| //===- FunctionSpecialization.cpp - Function Specialization ---------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/IPO/FunctionSpecialization.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/CodeMetrics.h" |
| #include "llvm/Analysis/ConstantFolding.h" |
| #include "llvm/Analysis/InlineCost.h" |
| #include "llvm/Analysis/InstructionSimplify.h" |
| #include "llvm/Analysis/TargetTransformInfo.h" |
| #include "llvm/Analysis/ValueLattice.h" |
| #include "llvm/Analysis/ValueLatticeUtils.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/Transforms/Scalar/SCCP.h" |
| #include "llvm/Transforms/Utils/Cloning.h" |
| #include "llvm/Transforms/Utils/SCCPSolver.h" |
| #include "llvm/Transforms/Utils/SizeOpts.h" |
| #include <cmath> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "function-specialization" |
| |
| STATISTIC(NumSpecsCreated, "Number of specializations created"); |
| |
| static cl::opt<bool> ForceSpecialization( |
| "force-specialization", cl::init(false), cl::Hidden, cl::desc( |
| "Force function specialization for every call site with a constant " |
| "argument")); |
| |
| static cl::opt<unsigned> MaxClones( |
| "funcspec-max-clones", cl::init(3), cl::Hidden, cl::desc( |
| "The maximum number of clones allowed for a single function " |
| "specialization")); |
| |
| static cl::opt<unsigned> |
| MaxDiscoveryIterations("funcspec-max-discovery-iterations", cl::init(100), |
| cl::Hidden, |
| cl::desc("The maximum number of iterations allowed " |
| "when searching for transitive " |
| "phis")); |
| |
| static cl::opt<unsigned> MaxIncomingPhiValues( |
| "funcspec-max-incoming-phi-values", cl::init(8), cl::Hidden, |
| cl::desc("The maximum number of incoming values a PHI node can have to be " |
| "considered during the specialization bonus estimation")); |
| |
| static cl::opt<unsigned> MaxBlockPredecessors( |
| "funcspec-max-block-predecessors", cl::init(2), cl::Hidden, cl::desc( |
| "The maximum number of predecessors a basic block can have to be " |
| "considered during the estimation of dead code")); |
| |
| static cl::opt<unsigned> MinFunctionSize( |
| "funcspec-min-function-size", cl::init(300), cl::Hidden, cl::desc( |
| "Don't specialize functions that have less than this number of " |
| "instructions")); |
| |
| static cl::opt<unsigned> MaxCodeSizeGrowth( |
| "funcspec-max-codesize-growth", cl::init(3), cl::Hidden, cl::desc( |
| "Maximum codesize growth allowed per function")); |
| |
| static cl::opt<unsigned> MinCodeSizeSavings( |
| "funcspec-min-codesize-savings", cl::init(20), cl::Hidden, cl::desc( |
| "Reject specializations whose codesize savings are less than this" |
| "much percent of the original function size")); |
| |
| static cl::opt<unsigned> MinLatencySavings( |
| "funcspec-min-latency-savings", cl::init(40), cl::Hidden, |
| cl::desc("Reject specializations whose latency savings are less than this" |
| "much percent of the original function size")); |
| |
| static cl::opt<unsigned> MinInliningBonus( |
| "funcspec-min-inlining-bonus", cl::init(300), cl::Hidden, cl::desc( |
| "Reject specializations whose inlining bonus is less than this" |
| "much percent of the original function size")); |
| |
| static cl::opt<bool> SpecializeOnAddress( |
| "funcspec-on-address", cl::init(false), cl::Hidden, cl::desc( |
| "Enable function specialization on the address of global values")); |
| |
| // Disabled by default as it can significantly increase compilation times. |
| // |
| // https://llvm-compile-time-tracker.com |
| // https://github.com/nikic/llvm-compile-time-tracker |
| static cl::opt<bool> SpecializeLiteralConstant( |
| "funcspec-for-literal-constant", cl::init(false), cl::Hidden, cl::desc( |
| "Enable specialization of functions that take a literal constant as an " |
| "argument")); |
| |
| bool InstCostVisitor::canEliminateSuccessor(BasicBlock *BB, BasicBlock *Succ, |
| DenseSet<BasicBlock *> &DeadBlocks) { |
| unsigned I = 0; |
| return all_of(predecessors(Succ), |
| [&I, BB, Succ, &DeadBlocks] (BasicBlock *Pred) { |
| return I++ < MaxBlockPredecessors && |
| (Pred == BB || Pred == Succ || DeadBlocks.contains(Pred)); |
| }); |
| } |
| |
| // Estimates the codesize savings due to dead code after constant propagation. |
| // \p WorkList represents the basic blocks of a specialization which will |
| // eventually become dead once we replace instructions that are known to be |
| // constants. The successors of such blocks are added to the list as long as |
| // the \p Solver found they were executable prior to specialization, and only |
| // if all their predecessors are dead. |
| Cost InstCostVisitor::estimateBasicBlocks( |
| SmallVectorImpl<BasicBlock *> &WorkList) { |
| Cost CodeSize = 0; |
| // Accumulate the instruction cost of each basic block weighted by frequency. |
| while (!WorkList.empty()) { |
| BasicBlock *BB = WorkList.pop_back_val(); |
| |
| // These blocks are considered dead as far as the InstCostVisitor |
