| //===- 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 |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This specialises functions with constant parameters (e.g. functions, |
| // globals). Constant parameters like function pointers and constant globals |
| // are propagated to the callee by specializing the function. |
| // |
| // Current limitations: |
| // - It does not yet handle integer ranges. |
| // - Only 1 argument per function is specialised, |
| // - The cost-model could be further looked into, |
| // - We are not yet caching analysis results. |
| // |
| // Ideas: |
| // - With a function specialization attribute for arguments, we could have |
| // a direct way to steer function specialization, avoiding the cost-model, |
| // and thus control compile-times / code-size. |
| // |
| // Todos: |
| // - Specializing recursive functions relies on running the transformation a |
| // number of times, which is controlled by option |
| // `func-specialization-max-iters`. Thus, increasing this value and the |
| // number of iterations, will linearly increase the number of times recursive |
| // functions get specialized, see also the discussion in |
| // https://reviews.llvm.org/D106426 for details. Perhaps there is a |
| // compile-time friendlier way to control/limit the number of specialisations |
| // for recursive functions. |
| // - Don't transform the function if there is no function specialization |
| // happens. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AssumptionCache.h" |
| #include "llvm/Analysis/CodeMetrics.h" |
| #include "llvm/Analysis/DomTreeUpdater.h" |
| #include "llvm/Analysis/InlineCost.h" |
| #include "llvm/Analysis/LoopInfo.h" |
| #include "llvm/Analysis/TargetLibraryInfo.h" |
| #include "llvm/Analysis/TargetTransformInfo.h" |
| #include "llvm/Transforms/Scalar/SCCP.h" |
| #include "llvm/Transforms/Utils/Cloning.h" |
| #include "llvm/Transforms/Utils/SizeOpts.h" |
| #include <cmath> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "function-specialization" |
| |
| STATISTIC(NumFuncSpecialized, "Number of functions specialized"); |
| |
| static cl::opt<bool> ForceFunctionSpecialization( |
| "force-function-specialization", cl::init(false), cl::Hidden, |
| cl::desc("Force function specialization for every call site with a " |
| "constant argument")); |
| |
| static cl::opt<unsigned> FuncSpecializationMaxIters( |
| "func-specialization-max-iters", cl::Hidden, |
| cl::desc("The maximum number of iterations function specialization is run"), |
| cl::init(1)); |
| |
| static cl::opt<unsigned> MaxConstantsThreshold( |
| "func-specialization-max-constants", cl::Hidden, |
| cl::desc("The maximum number of clones allowed for a single function " |
| "specialization"), |
| cl::init(3)); |
| |
| static cl::opt<unsigned> SmallFunctionThreshold( |
| "func-specialization-size-threshold", cl::Hidden, |
| cl::desc("Don't specialize functions that have less than this theshold " |
| "number of instructions"), |
| cl::init(100)); |
| |
| static cl::opt<unsigned> |
| AvgLoopIterationCount("func-specialization-avg-iters-cost", cl::Hidden, |
| cl::desc("Average loop iteration count cost"), |
| cl::init(10)); |
| |
| static cl::opt<bool> SpecializeOnAddresses( |
| "func-specialization-on-address", cl::init(false), cl::Hidden, |
| cl::desc("Enable function specialization on the address of global values")); |
| |
| // TODO: This needs checking to see the impact on compile-times, which is why |
| // this is off by default for now. |
| static cl::opt<bool> EnableSpecializationForLiteralConstant( |
| "function-specialization-for-literal-constant", cl::init(false), cl::Hidden, |
| cl::desc("Enable specialization of functions that take a literal constant " |
| "as an argument.")); |
| |
| // Helper to check if \p LV is either a constant or a constant |
| // range with a single element. This should cover exactly the same cases as the |
| // old ValueLatticeElement::isConstant() and is intended to be used in the |
| // transition to ValueLatticeElement. |
