brief Top-level driver for each loop: find store->load forwarding
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/// candidates, add run-time checks and perform transformation.
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bool processLoop() {
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DEBUG(dbgs() << "\nIn \"" << L->getHeader()->getParent()->getName()
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<< "\" checking " << *L << "\n");
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// Look for store-to-load forwarding cases across the
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// backedge. E.g.:
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//
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// loop:
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// %x = load %gep_i
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// = ... %x
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// store %y, %gep_i_plus_1
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//
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// =>
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//
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// ph:
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// %x.initial = load %gep_0
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// loop:
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// %x.storeforward = phi [%x.initial, %ph] [%y, %loop]
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// %x = load %gep_i <---- now dead
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// = ... %x.storeforward
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// store %y, %gep_i_plus_1
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// First start with store->load dependences.
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auto StoreToLoadDependences = findStoreToLoadDependences(LAI);
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if (StoreToLoadDependences.empty())
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return false;
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// Generate an index for each load and store according to the original
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// program order. This will be used later.
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InstOrder = LAI.getDepChecker().generateInstructionOrderMap();
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// To keep things simple for now, remove those where the load is potentially
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// fed by multiple stores.
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removeDependencesFromMultipleStores(StoreToLoadDependences);
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if (StoreToLoadDependences.empty())
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return false;
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// Filter the candidates further.
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SmallVector<StoreToLoadForwardingCandidate, 4> Candidates;
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unsigned NumForwarding = 0;
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for (const StoreToLoadForwardingCandidate Cand : StoreToLoadDependences) {
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DEBUG(dbgs() << "Candidate " << Cand);
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// Make sure that the stored values is available everywhere in the loop in
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// the next iteration.
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if (!doesStoreDominatesAllLatches(Cand.Store->getParent(), L, DT))
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continue;
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// Check whether the SCEV difference is the same as the induction step,
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// thus we load the value in the next iteration.
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if (!Cand.isDependenceDistanceOfOne(PSE, L))
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continue;
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++NumForwarding;
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DEBUG(dbgs()
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<< NumForwarding
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<< ". Valid store-to-load forwarding across the loop backedge\n");
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Candidates.push_back(Cand);
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}
|
|
if (Candidates.empty())
|
|
return false;
|
|
|
|
// Check intervening may-alias stores. These need runtime checks for alias
|
|
// disambiguation.
|
|
SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks =
|
|
collectMemchecks(Candidates);
|
|
|
|
// Too many checks are likely to outweigh the benefits of forwarding.
|
|
if (Checks.size() > Candidates.size() * CheckPerElim) {
|
|
DEBUG(dbgs() << "Too many run-time checks needed.\n");
|
|
return false;
|
|
}
|
|
|
|
if (LAI.PSE.getUnionPredicate().getComplexity() >
|
|
LoadElimSCEVCheckThreshold) {
|
|
DEBUG(dbgs() << "Too many SCEV run-time checks needed.\n");
|
|
return false;
|
|
}
|
|
|
|
if (!Checks.empty() || !LAI.PSE.getUnionPredicate().isAlwaysTrue()) {
|
|
if (L->getHeader()->getParent()->optForSize()) {
|
|
DEBUG(dbgs() << "Versioning is needed but not allowed when optimizing "
|
|
"for size.\n");
|
|
return false;
|
|
}
|
|
|
|
// Point of no-return, start the transformation. First, version the loop
|
|
// if necessary.
|
|
|
|
LoopVersioning LV(LAI, L, LI, DT, PSE.getSE(), false);
|
|
LV.setAliasChecks(std::move(Checks));
|
|
LV.setSCEVChecks(LAI.PSE.getUnionPredicate());
|
|
LV.versionLoop();
|
|
}
|
|
|
|
// Next, propagate the value stored by the store to the users of the load.
|
|
// Also for the first iteration, generate the initial value of the load.
|
|
SCEVExpander SEE(*PSE.getSE(), L->getHeader()->getModule()->getDataLayout(),
|
|
"storeforward");
|
|
for (const auto &Cand : Candidates)
|
|
propagateStoredValueToLoadUsers(Cand, SEE);
|
|
NumLoopLoadEliminted += NumForwarding;
|
|
|
|
return true;
|
|
}
|
|
|
|
private:
|
|
Loop *L;
|
|
|
|
/// \brief Maps the load/store instructions to their index according to
|
|
/// program order.
|
|
DenseMap<Instruction *, unsigned> InstOrder;
|
|
|
|
// Analyses used.
|
|
LoopInfo *LI;
|
|
const LoopAccessInfo &LAI;
|
|
DominatorTree *DT;
|
|
PredicatedScalarEvolution PSE;
|
|
};
|
|
|
|
/// \brief The pass. Most of the work is delegated to the per-loop
|
|
/// LoadEliminationForLoop class.
|
|
class LoopLoadElimination : public FunctionPass {
|
|
public:
|
|
LoopLoadElimination() : FunctionPass(ID) {
|
|
initializeLoopLoadEliminationPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnFunction(Function &F) override {
|
|
auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
|
|
auto *LAA = &getAnalysis<LoopAccessAnalysis>();
|
|
auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
|
|
|
// Build up a worklist of inner-loops to vectorize. This is necessary as the
|
|
// act of distributing a loop creates new loops and can invalidate iterators
|
|
// across the loops.
|
|
SmallVector<Loop *, 8> Worklist;
|
|
|
|
for (Loop *TopLevelLoop : *LI)
|
|
for (Loop *L : depth_first(TopLevelLoop))
|
|
// We only handle inner-most loops.
|
|
if (L->empty())
|
|
Worklist.push_back(L);
|
|
|
|
// Now walk the identified inner loops.
|
|
bool Changed = false;
|
|
for (Loop *L : Worklist) {
|
|
const LoopAccessInfo &LAI = LAA->getInfo(L, ValueToValueMap());
|
|
// The actual work is performed by LoadEliminationForLoop.
|
|
LoadEliminationForLoop LEL(L, LI, LAI, DT);
|
|
Changed |= LEL.processLoop();
|
|
}
|
|
|
|
// Process each loop nest in the function.
|
|
return Changed;
|
|
}
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.addRequiredID(LoopSimplifyID);
|
|
AU.addRequired<LoopInfoWrapperPass>();
|
|
AU.addPreserved<LoopInfoWrapperPass>();
|
|
AU.addRequired<LoopAccessAnalysis>();
|
|
AU.addRequired<ScalarEvolutionWrapperPass>();
|
|
AU.addRequired<DominatorTreeWrapperPass>();
|
|
AU.addPreserved<DominatorTreeWrapperPass>();
|
|
}
|
|
|
|
static char ID;
|
|
};
|
|
}
|
|
|
|
char LoopLoadElimination::ID;
|
|
static const char LLE_name[] = "Loop Load Elimination";
|
|
|
|
INITIALIZE_PASS_BEGIN(LoopLoadElimination, LLE_OPTION, LLE_name, false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(LoopAccessAnalysis)
|
|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
|
|
INITIALIZE_PASS_END(LoopLoadElimination, LLE_OPTION, LLE_name, false, false)
|
|
|
|
namespace llvm {
|
|
FunctionPass *createLoopLoadEliminationPass() {
|
|
return new LoopLoadElimination();
|
|
}
|
|
}
|