| // is concerned. They haven't been proven dead yet by the Solver, |
| // but may become if we propagate the specialization arguments. |
| if (!DeadBlocks.insert(BB).second) |
| continue; |
| |
| for (Instruction &I : *BB) { |
| // Disregard SSA copies. |
| if (auto *II = dyn_cast<IntrinsicInst>(&I)) |
| if (II->getIntrinsicID() == Intrinsic::ssa_copy) |
| continue; |
| // If it's a known constant we have already accounted for it. |
| if (KnownConstants.contains(&I)) |
| continue; |
| |
| Cost C = TTI.getInstructionCost(&I, TargetTransformInfo::TCK_CodeSize); |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: CodeSize " << C |
| << " for user " << I << "\n"); |
| CodeSize += C; |
| } |
| |
| // Keep adding dead successors to the list as long as they are |
| // executable and only reachable from dead blocks. |
| for (BasicBlock *SuccBB : successors(BB)) |
| if (isBlockExecutable(SuccBB) && |
| canEliminateSuccessor(BB, SuccBB, DeadBlocks)) |
| WorkList.push_back(SuccBB); |
| } |
| return CodeSize; |
| } |
| |
| static Constant *findConstantFor(Value *V, ConstMap &KnownConstants) { |
| if (auto *C = dyn_cast<Constant>(V)) |
| return C; |
| return KnownConstants.lookup(V); |
| } |
| |
| Bonus InstCostVisitor::getBonusFromPendingPHIs() { |
| Bonus B; |
| while (!PendingPHIs.empty()) { |
| Instruction *Phi = PendingPHIs.pop_back_val(); |
| // The pending PHIs could have been proven dead by now. |
| if (isBlockExecutable(Phi->getParent())) |
| B += getUserBonus(Phi); |
| } |
| return B; |
| } |
| |
| /// Compute a bonus for replacing argument \p A with constant \p C. |
| Bonus InstCostVisitor::getSpecializationBonus(Argument *A, Constant *C) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Analysing bonus for constant: " |
| << C->getNameOrAsOperand() << "\n"); |
| Bonus B; |
| for (auto *U : A->users()) |
| if (auto *UI = dyn_cast<Instruction>(U)) |
| if (isBlockExecutable(UI->getParent())) |
| B += getUserBonus(UI, A, C); |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Accumulated bonus {CodeSize = " |
| << B.CodeSize << ", Latency = " << B.Latency |
| << "} for argument " << *A << "\n"); |
| return B; |
| } |
| |
| Bonus InstCostVisitor::getUserBonus(Instruction *User, Value *Use, Constant *C) { |
| // We have already propagated a constant for this user. |
| if (KnownConstants.contains(User)) |
| return {0, 0}; |
| |
| // Cache the iterator before visiting. |
| LastVisited = Use ? KnownConstants.insert({Use, C}).first |
| : KnownConstants.end(); |
| |
| Cost CodeSize = 0; |
| if (auto *I = dyn_cast<SwitchInst>(User)) { |
| CodeSize = estimateSwitchInst(*I); |
| } else if (auto *I = dyn_cast<BranchInst>(User)) { |
| CodeSize = estimateBranchInst(*I); |
| } else { |
| C = visit(*User); |
| if (!C) |
| return {0, 0}; |
| } |
| |
| // Even though it doesn't make sense to bind switch and branch instructions |
| // with a constant, unlike any other instruction type, it prevents estimating |
| // their bonus multiple times. |
| KnownConstants.insert({User, C}); |
| |
| CodeSize += TTI.getInstructionCost(User, TargetTransformInfo::TCK_CodeSize); |
| |
| uint64_t Weight = BFI.getBlockFreq(User->getParent()).getFrequency() / |
| BFI.getEntryFreq().getFrequency(); |
| |
| Cost Latency = Weight * |
| TTI.getInstructionCost(User, TargetTransformInfo::TCK_Latency); |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: {CodeSize = " << CodeSize |
| << ", Latency = " << Latency << "} for user " |
| << *User << "\n"); |
| |
| Bonus B(CodeSize, Latency); |
| for (auto *U : User->users()) |
| if (auto *UI = dyn_cast<Instruction>(U)) |
| if (UI != User && isBlockExecutable(UI->getParent())) |
| B += getUserBonus(UI, User, C); |
| |
| return B; |
| } |
| |
| Cost InstCostVisitor::estimateSwitchInst(SwitchInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| if (I.getCondition() != LastVisited->first) |
| return 0; |
| |
| auto *C = dyn_cast<ConstantInt>(LastVisited->second); |
| if (!C) |
| return 0; |
| |
| BasicBlock *Succ = I.findCaseValue(C)->getCaseSuccessor(); |
| // Initialize the worklist with the dead basic blocks. These are the |
| // destination labels which are different from the one corresponding |
| // to \p C. They should be executable and have a unique predecessor. |
| SmallVector<BasicBlock *> WorkList; |
| for (const auto &Case : I.cases()) { |
| BasicBlock *BB = Case.getCaseSuccessor(); |
| if (BB != Succ && isBlockExecutable(BB) && |
| canEliminateSuccessor(I.getParent(), BB, DeadBlocks)) |
| WorkList.push_back(BB); |
| } |
| |
| return estimateBasicBlocks(WorkList); |
| } |
| |
| Cost InstCostVisitor::estimateBranchInst(BranchInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| if (I.getCondition() != LastVisited->first) |
| return 0; |
| |
| BasicBlock *Succ = I.getSuccessor(LastVisited->second->isOneValue()); |
| // Initialize the worklist with the dead successor as long as |
| // it is executable and has a unique predecessor. |
| SmallVector<BasicBlock *> WorkList; |
| if (isBlockExecutable(Succ) && |