| static bool isConstant(const ValueLatticeElement &LV) { |
| return LV.isConstant() || |
| (LV.isConstantRange() && LV.getConstantRange().isSingleElement()); |
| } |
| |
| // Helper to check if \p LV is either overdefined or a constant int. |
| static bool isOverdefined(const ValueLatticeElement &LV) { |
| return !LV.isUnknownOrUndef() && !isConstant(LV); |
| } |
| |
| static Constant *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; |
| } |
| return dyn_cast_or_null<Constant>(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. |
| static Constant *getConstantStackValue(CallInst *Call, Value *Val, |
| SCCPSolver &Solver) { |
| 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 |
| // } |
| // |
| static void constantArgPropagation(SmallVectorImpl<Function *> &WorkList, |
| Module &M, SCCPSolver &Solver) { |
| // Iterate over the argument tracked functions see if there |
| // are any new constant values for the call instruction via |
| // stack variables. |
| for (auto *F : WorkList) { |
| // TODO: Generalize for any read only arguments. |
| if (F->arg_size() != 1) |
| continue; |
| |
| auto &Arg = *F->arg_begin(); |
| if (!Arg.onlyReadsMemory() || !Arg.getType()->isPointerTy()) |
| continue; |
| |
| for (auto *User : F->users()) { |
| auto *Call = dyn_cast<CallInst>(User); |
| if (!Call) |
| break; |
| auto *ArgOp = Call->getArgOperand(0); |
| auto *ArgOpType = ArgOp->getType(); |
| auto *ConstVal = getConstantStackValue(Call, ArgOp, Solver); |
| if (!ConstVal) |
| break; |
| |
| Value *GV = new GlobalVariable(M, ConstVal->getType(), true, |
| GlobalValue::InternalLinkage, ConstVal, |
| "funcspec.arg"); |
| |
| if (ArgOpType != ConstVal->getType()) |
| GV = ConstantExpr::getBitCast(cast<Constant>(GV), ArgOp->getType()); |
| |
| Call->setArgOperand(0, GV); |
| |
| // Add the changed CallInst to Solver Worklist |
| Solver.visitCall(*Call); |
| } |
| } |
| } |
| |
| // ssa_copy intrinsics are introduced by the SCCP solver. These intrinsics |
| // interfere with the constantArgPropagation 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(); |
| } |
| } |
| } |
| |
| static void removeSSACopy(Module &M) { |
| for (Function &F : M) |
| removeSSACopy(F); |
| } |
| |
| namespace { |
| class FunctionSpecializer { |
| |
| /// The IPSCCP Solver. |
| SCCPSolver &Solver; |
| |
| /// Analyses used to help determine if a function should be specialized. |
| std::function<AssumptionCache &(Function &)> GetAC; |
| std::function<TargetTransformInfo &(Function &)> GetTTI; |
| std::function<TargetLibraryInfo &(Function &)> GetTLI; |
| |
| SmallPtrSet<Function *, 2> SpecializedFuncs; |
| |
| public: |
| FunctionSpecializer(SCCPSolver &Solver, |
| std::function<AssumptionCache &(Function &)> GetAC, |
| std::function<TargetTransformInfo &(Function &)> GetTTI, |
| std::function<TargetLibraryInfo &(Function &)> GetTLI) |
| : Solver(Solver), GetAC(GetAC), GetTTI(GetTTI), GetTLI(GetTLI) {} |
| |
| /// Attempt to specialize functions in the module to enable constant |
| /// propagation across function boundaries. |
| /// |
| /// \returns true if at least one function is specialized. |
| bool |
| specializeFunctions(SmallVectorImpl<Function *> &FuncDecls, |
| SmallVectorImpl<Function *> &CurrentSpecializations) { |
| |
| // Attempt to specialize the argument-tracked functions. |
| bool Changed = false; |
| for (auto *F : FuncDecls) { |
| if (specializeFunction(F, CurrentSpecializations)) { |
| Changed = true; |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Can specialize this func.\n"); |
| } else { |
| LLVM_DEBUG( |
| dbgs() << "FnSpecialization: Cannot specialize this func.\n"); |
| } |
| } |
| |
| for (auto *SpecializedFunc : CurrentSpecializations) { |
| SpecializedFuncs.insert(SpecializedFunc); |
| |
| // Initialize the state of the newly created functions, marking them |
| // argument-tracked and executable. |
| if (SpecializedFunc->hasExactDefinition() && |
| !SpecializedFunc->hasFnAttribute(Attribute::Naked)) |
| Solver.addTrackedFunction(SpecializedFunc); |
| Solver.addArgumentTrackedFunction(SpecializedFunc); |
