FunctionSpecialization.h

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//===- FunctionSpecialization.h - 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. Constant parameters
// like function pointers and constant globals are propagated to the callee by
// specializing the function. The main benefit of this pass at the moment is
// that indirect calls are transformed into direct calls, which provides inline
// opportunities that the inliner would not have been able to achieve. That's
// why function specialisation is run before the inliner in the optimisation
// pipeline; that is by design. Otherwise, we would only benefit from constant
// passing, which is a valid use-case too, but hasn't been explored much in
// terms of performance uplifts, cost-model and compile-time impact.
//
// Current limitations:
// - It does not yet handle integer ranges. We do support "literal constants",
//   but that's off by default under an option.
// - The cost-model could be further looked into (it mainly focuses on inlining
//   benefits),
//
// 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 function specialization does not trigger;
//   the SCCPSolver may make IR changes.
//
// References:
// - 2021 LLVM Dev Mtg “Introducing function specialisation, and can we enable
//   it by default?”, https://www.youtube.com/watch?v=zJiCjeXgV5Q
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_IPO_FUNCTIONSPECIALIZATION_H
#define LLVM_TRANSFORMS_IPO_FUNCTIONSPECIALIZATION_H
 
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/TargetTransformInfo.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"
 
using namespace llvm;
 
namespace llvm {
// Specialization signature, used to uniquely designate a specialization within
// a function.
struct SpecSig {
  // Hashing support, used to distinguish between ordinary, empty, or tombstone
  // keys.
  unsigned Key = 0;
  SmallVector<ArgInfo, 4> Args;
 
  bool operator==(const SpecSig &Other) const {
    if (Key != Other.Key || Args.size() != Other.Args.size())
      return false;
    for (size_t I = 0; I < Args.size(); ++I)
      if (Args[I] != Other.Args[I])
        return false;
    return true;
  }
 
  friend hash_code hash_value(const SpecSig &S) {
    return hash_combine(hash_value(S.Key),
                        hash_combine_range(S.Args.begin(), S.Args.end()));
  }
};
 
// Specialization instance.
struct Spec {
  // Original function.
  Function *F;
 
  // Cloned function, a specialized version of the original one.
  Function *Clone = nullptr;
 
  // Specialization signature.
  SpecSig Sig;
 
  // Profitability of the specialization.
  InstructionCost Gain;
 
  // List of call sites, matching this specialization.
  SmallVector<CallBase *> CallSites;
 
  Spec(Function *F, const SpecSig &S, InstructionCost G)
      : F(F), Sig(S), Gain(G) {}
  Spec(Function *F, const SpecSig &&S, InstructionCost G)
      : F(F), Sig(S), Gain(G) {}
};
 
// Map of potential specializations for each function. The FunctionSpecializer
// keeps the discovered specialisation opportunities for the module in a single
// vector, where the specialisations of each function form a contiguous range.
// This map's value is the beginning and the end of that range.
using SpecMap = DenseMap<Function *, std::pair<unsigned, unsigned>>;
 
class FunctionSpecializer {
 
  /// The IPSCCP Solver.
  SCCPSolver &Solver;
 
  Module &M;
 
  /// Analysis manager, needed to invalidate analyses.
  FunctionAnalysisManager *FAM;
 
  /// Analyses used to help determine if a function should be specialized.
  std::function<const TargetLibraryInfo &(Function &)> GetTLI;
  std::function<TargetTransformInfo &(Function &)> GetTTI;
  std::function<AssumptionCache &(Function &)> GetAC;
 
  SmallPtrSet<Function *, 32> Specializations;
  SmallPtrSet<Function *, 32> FullySpecialized;
  DenseMap<Function *, CodeMetrics> FunctionMetrics;
 
public:
  FunctionSpecializer(
      SCCPSolver &Solver, Module &M, FunctionAnalysisManager *FAM,
      std::function<const TargetLibraryInfo &(Function &)> GetTLI,
      std::function<TargetTransformInfo &(Function &)> GetTTI,
      std::function<AssumptionCache &(Function &)> GetAC)
      : Solver(Solver), M(M), FAM(FAM), GetTLI(GetTLI), GetTTI(GetTTI),
        GetAC(GetAC) {}
 
  ~FunctionSpecializer();
 
  bool isClonedFunction(Function *F) { return Specializations.count(F); }
 
  bool run();
 
private:
  Constant *getPromotableAlloca(AllocaInst *Alloca, CallInst *Call);
 
  /// 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 *getConstantStackValue(CallInst *Call, Value *Val);
 
  /// Iterate over the argument tracked functions see if there
  /// are any new constant values for the call instruction via
  /// stack variables.
  void promoteConstantStackValues();
 
  /// Clean up fully specialized functions.
  void removeDeadFunctions();
 
  /// Remove any ssa_copy intrinsics that may have been introduced.
  void cleanUpSSA();
 
  // Compute the code metrics for function \p F.
  CodeMetrics &analyzeFunction(Function *F);
 
  /// @brief  Find potential specialization opportunities.
  /// @param F Function to specialize
  /// @param Cost Cost of specializing a function. Final gain is this cost
  /// minus benefit
  /// @param AllSpecs A vector to add potential specializations to.
  /// @param SM  A map for a function's specialisation range
  /// @return True, if any potential specializations were found
  bool findSpecializations(Function *F, InstructionCost Cost,
                           SmallVectorImpl<Spec> &AllSpecs, SpecMap &SM);
 
  bool isCandidateFunction(Function *F);
 
  /// @brief Create a specialization of \p F and prime the SCCPSolver
  /// @param F Function to specialize
  /// @param S Which specialization to create
  /// @return The new, cloned function
  Function *createSpecialization(Function *F, const SpecSig &S);
 
  /// Compute and return the cost of specializing function \p F.
  InstructionCost getSpecializationCost(Function *F);
 
  /// Compute a bonus for replacing argument \p A with constant \p C.
  InstructionCost getSpecializationBonus(Argument *A, Constant *C,
                                         const LoopInfo &LI);
 
  /// Determine if it is possible to specialise the function for constant values
  /// of the formal parameter \p A.
  bool isArgumentInteresting(Argument *A);
 
  /// Check if the value \p V  (an actual argument) is a constant or can only
  /// have a constant value. Return that constant.
  Constant *getCandidateConstant(Value *V);
 
  /// @brief Find and update calls to \p F, which match a specialization
  /// @param F Orginal function
  /// @param Begin Start of a range of possibly matching specialisations
  /// @param End End of a range (exclusive) of possibly matching specialisations
  void updateCallSites(Function *F, const Spec *Begin, const Spec *End);
};
} // namespace llvm
 
#endif // LLVM_TRANSFORMS_IPO_FUNCTIONSPECIALIZATION_H