| canEliminateSuccessor(I.getParent(), Succ, DeadBlocks)) |
| WorkList.push_back(Succ); |
| |
| return estimateBasicBlocks(WorkList); |
| } |
| |
| bool InstCostVisitor::discoverTransitivelyIncomingValues( |
| Constant *Const, PHINode *Root, DenseSet<PHINode *> &TransitivePHIs) { |
| |
| SmallVector<PHINode *, 64> WorkList; |
| WorkList.push_back(Root); |
| unsigned Iter = 0; |
| |
| while (!WorkList.empty()) { |
| PHINode *PN = WorkList.pop_back_val(); |
| |
| if (++Iter > MaxDiscoveryIterations || |
| PN->getNumIncomingValues() > MaxIncomingPhiValues) |
| return false; |
| |
| if (!TransitivePHIs.insert(PN).second) |
| continue; |
| |
| for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) { |
| Value *V = PN->getIncomingValue(I); |
| |
| // Disregard self-references and dead incoming values. |
| if (auto *Inst = dyn_cast<Instruction>(V)) |
| if (Inst == PN || DeadBlocks.contains(PN->getIncomingBlock(I))) |
| continue; |
| |
| if (Constant *C = findConstantFor(V, KnownConstants)) { |
| // Not all incoming values are the same constant. Bail immediately. |
| if (C != Const) |
| return false; |
| continue; |
| } |
| |
| if (auto *Phi = dyn_cast<PHINode>(V)) { |
| WorkList.push_back(Phi); |
| continue; |
| } |
| |
| // We can't reason about anything else. |
| return false; |
| } |
| } |
| return true; |
| } |
| |
| Constant *InstCostVisitor::visitPHINode(PHINode &I) { |
| if (I.getNumIncomingValues() > MaxIncomingPhiValues) |
| return nullptr; |
| |
| bool Inserted = VisitedPHIs.insert(&I).second; |
| Constant *Const = nullptr; |
| bool HaveSeenIncomingPHI = false; |
| |
| for (unsigned Idx = 0, E = I.getNumIncomingValues(); Idx != E; ++Idx) { |
| Value *V = I.getIncomingValue(Idx); |
| |
| // Disregard self-references and dead incoming values. |
| if (auto *Inst = dyn_cast<Instruction>(V)) |
| if (Inst == &I || DeadBlocks.contains(I.getIncomingBlock(Idx))) |
| continue; |
| |
| if (Constant *C = findConstantFor(V, KnownConstants)) { |
| if (!Const) |
| Const = C; |
| // Not all incoming values are the same constant. Bail immediately. |
| if (C != Const) |
| return nullptr; |
| continue; |
| } |
| |
| if (Inserted) { |
| // First time we are seeing this phi. We will retry later, after |
| // all the constant arguments have been propagated. Bail for now. |
| PendingPHIs.push_back(&I); |
| return nullptr; |
| } |
| |
| if (isa<PHINode>(V)) { |
| // Perhaps it is a Transitive Phi. We will confirm later. |
| HaveSeenIncomingPHI = true; |
| continue; |
| } |
| |
| // We can't reason about anything else. |
| return nullptr; |
| } |
| |
| if (!Const) |
| return nullptr; |
| |
| if (!HaveSeenIncomingPHI) |
| return Const; |
| |
| DenseSet<PHINode *> TransitivePHIs; |
| if (!discoverTransitivelyIncomingValues(Const, &I, TransitivePHIs)) |
| return nullptr; |
| |
| return Const; |
| } |
| |
| Constant *InstCostVisitor::visitFreezeInst(FreezeInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| if (isGuaranteedNotToBeUndefOrPoison(LastVisited->second)) |
| return LastVisited->second; |
| return nullptr; |
| } |
| |
| Constant *InstCostVisitor::visitCallBase(CallBase &I) { |
| Function *F = I.getCalledFunction(); |
| if (!F || !canConstantFoldCallTo(&I, F)) |
| return nullptr; |
| |
| SmallVector<Constant *, 8> Operands; |
| Operands.reserve(I.getNumOperands()); |
| |
| for (unsigned Idx = 0, E = I.getNumOperands() - 1; Idx != E; ++Idx) { |
| Value *V = I.getOperand(Idx); |
| Constant *C = findConstantFor(V, KnownConstants); |
| if (!C) |
| return nullptr; |
| Operands.push_back(C); |
| } |
| |
| auto Ops = ArrayRef(Operands.begin(), Operands.end()); |
| return ConstantFoldCall(&I, F, Ops); |
| } |
| |
| Constant *InstCostVisitor::visitLoadInst(LoadInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| if (isa<ConstantPointerNull>(LastVisited->second)) |
| return nullptr; |
| return ConstantFoldLoadFromConstPtr(LastVisited->second, I.getType(), DL); |
| } |
| |
| Constant *InstCostVisitor::visitGetElementPtrInst(GetElementPtrInst &I) { |
| SmallVector<Constant *, 8> Operands; |
| Operands.reserve(I.getNumOperands()); |
| |
| for (unsigned Idx = 0, E = I.getNumOperands(); Idx != E; ++Idx) { |
| Value *V = I.getOperand(Idx); |
| Constant *C = findConstantFor(V, KnownConstants); |
| if (!C) |
| return nullptr; |
| Operands.push_back(C); |
| } |
| |
| auto Ops = ArrayRef(Operands.begin(), Operands.end()); |
| return ConstantFoldInstOperands(&I, Ops, DL); |
| } |
| |
| Constant *InstCostVisitor::visitSelectInst(SelectInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| if (I.getCondition() != LastVisited->first) |
| return nullptr; |
| |
| Value *V = LastVisited->second->isZeroValue() ? I.getFalseValue() |
| : I.getTrueValue(); |
| Constant *C = findConstantFor(V, KnownConstants); |
| return C; |
| } |
| |
| Constant *InstCostVisitor::visitCastInst(CastInst &I) { |
| return ConstantFoldCastOperand(I.getOpcode(), LastVisited->second, |
| I.getType(), DL); |
| } |
| |