| FuncDecls.push_back(SpecializedFunc); |
| Solver.markBlockExecutable(&SpecializedFunc->front()); |
| |
| // Replace the function arguments for the specialized functions. |
| for (Argument &Arg : SpecializedFunc->args()) |
| if (!Arg.use_empty() && tryToReplaceWithConstant(&Arg)) |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Replaced constant argument: " |
| << Arg.getName() << "\n"); |
| } |
| |
| NumFuncSpecialized += NbFunctionsSpecialized; |
| return Changed; |
| } |
| |
| bool tryToReplaceWithConstant(Value *V) { |
| if (!V->getType()->isSingleValueType() || isa<CallBase>(V) || |
| V->user_empty()) |
| return false; |
| |
| const ValueLatticeElement &IV = Solver.getLatticeValueFor(V); |
| if (isOverdefined(IV)) |
| return false; |
| auto *Const = |
| isConstant(IV) ? Solver.getConstant(IV) : UndefValue::get(V->getType()); |
| V->replaceAllUsesWith(Const); |
| |
| for (auto *U : Const->users()) |
| if (auto *I = dyn_cast<Instruction>(U)) |
| if (Solver.isBlockExecutable(I->getParent())) |
| Solver.visit(I); |
| |
| // Remove the instruction from Block and Solver. |
| if (auto *I = dyn_cast<Instruction>(V)) { |
| if (I->isSafeToRemove()) { |
| I->eraseFromParent(); |
| Solver.removeLatticeValueFor(I); |
| } |
| } |
| return true; |
| } |
| |
| private: |
| // The number of functions specialised, used for collecting statistics and |
| // also in the cost model. |
| unsigned NbFunctionsSpecialized = 0; |
| |
| /// Clone the function \p F and remove the ssa_copy intrinsics added by |
| /// the SCCPSolver in the cloned version. |
| Function *cloneCandidateFunction(Function *F) { |
| ValueToValueMapTy EmptyMap; |
| Function *Clone = CloneFunction(F, EmptyMap); |
| removeSSACopy(*Clone); |
| return Clone; |
| } |
| |
| /// This function decides whether to specialize function \p F based on the |
| /// known constant values its arguments can take on. Specialization is |
| /// performed on the first interesting argument. Specializations based on |
| /// additional arguments will be evaluated on following iterations of the |
| /// main IPSCCP solve loop. \returns true if the function is specialized and |
| /// false otherwise. |
| bool specializeFunction(Function *F, |
| SmallVectorImpl<Function *> &Specializations) { |
| |
| // Do not specialize the cloned function again. |
| if (SpecializedFuncs.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"); |
| |
| // Determine if it would be profitable to create a specialization of the |
| // function where the argument takes on the given constant value. If so, |
| // add the constant to Constants. |
| auto FnSpecCost = getSpecializationCost(F); |
| if (!FnSpecCost.isValid()) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Invalid specialisation cost.\n"); |
| return false; |
| } |
| |
| LLVM_DEBUG(dbgs() << "FnSpecialization: func specialisation cost: "; |
| FnSpecCost.print(dbgs()); dbgs() << "\n"); |
| |
| // Determine if we should specialize the function based on the values the |
| // argument can take on. If specialization is not profitable, we continue |
| // on to the next argument. |
| for (Argument &A : F->args()) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Analysing arg: " << A.getName() |
| << "\n"); |
| // True if this will be a partial specialization. We will need to keep |
| // the original function around in addition to the added specializations. |
| bool IsPartial = true; |
| |
| // Determine if this argument is interesting. If we know the argument can |
| // take on any constant values, they are collected in Constants. If the |
| // argument can only ever equal a constant value in Constants, the |
| // function will be completely specialized, and the IsPartial flag will |
| // be set to false by isArgumentInteresting (that function only adds |
| // values to the Constants list that are deemed profitable). |
| SmallVector<Constant *, 4> Constants; |
| if (!isArgumentInteresting(&A, Constants, FnSpecCost, IsPartial)) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Argument is not interesting\n"); |