| Constant *InstCostVisitor::visitCmpInst(CmpInst &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| bool Swap = I.getOperand(1) == LastVisited->first; |
| Value *V = Swap ? I.getOperand(0) : I.getOperand(1); |
| Constant *Other = findConstantFor(V, KnownConstants); |
| if (!Other) |
| return nullptr; |
| |
| Constant *Const = LastVisited->second; |
| return Swap ? |
| ConstantFoldCompareInstOperands(I.getPredicate(), Other, Const, DL) |
| : ConstantFoldCompareInstOperands(I.getPredicate(), Const, Other, DL); |
| } |
| |
| Constant *InstCostVisitor::visitUnaryOperator(UnaryOperator &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| return ConstantFoldUnaryOpOperand(I.getOpcode(), LastVisited->second, DL); |
| } |
| |
| Constant *InstCostVisitor::visitBinaryOperator(BinaryOperator &I) { |
| assert(LastVisited != KnownConstants.end() && "Invalid iterator!"); |
| |
| bool Swap = I.getOperand(1) == LastVisited->first; |
| Value *V = Swap ? I.getOperand(0) : I.getOperand(1); |
| Constant *Other = findConstantFor(V, KnownConstants); |
| if (!Other) |
| return nullptr; |
| |
| Constant *Const = LastVisited->second; |
| return dyn_cast_or_null<Constant>(Swap ? |
| simplifyBinOp(I.getOpcode(), Other, Const, SimplifyQuery(DL)) |
| : simplifyBinOp(I.getOpcode(), Const, Other, SimplifyQuery(DL))); |
| } |
| |
| Constant *FunctionSpecializer::getPromotableAlloca(AllocaInst *Alloca, |
| CallInst *Call) { |
| Value *StoreValue = nullptr; |
| for (auto *User : Alloca->users()) { |
| // We can't use llvm::isAllocaPromotable() as that would fail because of |
| // the usage in the CallInst, which is what we check here. |
| if (User == Call) |
| continue; |
| if (auto *Bitcast = dyn_cast<BitCastInst>(User)) { |
| if (!Bitcast->hasOneUse() || *Bitcast->user_begin() != Call) |
| return nullptr; |
| continue; |
| } |
| |
| if (auto *Store = dyn_cast<StoreInst>(User)) { |
| // This is a duplicate store, bail out. |
| if (StoreValue || Store->isVolatile()) |
| return nullptr; |
| StoreValue = Store->getValueOperand(); |
| continue; |
| } |
| // Bail if there is any other unknown usage. |
| return nullptr; |
| } |
| |
| if (!StoreValue) |
| return nullptr; |
| |
| return getCandidateConstant(StoreValue); |
| } |
| |
| // A constant stack value is an AllocaInst that has a single constant |
| // value stored to it. Return this constant if such an alloca stack value |
| // is a function argument. |
| Constant *FunctionSpecializer::getConstantStackValue(CallInst *Call, |
| Value *Val) { |
| if (!Val) |
| return nullptr; |
| Val = Val->stripPointerCasts(); |
| if (auto *ConstVal = dyn_cast<ConstantInt>(Val)) |
| return ConstVal; |
| auto *Alloca = dyn_cast<AllocaInst>(Val); |
| if (!Alloca || !Alloca->getAllocatedType()->isIntegerTy()) |
| return nullptr; |
| return getPromotableAlloca(Alloca, Call); |
| } |
| |
| // To support specializing recursive functions, it is important to propagate |
| // constant arguments because after a first iteration of specialisation, a |
| // reduced example may look like this: |
| // |
| // define internal void @RecursiveFn(i32* arg1) { |
| // %temp = alloca i32, align 4 |
| // store i32 2 i32* %temp, align 4 |
| // call void @RecursiveFn.1(i32* nonnull %temp) |
| // ret void |
| // } |
| // |
| // Before a next iteration, we need to propagate the constant like so |
| // which allows further specialization in next iterations. |
| // |
| // @funcspec.arg = internal constant i32 2 |
| // |
| // define internal void @someFunc(i32* arg1) { |
| // call void @otherFunc(i32* nonnull @funcspec.arg) |
| // ret void |
| // } |
| // |
| // See if there are any new constant values for the callers of \p F via |
| // stack variables and promote them to global variables. |
| void FunctionSpecializer::promoteConstantStackValues(Function *F) { |
| for (User *U : F->users()) { |
| |
| auto *Call = dyn_cast<CallInst>(U); |
| if (!Call) |
| continue; |
| |
| if (!Solver.isBlockExecutable(Call->getParent())) |
| continue; |
| |
| for (const Use &U : Call->args()) { |
| unsigned Idx = Call->getArgOperandNo(&U); |
| Value *ArgOp = Call->getArgOperand(Idx); |
| Type *ArgOpType = ArgOp->getType(); |
| |
| if (!Call->onlyReadsMemory(Idx) || !ArgOpType->isPointerTy()) |
| continue; |
| |
| auto *ConstVal = getConstantStackValue(Call, ArgOp); |
| if (!ConstVal) |
| continue; |
| |
| Value *GV = new GlobalVariable(M, ConstVal->getType(), true, |
| GlobalValue::InternalLinkage, ConstVal, |
| "specialized.arg." + Twine(++NGlobals)); |
| Call->setArgOperand(Idx, GV); |
| } |
| } |
| } |
| |
| // ssa_copy intrinsics are introduced by the SCCP solver. These intrinsics |
| // interfere with the promoteConstantStackValues() optimization. |
| static void removeSSACopy(Function &F) { |
| for (BasicBlock &BB : F) { |
| for (Instruction &Inst : llvm::make_early_inc_range(BB)) { |
| auto *II = dyn_cast<IntrinsicInst>(&Inst); |
| if (!II) |
| continue; |
| if (II->getIntrinsicID() != Intrinsic::ssa_copy) |
| continue; |
| Inst.replaceAllUsesWith(II->getOperand(0)); |
| Inst.eraseFromParent(); |
| } |
| } |
| } |
| |
| /// Remove any ssa_copy intrinsics that may have been introduced. |
| void FunctionSpecializer::cleanUpSSA() { |
| for (Function *F : Specializations) |
| removeSSACopy(*F); |
| } |
| |
| |
| template <> struct llvm::DenseMapInfo<SpecSig> { |
| static inline SpecSig getEmptyKey() { return {~0U, {}}; } |
| |
| static inline SpecSig getTombstoneKey() { return {~1U, {}}; } |
| |
| static unsigned getHashValue(const SpecSig &S) { |
| return static_cast<unsigned>(hash_value(S)); |
| } |
| |
| static bool isEqual(const SpecSig &LHS, const SpecSig &RHS) { |
| return LHS == RHS; |
| } |
| }; |
| |
| FunctionSpecializer::~FunctionSpecializer() { |
| LLVM_DEBUG( |
| if (NumSpecsCreated > 0) |
| dbgs() << "FnSpecialization: Created " << NumSpecsCreated |
| << " specializations in module " << M.getName() << "\n"); |
| // Eliminate dead code. |
| removeDeadFunctions(); |
| cleanUpSSA(); |
| } |
| |
| /// Attempt to specialize functions in the module to enable constant |
| /// propagation across function boundaries. |
| /// |
| /// \returns true if at least one function is specialized. |
| bool FunctionSpecializer::run() { |
| // Find possible specializations for each function. |
| SpecMap SM; |
| SmallVector<Spec, 32> AllSpecs; |
| unsigned NumCandidates = 0; |
| for (Function &F : M) { |
| if (!isCandidateFunction(&F)) |
| continue; |
| |
| auto [It, Inserted] = FunctionMetrics.try_emplace(&F); |
| CodeMetrics &Metrics = It->second; |
| //Analyze the function. |
| if (Inserted) { |
| SmallPtrSet<const Value *, 32> EphValues; |
| CodeMetrics::collectEphemeralValues(&F, &GetAC(F), EphValues); |
| for (BasicBlock &BB : F) |
| Metrics.analyzeBasicBlock(&BB, GetTTI(F), EphValues); |
| } |
| |
| // If the code metrics reveal that we shouldn't duplicate the function, |
| // or if the code size implies that this function is easy to get inlined, |
| // then we shouldn't specialize it. |
| if (Metrics.notDuplicatable || !Metrics.NumInsts.isValid() || |
| (!ForceSpecialization && !F.hasFnAttribute(Attribute::NoInline) && |
| Metrics.NumInsts < MinFunctionSize)) |
| continue; |
| |
| // TODO: For now only consider recursive functions when running multiple |
| // times. This should change if specialization on literal constants gets |
| // enabled. |
| if (!Inserted && !Metrics.isRecursive && !SpecializeLiteralConstant) |
| continue; |
| |
| int64_t Sz = *Metrics.NumInsts.getValue(); |
| assert(Sz > 0 && "CodeSize should be positive"); |
| // It is safe to down cast from int64_t, NumInsts is always positive. |
| unsigned FuncSize = static_cast<unsigned>(Sz); |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Specialization cost for " |
| << F.getName() << " is " << FuncSize << "\n"); |
| |
| if (Inserted && Metrics.isRecursive) |
| promoteConstantStackValues(&F); |
| |
| if (!findSpecializations(&F, FuncSize, AllSpecs, SM)) { |
| LLVM_DEBUG( |
| dbgs() << "FnSpecialization: No possible specializations found for " |
| << F.getName() << "\n"); |
| continue; |
| } |
| |
| ++NumCandidates; |
| } |
| |
| if (!NumCandidates) { |
| LLVM_DEBUG( |
| dbgs() |
| << "FnSpecialization: No possible specializations found in module\n"); |
| return false; |
| } |
| |
| // Choose the most profitable specialisations, which fit in the module |
| // specialization budget, which is derived from maximum number of |
| // specializations per specialization candidate function. |
| auto CompareScore = [&AllSpecs](unsigned I, unsigned J) { |
| return AllSpecs[I].Score > AllSpecs[J].Score; |
| }; |
| const unsigned NSpecs = |
| std::min(NumCandidates * MaxClones, unsigned(AllSpecs.size())); |
| SmallVector<unsigned> BestSpecs(NSpecs + 1); |
| std::iota(BestSpecs.begin(), BestSpecs.begin() + NSpecs, 0); |
| if (AllSpecs.size() > NSpecs) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Number of candidates exceed " |
| << "the maximum number of clones threshold.\n" |
| << "FnSpecialization: Specializing the " |
| << NSpecs |
| << " most profitable candidates.\n"); |
| std::make_heap(BestSpecs.begin(), BestSpecs.begin() + NSpecs, CompareScore); |
| for (unsigned I = NSpecs, N = AllSpecs.size(); I < N; ++I) { |
| BestSpecs[NSpecs] = I; |
| std::push_heap(BestSpecs.begin(), BestSpecs.end(), CompareScore); |
| std::pop_heap(BestSpecs.begin(), BestSpecs.end(), CompareScore); |
| } |
| } |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: List of specializations \n"; |
| for (unsigned I = 0; I < NSpecs; ++I) { |
| const Spec &S = AllSpecs[BestSpecs[I]]; |
| dbgs() << "FnSpecialization: Function " << S.F->getName() |
| << " , score " << S.Score << "\n"; |
| for (const ArgInfo &Arg : S.Sig.Args) |
| dbgs() << "FnSpecialization: FormalArg = " |
| << Arg.Formal->getNameOrAsOperand() |
| << ", ActualArg = " << Arg.Actual->getNameOrAsOperand() |
| << "\n"; |
| }); |
| |
| // Create the chosen specializations. |
| SmallPtrSet<Function *, 8> OriginalFuncs; |