| continue; |
| } |
| |
| assert(!Constants.empty() && "No constants on which to specialize"); |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Argument is interesting!\n" |
| << "FnSpecialization: Specializing '" << F->getName() |
| << "' on argument: " << A << "\n" |
| << "FnSpecialization: Constants are:\n\n"; |
| for (unsigned I = 0; I < Constants.size(); ++I) dbgs() |
| << *Constants[I] << "\n"; |
| dbgs() << "FnSpecialization: End of constants\n\n"); |
| |
| // Create a version of the function in which the argument is marked |
| // constant with the given value. |
| for (auto *C : Constants) { |
| // Clone the function. We leave the ValueToValueMap empty to allow |
| // IPSCCP to propagate the constant arguments. |
| Function *Clone = cloneCandidateFunction(F); |
| Argument *ClonedArg = Clone->arg_begin() + A.getArgNo(); |
| |
| // Rewrite calls to the function so that they call the clone instead. |
| rewriteCallSites(F, Clone, *ClonedArg, C); |
| |
| // 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.markArgInFuncSpecialization(F, ClonedArg, C); |
| |
| // Mark all the specialized functions |
| Specializations.push_back(Clone); |
| NbFunctionsSpecialized++; |
| } |
| |
| // If the function has been completely specialized, the original function |
| // is no longer needed. Mark it unreachable. |
| if (!IsPartial) |
| Solver.markFunctionUnreachable(F); |
| |
| // FIXME: Only one argument per function. |
| return true; |
| } |
| |
| return false; |
| } |
| |
| /// Compute the cost of specializing function \p F. |
| InstructionCost getSpecializationCost(Function *F) { |
| // Compute the code metrics for the function. |
| SmallPtrSet<const Value *, 32> EphValues; |
| CodeMetrics::collectEphemeralValues(F, &(GetAC)(*F), EphValues); |
| CodeMetrics Metrics; |
| for (BasicBlock &BB : *F) |
| Metrics.analyzeBasicBlock(&BB, (GetTTI)(*F), EphValues); |
| |
| // If the code metrics reveal that we shouldn't duplicate the function, we |
| // shouldn't specialize it. Set the specialization cost to Invalid. |
| // Or if the lines of codes implies that this function is easy to get |
| // inlined so that we shouldn't specialize it. |
| if (Metrics.notDuplicatable || |
| (!ForceFunctionSpecialization && |
| Metrics.NumInsts < SmallFunctionThreshold)) { |
| InstructionCost C{}; |
| C.setInvalid(); |
| return C; |
| } |
| |
| // Otherwise, set the specialization cost to be the cost of all the |
| // instructions in the function and penalty for specializing more functions. |
| unsigned Penalty = NbFunctionsSpecialized + 1; |
| return Metrics.NumInsts * InlineConstants::InstrCost * Penalty; |
| } |
| |
| InstructionCost getUserBonus(User *U, llvm::TargetTransformInfo &TTI, |
| LoopInfo &LI) { |
| auto *I = dyn_cast_or_null<Instruction>(U); |
| // If not an instruction we do not know how to evaluate. |
| // Keep minimum possible cost for now so that it doesnt affect |
| // specialization. |
| if (!I) |
| return std::numeric_limits<unsigned>::min(); |
| |
| auto Cost = TTI.getUserCost(U, TargetTransformInfo::TCK_SizeAndLatency); |
| |
| // Traverse recursively if there are more uses. |
| // TODO: Any other instructions to be added here? |
| if (I->mayReadFromMemory() || I->isCast()) |
| for (auto *User : I->users()) |
| Cost += getUserBonus(User, TTI, LI); |
| |
| // Increase the cost if it is inside the loop. |
| auto LoopDepth = LI.getLoopDepth(I->getParent()); |
| Cost *= std::pow((double)AvgLoopIterationCount, LoopDepth); |
| return Cost; |
| } |
| |
| /// Compute a bonus for replacing argument \p A with constant \p C. |
| InstructionCost getSpecializationBonus(Argument *A, Constant *C) { |
| Function *F = A->getParent(); |
| DominatorTree DT(*F); |
| LoopInfo LI(DT); |
| auto &TTI = (GetTTI)(*F); |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Analysing bonus for: " << *A |
| << "\n"); |
| |
| InstructionCost TotalCost = 0; |
| for (auto *U : A->users()) { |
| TotalCost += getUserBonus(U, TTI, LI); |
| LLVM_DEBUG(dbgs() << "FnSpecialization: User cost "; |