| SmallVector<Function *> Clones; |
| for (unsigned I = 0; I < NSpecs; ++I) { |
| Spec &S = AllSpecs[BestSpecs[I]]; |
| S.Clone = createSpecialization(S.F, S.Sig); |
| |
| // Update the known call sites to call the clone. |
| for (CallBase *Call : S.CallSites) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Redirecting " << *Call |
| << " to call " << S.Clone->getName() << "\n"); |
| Call->setCalledFunction(S.Clone); |
| } |
| |
| Clones.push_back(S.Clone); |
| OriginalFuncs.insert(S.F); |
| } |
| |
| Solver.solveWhileResolvedUndefsIn(Clones); |
| |
| // Update the rest of the call sites - these are the recursive calls, calls |
| // to discarded specialisations and calls that may match a specialisation |
| // after the solver runs. |
| for (Function *F : OriginalFuncs) { |
| auto [Begin, End] = SM[F]; |
| updateCallSites(F, AllSpecs.begin() + Begin, AllSpecs.begin() + End); |
| } |
| |
| for (Function *F : Clones) { |
| if (F->getReturnType()->isVoidTy()) |
| continue; |
| if (F->getReturnType()->isStructTy()) { |
| auto *STy = cast<StructType>(F->getReturnType()); |
| if (!Solver.isStructLatticeConstant(F, STy)) |
| continue; |
| } else { |
| auto It = Solver.getTrackedRetVals().find(F); |
| assert(It != Solver.getTrackedRetVals().end() && |
| "Return value ought to be tracked"); |
| if (SCCPSolver::isOverdefined(It->second)) |
| continue; |
| } |
| for (User *U : F->users()) { |
| if (auto *CS = dyn_cast<CallBase>(U)) { |
| //The user instruction does not call our function. |
| if (CS->getCalledFunction() != F) |
| continue; |
| Solver.resetLatticeValueFor(CS); |
| } |
| } |
| } |
| |
| // Rerun the solver to notify the users of the modified callsites. |
| Solver.solveWhileResolvedUndefs(); |
| |
| for (Function *F : OriginalFuncs) |
| if (FunctionMetrics[F].isRecursive) |
| promoteConstantStackValues(F); |
| |
| return true; |
| } |
| |
| void FunctionSpecializer::removeDeadFunctions() { |
| for (Function *F : FullySpecialized) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Removing dead function " |
| << F->getName() << "\n"); |
| if (FAM) |
| FAM->clear(*F, F->getName()); |
| F->eraseFromParent(); |
| } |
| FullySpecialized.clear(); |
| } |
| |
| /// Clone the function \p F and remove the ssa_copy intrinsics added by |
| /// the SCCPSolver in the cloned version. |
| static Function *cloneCandidateFunction(Function *F, unsigned NSpecs) { |
| ValueToValueMapTy Mappings; |
| Function *Clone = CloneFunction(F, Mappings); |
| Clone->setName(F->getName() + ".specialized." + Twine(NSpecs)); |
| removeSSACopy(*Clone); |
| return Clone; |
| } |
| |
| bool FunctionSpecializer::findSpecializations(Function *F, unsigned FuncSize, |
| SmallVectorImpl<Spec> &AllSpecs, |
| SpecMap &SM) { |
| // A mapping from a specialisation signature to the index of the respective |
| // entry in the all specialisation array. Used to ensure uniqueness of |
| // specialisations. |
| DenseMap<SpecSig, unsigned> UniqueSpecs; |
| |
| // Get a list of interesting arguments. |
| SmallVector<Argument *> Args; |
| for (Argument &Arg : F->args()) |
| if (isArgumentInteresting(&Arg)) |
| Args.push_back(&Arg); |
| |
| if (Args.empty()) |
| return false; |
| |
| for (User *U : F->users()) { |
| if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
| continue; |
| auto &CS = *cast<CallBase>(U); |
| |
| // The user instruction does not call our function. |
| if (CS.getCalledFunction() != F) |
| continue; |
| |
| // If the call site has attribute minsize set, that callsite won't be |
| // specialized. |
| if (CS.hasFnAttr(Attribute::MinSize)) |
| continue; |
| |
| // If the parent of the call site will never be executed, we don't need |
| // to worry about the passed value. |
| if (!Solver.isBlockExecutable(CS.getParent())) |
| continue; |
| |
| // Examine arguments and create a specialisation candidate from the |
| // constant operands of this call site. |
| SpecSig S; |
| for (Argument *A : Args) { |
| Constant *C = getCandidateConstant(CS.getArgOperand(A->getArgNo())); |
| if (!C) |
| continue; |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Found interesting argument " |
| << A->getName() << " : " << C->getNameOrAsOperand() |
| << "\n"); |
| S.Args.push_back({A, C}); |
| } |
| |
| if (S.Args.empty()) |
| continue; |
| |
| // Check if we have encountered the same specialisation already. |
| if (auto It = UniqueSpecs.find(S); It != UniqueSpecs.end()) { |
| // Existing specialisation. Add the call to the list to rewrite, unless |
| // it's a recursive call. A specialisation, generated because of a |
| // recursive call may end up as not the best specialisation for all |
| // the cloned instances of this call, which result from specialising |
| // functions. Hence we don't rewrite the call directly, but match it with |
| // the best specialisation once all specialisations are known. |
| if (CS.getFunction() == F) |
| continue; |
| const unsigned Index = It->second; |
| AllSpecs[Index].CallSites.push_back(&CS); |
| } else { |
| // Calculate the specialisation gain. |
| Bonus B; |
| unsigned Score = 0; |
| InstCostVisitor Visitor = getInstCostVisitorFor(F); |
| for (ArgInfo &A : S.Args) { |
| B += Visitor.getSpecializationBonus(A.Formal, A.Actual); |
| Score += getInliningBonus(A.Formal, A.Actual); |
| } |
| B += Visitor.getBonusFromPendingPHIs(); |
| |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Specialization bonus {CodeSize = " |
| << B.CodeSize << ", Latency = " << B.Latency |
| << ", Inlining = " << Score << "}\n"); |
| |
| FunctionGrowth[F] += FuncSize - B.CodeSize; |
| |
| auto IsProfitable = [](Bonus &B, unsigned Score, unsigned FuncSize, |
| unsigned FuncGrowth) -> bool { |
| // No check required. |
| if (ForceSpecialization) |
| return true; |
| // Minimum inlining bonus. |
| if (Score > MinInliningBonus * FuncSize / 100) |
| return true; |
| // Minimum codesize savings. |
| if (B.CodeSize < MinCodeSizeSavings * FuncSize / 100) |
| return false; |
| // Minimum latency savings. |
| if (B.Latency < MinLatencySavings * FuncSize / 100) |
| return false; |
| // Maximum codesize growth. |
| if (FuncGrowth / FuncSize > MaxCodeSizeGrowth) |
| return false; |
| return true; |
| }; |
| |
| // Discard unprofitable specialisations. |
| if (!IsProfitable(B, Score, FuncSize, FunctionGrowth[F])) |
| continue; |
| |
| // Create a new specialisation entry. |
| Score += std::max(B.CodeSize, B.Latency); |
| auto &Spec = AllSpecs.emplace_back(F, S, Score); |
| if (CS.getFunction() != F) |
| Spec.CallSites.push_back(&CS); |
| const unsigned Index = AllSpecs.size() - 1; |
| UniqueSpecs[S] = Index; |
| if (auto [It, Inserted] = SM.try_emplace(F, Index, Index + 1); !Inserted) |
| It->second.second = Index + 1; |
| } |
| } |
| |
| return !UniqueSpecs.empty(); |
| } |
| |
| bool FunctionSpecializer::isCandidateFunction(Function *F) { |
| if (F->isDeclaration() || F->arg_empty()) |
| return false; |
| |
| if (F->hasFnAttribute(Attribute::NoDuplicate)) |
| return false; |
| |
| // Do not specialize the cloned function again. |
| if (Specializations.contains(F)) |
| return false; |
| |
| // If we're optimizing the function for size, we shouldn't specialize it. |
| if (F->hasOptSize() || |
| shouldOptimizeForSize(F, nullptr, nullptr, PGSOQueryType::IRPass)) |
| return false; |
| |
| // Exit if the function is not executable. There's no point in specializing |
| // a dead function. |
| if (!Solver.isBlockExecutable(&F->getEntryBlock())) |
| return false; |
| |
| // It wastes time to specialize a function which would get inlined finally. |
| if (F->hasFnAttribute(Attribute::AlwaysInline)) |
| return false; |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Try function: " << F->getName() |
| << "\n"); |
| return true; |
| } |
| |
| Function *FunctionSpecializer::createSpecialization(Function *F, |
| const SpecSig &S) { |
| Function *Clone = cloneCandidateFunction(F, Specializations.size() + 1); |
| |
| // The original function does not neccessarily have internal linkage, but the |
| // clone must. |
| Clone->setLinkage(GlobalValue::InternalLinkage); |
| |
| // Initialize the lattice state of the arguments of the function clone, |
| // marking the argument on which we specialized the function constant |
| // with the given value. |
| Solver.setLatticeValueForSpecializationArguments(Clone, S.Args); |
| Solver.markBlockExecutable(&Clone->front()); |
| Solver.addArgumentTrackedFunction(Clone); |
| Solver.addTrackedFunction(Clone); |
| |
| // Mark all the specialized functions |
| Specializations.insert(Clone); |
| ++NumSpecsCreated; |
| |
| return Clone; |
| } |
| |
| /// Compute the inlining bonus for replacing argument \p A with constant \p C. |
| /// The below heuristic is only concerned with exposing inlining |
| /// opportunities via indirect call promotion. If the argument is not a |
| /// (potentially casted) function pointer, give up. |
| unsigned FunctionSpecializer::getInliningBonus(Argument *A, Constant *C) { |
| Function *CalledFunction = dyn_cast<Function>(C->stripPointerCasts()); |
| if (!CalledFunction) |
| return 0; |
| |
| // Get TTI for the called function (used for the inline cost). |
| auto &CalleeTTI = (GetTTI)(*CalledFunction); |
| |
| // Look at all the call sites whose called value is the argument. |
| // Specializing the function on the argument would allow these indirect |
| // calls to be promoted to direct calls. If the indirect call promotion |
| // would likely enable the called function to be inlined, specializing is a |
| // good idea. |
| int InliningBonus = 0; |
| for (User *U : A->users()) { |
| if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
| continue; |
| auto *CS = cast<CallBase>(U); |
| if (CS->getCalledOperand() != A) |
| continue; |
| if (CS->getFunctionType() != CalledFunction->getFunctionType()) |
| continue; |
| |
| // Get the cost of inlining the called function at this call site. Note |
| // that this is only an estimate. The called function may eventually |