| TotalCost.print(dbgs()); dbgs() << " for: " << *U << "\n"); |
| } |
| |
| // The below heuristic is only concerned with exposing inlining |
| // opportunities via indirect call promotion. If the argument is not a |
| // function pointer, give up. |
| if (!isa<PointerType>(A->getType()) || |
| !isa<FunctionType>(A->getType()->getPointerElementType())) |
| return TotalCost; |
| |
| // Since the argument is a function pointer, its incoming constant values |
| // should be functions or constant expressions. The code below attempts to |
| // look through cast expressions to find the function that will be called. |
| Value *CalledValue = C; |
| while (isa<ConstantExpr>(CalledValue) && |
| cast<ConstantExpr>(CalledValue)->isCast()) |
| CalledValue = cast<User>(CalledValue)->getOperand(0); |
| Function *CalledFunction = dyn_cast<Function>(CalledValue); |
| if (!CalledFunction) |
| return TotalCost; |
| |
| // 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 Bonus = 0; |
| for (User *U : A->users()) { |
| if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
| continue; |
| auto *CS = cast<CallBase>(U); |
| if (CS->getCalledOperand() != A) |
| 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()) |
| Bonus += Params.DefaultThreshold; |
| else if (IC.isVariable() && IC.getCostDelta() > 0) |
| Bonus += IC.getCostDelta(); |
| } |
| |
| return TotalCost + Bonus; |
| } |
| |
| /// Determine if we should specialize a function based on the incoming values |
| /// of the given argument. |
| /// |
| /// This function implements the goal-directed heuristic. It determines if |
| /// specializing the function based on the incoming values of argument \p A |
| /// would result in any significant optimization opportunities. If |
| /// optimization opportunities exist, the constant values of \p A on which to |
| /// specialize the function are collected in \p Constants. If the values in |
| /// \p Constants represent the complete set of values that \p A can take on, |
| /// the function will be completely specialized, and the \p IsPartial flag is |
| /// set to false. |
| /// |
| /// \returns true if the function should be specialized on the given |
| /// argument. |
| bool isArgumentInteresting(Argument *A, |
| SmallVectorImpl<Constant *> &Constants, |
| const InstructionCost &FnSpecCost, |
| bool &IsPartial) { |
| // For now, don't attempt to specialize functions based on the values of |
| // composite types. |
| if (!A->getType()->isSingleValueType() || A->user_empty()) |
| return false; |
| |
| // If the argument isn't overdefined, there's nothing to do. It should |
| // already be constant. |
| if (!Solver.getLatticeValueFor(A).isOverdefined()) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: nothing to do, arg is already " |
| << "constant?\n"); |
| return false; |
| } |
| |
| // Collect the constant values that the argument can take on. If the |
| // argument can't take on any constant values, we aren't going to |
| // specialize the function. While it's possible to specialize the function |
| // based on non-constant arguments, there's likely not much benefit to |
| // constant propagation in doing so. |
| // |
| // TODO 1: currently it won't specialize if there are over the threshold of |
| // calls using the same argument, e.g foo(a) x 4 and foo(b) x 1, but it |
| // might be beneficial to take the occurrences into account in the cost |
| // model, so we would need to find the unique constants. |
| // |
| // TODO 2: this currently does not support constants, i.e. integer ranges. |
| // |
| SmallVector<Constant *, 4> PossibleConstants; |
| bool AllConstant = getPossibleConstants(A, PossibleConstants); |
| if (PossibleConstants.empty()) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: no possible constants found\n"); |
| return false; |
| } |
| if (PossibleConstants.size() > MaxConstantsThreshold) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: number of constants found exceed " |
| << "the maximum number of constants threshold.\n"); |
| return false; |
| } |
| |