| // change in a way that leads to it not being inlined here, even though |
| // inlining looks profitable now. For example, one of its called |
| // functions may be inlined into it, making the called function too large |
| // to be inlined into this call site. |
| // |
| // We apply a boost for performing indirect call promotion by increasing |
| // the default threshold by the threshold for indirect calls. |
| auto Params = getInlineParams(); |
| Params.DefaultThreshold += InlineConstants::IndirectCallThreshold; |
| InlineCost IC = |
| getInlineCost(*CS, CalledFunction, Params, CalleeTTI, GetAC, GetTLI); |
| |
| // We clamp the bonus for this call to be between zero and the default |
| // threshold. |
| if (IC.isAlways()) |
| InliningBonus += Params.DefaultThreshold; |
| else if (IC.isVariable() && IC.getCostDelta() > 0) |
| InliningBonus += IC.getCostDelta(); |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Inlining bonus " << InliningBonus |
| << " for user " << *U << "\n"); |
| } |
| |
| return InliningBonus > 0 ? static_cast<unsigned>(InliningBonus) : 0; |
| } |
| |
| /// Determine if it is possible to specialise the function for constant values |
| /// of the formal parameter \p A. |
| bool FunctionSpecializer::isArgumentInteresting(Argument *A) { |
| // No point in specialization if the argument is unused. |
| if (A->user_empty()) |
| return false; |
| |
| Type *Ty = A->getType(); |
| if (!Ty->isPointerTy() && (!SpecializeLiteralConstant || |
| (!Ty->isIntegerTy() && !Ty->isFloatingPointTy() && !Ty->isStructTy()))) |
| return false; |
| |
| // SCCP solver does not record an argument that will be constructed on |
| // stack. |
| if (A->hasByValAttr() && !A->getParent()->onlyReadsMemory()) |
| return false; |
| |
| // For non-argument-tracked functions every argument is overdefined. |
| if (!Solver.isArgumentTrackedFunction(A->getParent())) |
| return true; |
| |
| // Check the lattice value and decide if we should attemt to specialize, |
| // based on this argument. No point in specialization, if the lattice value |
| // is already a constant. |
| bool IsOverdefined = Ty->isStructTy() |
| ? any_of(Solver.getStructLatticeValueFor(A), SCCPSolver::isOverdefined) |
| : SCCPSolver::isOverdefined(Solver.getLatticeValueFor(A)); |
| |
| LLVM_DEBUG( |
| if (IsOverdefined) |
| dbgs() << "FnSpecialization: Found interesting parameter " |
| << A->getNameOrAsOperand() << "\n"; |
| else |
| dbgs() << "FnSpecialization: Nothing to do, parameter " |
| << A->getNameOrAsOperand() << " is already constant\n"; |
| ); |
| return IsOverdefined; |
| } |
| |
| /// Check if the value \p V (an actual argument) is a constant or can only |
| /// have a constant value. Return that constant. |
| Constant *FunctionSpecializer::getCandidateConstant(Value *V) { |
| if (isa<PoisonValue>(V)) |
| return nullptr; |
| |
| // Select for possible specialisation values that are constants or |
| // are deduced to be constants or constant ranges with a single element. |
| Constant *C = dyn_cast<Constant>(V); |
| if (!C) |
| C = Solver.getConstantOrNull(V); |
| |
| // Don't specialize on (anything derived from) the address of a non-constant |
| // global variable, unless explicitly enabled. |
| if (C && C->getType()->isPointerTy() && !C->isNullValue()) |
| if (auto *GV = dyn_cast<GlobalVariable>(getUnderlyingObject(C)); |
| GV && !(GV->isConstant() || SpecializeOnAddress)) |
| return nullptr; |
| |
| return C; |
| } |
| |
| void FunctionSpecializer::updateCallSites(Function *F, const Spec *Begin, |
| const Spec *End) { |
| // Collect the call sites that need updating. |
| SmallVector<CallBase *> ToUpdate; |
| for (User *U : F->users()) |
| if (auto *CS = dyn_cast<CallBase>(U); |
| CS && CS->getCalledFunction() == F && |
| Solver.isBlockExecutable(CS->getParent())) |
| ToUpdate.push_back(CS); |
| |
| unsigned NCallsLeft = ToUpdate.size(); |
| for (CallBase *CS : ToUpdate) { |
| bool ShouldDecrementCount = CS->getFunction() == F; |
| |
| // Find the best matching specialisation. |
| const Spec *BestSpec = nullptr; |
| for (const Spec &S : make_range(Begin, End)) { |
| if (!S.Clone || (BestSpec && S.Score <= BestSpec->Score)) |
| continue; |
| |
| if (any_of(S.Sig.Args, [CS, this](const ArgInfo &Arg) { |
| unsigned ArgNo = Arg.Formal->getArgNo(); |
| return getCandidateConstant(CS->getArgOperand(ArgNo)) != Arg.Actual; |
| })) |
| continue; |
| |
| BestSpec = &S; |
| } |
| |
| if (BestSpec) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Redirecting " << *CS |
| << " to call " << BestSpec->Clone->getName() << "\n"); |
| CS->setCalledFunction(BestSpec->Clone); |
| ShouldDecrementCount = true; |
| } |
| |
| if (ShouldDecrementCount) |
| --NCallsLeft; |
| } |
| |
| // If the function has been completely specialized, the original function |
| // is no longer needed. Mark it unreachable. |
| if (NCallsLeft == 0 && Solver.isArgumentTrackedFunction(F)) { |
| Solver.markFunctionUnreachable(F); |
| FullySpecialized.insert(F); |
| } |
| } |