| for (auto *C : PossibleConstants) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Constant: " << *C << "\n"); |
| if (ForceFunctionSpecialization) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Forced!\n"); |
| Constants.push_back(C); |
| continue; |
| } |
| if (getSpecializationBonus(A, C) > FnSpecCost) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: profitable!\n"); |
| Constants.push_back(C); |
| } else { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: not profitable\n"); |
| } |
| } |
| |
| // None of the constant values the argument can take on were deemed good |
| // candidates on which to specialize the function. |
| if (Constants.empty()) |
| return false; |
| |
| // This will be a partial specialization if some of the constants were |
| // rejected due to their profitability. |
| IsPartial = !AllConstant || PossibleConstants.size() != Constants.size(); |
| |
| return true; |
| } |
| |
| /// Collect in \p Constants all the constant values that argument \p A can |
| /// take on. |
| /// |
| /// \returns true if all of the values the argument can take on are constant |
| /// (e.g., the argument's parent function cannot be called with an |
| /// overdefined value). |
| bool getPossibleConstants(Argument *A, |
| SmallVectorImpl<Constant *> &Constants) { |
| Function *F = A->getParent(); |
| bool AllConstant = true; |
| |
| // Iterate over all the call sites of the argument's parent function. |
| for (User *U : F->users()) { |
| if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
| continue; |
| auto &CS = *cast<CallBase>(U); |
| // If the call site has attribute minsize set, that callsite won't be |
| // specialized. |
| if (CS.hasFnAttr(Attribute::MinSize)) { |
| AllConstant = false; |
| 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; |
| |
| auto *V = CS.getArgOperand(A->getArgNo()); |
| if (isa<PoisonValue>(V)) |
| return false; |
| |
| // For now, constant expressions are fine but only if they are function |
| // calls. |
| if (auto *CE = dyn_cast<ConstantExpr>(V)) |
| if (!isa<Function>(CE->getOperand(0))) |
| return false; |
| |
| // TrackValueOfGlobalVariable only tracks scalar global variables. |
| if (auto *GV = dyn_cast<GlobalVariable>(V)) { |
| // Check if we want to specialize on the address of non-constant |
| // global values. |
| if (!GV->isConstant()) |
| if (!SpecializeOnAddresses) |
| return false; |
| |
| if (!GV->getValueType()->isSingleValueType()) |
| return false; |
| } |
| |
| if (isa<Constant>(V) && (Solver.getLatticeValueFor(V).isConstant() || |
| EnableSpecializationForLiteralConstant)) |
| Constants.push_back(cast<Constant>(V)); |
| else |
| AllConstant = false; |
| } |
| |
| // If the argument can only take on constant values, AllConstant will be |
| // true. |
| return AllConstant; |
| } |
| |
| /// Rewrite calls to function \p F to call function \p Clone instead. |
| /// |
| /// This function modifies calls to function \p F whose argument at index \p |
| /// ArgNo is equal to constant \p C. The calls are rewritten to call function |
| /// \p Clone instead. |
| /// |
| /// Callsites that have been marked with the MinSize function attribute won't |
| /// be specialized and rewritten. |
| void rewriteCallSites(Function *F, Function *Clone, Argument &Arg, |
| Constant *C) { |
| unsigned ArgNo = Arg.getArgNo(); |
| SmallVector<CallBase *, 4> CallSitesToRewrite; |
| for (auto *U : F->users()) { |
| if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
| continue; |
| auto &CS = *cast<CallBase>(U); |
| if (!CS.getCalledFunction() || CS.getCalledFunction() != F) |
| continue; |
| CallSitesToRewrite.push_back(&CS); |
| } |
| for (auto *CS : CallSitesToRewrite) { |
| if ((CS->getFunction() == Clone && CS->getArgOperand(ArgNo) == &Arg) || |
| CS->getArgOperand(ArgNo) == C) { |
| CS->setCalledFunction(Clone); |
| Solver.markOverdefined(CS); |
| } |
| } |
| } |
| }; |
| } // namespace |
| |
| bool llvm::runFunctionSpecialization( |
| Module &M, const DataLayout &DL, |
| std::function<TargetLibraryInfo &(Function &)> GetTLI, |
| std::function<TargetTransformInfo &(Function &)> GetTTI, |
| std::function<AssumptionCache &(Function &)> GetAC, |
| function_ref<AnalysisResultsForFn(Function &)> GetAnalysis) { |
| SCCPSolver Solver(DL, GetTLI, M.getContext()); |
| FunctionSpecializer FS(Solver, GetAC, GetTTI, GetTLI); |
| bool Changed = false; |
| |
| // Loop over all functions, marking arguments to those with their addresses |
| // taken or that are external as overdefined. |
| for (Function &F : M) { |
| if (F.isDeclaration()) |
| continue; |
| if (F.hasFnAttribute(Attribute::NoDuplicate)) |
| continue; |
| |
| LLVM_DEBUG(dbgs() << "\nFnSpecialization: Analysing decl: " << F.getName() |
| << "\n"); |
| Solver.addAnalysis(F, GetAnalysis(F)); |
| |
| // Determine if we can track the function's arguments. If so, add the |
| // function to the solver's set of argument-tracked functions. |
| if (canTrackArgumentsInterprocedurally(&F)) { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Can track arguments\n"); |
| Solver.addArgumentTrackedFunction(&F); |
| continue; |
| } else { |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Can't track arguments!\n" |
| << "FnSpecialization: Doesn't have local linkage, or " |
| << "has its address taken\n"); |
| } |
| |
| // Assume the function is called. |
| Solver.markBlockExecutable(&F.front()); |
| |
| // Assume nothing about the incoming arguments. |
| for (Argument &AI : F.args()) |
| Solver.markOverdefined(&AI); |
| } |
| |
| // Determine if we can track any of the module's global variables. If so, add |
| // the global variables we can track to the solver's set of tracked global |
| // variables. |
| for (GlobalVariable &G : M.globals()) { |
| G.removeDeadConstantUsers(); |
| if (canTrackGlobalVariableInterprocedurally(&G)) |
| Solver.trackValueOfGlobalVariable(&G); |
| } |
| |
| auto &TrackedFuncs = Solver.getArgumentTrackedFunctions(); |
| SmallVector<Function *, 16> FuncDecls(TrackedFuncs.begin(), |
| TrackedFuncs.end()); |
| |
| // No tracked functions, so nothing to do: don't run the solver and remove |
| // the ssa_copy intrinsics that may have been introduced. |
| if (TrackedFuncs.empty()) { |
| removeSSACopy(M); |
| return false; |
| } |
| |
| // Solve for constants. |
| auto RunSCCPSolver = [&](auto &WorkList) { |
| bool ResolvedUndefs = true; |
| |
| while (ResolvedUndefs) { |
| // Not running the solver unnecessary is checked in regression test |
| // nothing-to-do.ll, so if this debug message is changed, this regression |
| // test needs updating too. |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Running solver\n"); |
| |
| Solver.solve(); |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Resolving undefs\n"); |
| ResolvedUndefs = false; |
| for (Function *F : WorkList) |
| if (Solver.resolvedUndefsIn(*F)) |
| ResolvedUndefs = true; |
| } |
| |
| for (auto *F : WorkList) { |
| for (BasicBlock &BB : *F) { |
| if (!Solver.isBlockExecutable(&BB)) |
| continue; |
| // FIXME: The solver may make changes to the function here, so set |
| // Changed, even if later function specialization does not trigger. |
| for (auto &I : make_early_inc_range(BB)) |
| Changed |= FS.tryToReplaceWithConstant(&I); |
| } |
| } |
| }; |
| |
| #ifndef NDEBUG |
| LLVM_DEBUG(dbgs() << "FnSpecialization: Worklist fn decls:\n"); |
| for (auto *F : FuncDecls) |
| LLVM_DEBUG(dbgs() << "FnSpecialization: *) " << F->getName() << "\n"); |
| #endif |
| |
| // Initially resolve the constants in all the argument tracked functions. |
| RunSCCPSolver(FuncDecls); |
| |
| SmallVector<Function *, 2> CurrentSpecializations; |
| unsigned I = 0; |
| while (FuncSpecializationMaxIters != I++ && |
| FS.specializeFunctions(FuncDecls, CurrentSpecializations)) { |
| |
| // Run the solver for the specialized functions. |
| RunSCCPSolver(CurrentSpecializations); |
| |
| // Replace some unresolved constant arguments. |
| constantArgPropagation(FuncDecls, M, Solver); |
| |
| CurrentSpecializations.clear(); |
| Changed = true; |
| } |
| |
| // Clean up the IR by removing ssa_copy intrinsics. |
| removeSSACopy(M); |
| return Changed; |
| } |
