source/opt/register_pressure.h

// Copyright (c) 2018 Google LLC.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef SOURCE_OPT_REGISTER_PRESSURE_H_
#define SOURCE_OPT_REGISTER_PRESSURE_H_

#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include "source/opt/function.h"
#include "source/opt/types.h"

namespace spvtools {
namespace opt {

class IRContext;
class Loop;
class LoopDescriptor;

// Handles the register pressure of a function for different regions (function,
// loop, basic block). It also contains some utilities to foresee the register
// pressure following code transformations.
class RegisterLiveness {
 public:
  // Classification of SSA registers.
  struct RegisterClass {
    analysis::Type* type_;
    bool is_uniform_;

    bool operator==(const RegisterClass& rhs) const {
      return std::tie(type_, is_uniform_) ==
             std::tie(rhs.type_, rhs.is_uniform_);
    }
  };

  struct RegionRegisterLiveness {
    using LiveSet = std::unordered_set<Instruction*>;
    using RegClassSetTy = std::vector<std::pair<RegisterClass, size_t>>;

    // SSA register live when entering the basic block.
    LiveSet live_in_;
    // SSA register live when exiting the basic block.
    LiveSet live_out_;

    // Maximum number of required registers.
    size_t used_registers_;
    // Break down of the number of required registers per class of register.
    RegClassSetTy registers_classes_;

    void Clear() {
      live_out_.clear();
      live_in_.clear();
      used_registers_ = 0;
      registers_classes_.clear();
    }

    void AddRegisterClass(const RegisterClass& reg_class) {
      auto it = std::find_if(
          registers_classes_.begin(), registers_classes_.end(),
          [&reg_class](const std::pair<RegisterClass, size_t>& class_count) {
            return class_count.first == reg_class;
          });
      if (it != registers_classes_.end()) {
        it->second++;
      } else {
        registers_classes_.emplace_back(std::move(reg_class),
                                        static_cast<size_t>(1));
      }
    }

    void AddRegisterClass(Instruction* insn);
  };

  RegisterLiveness(IRContext* context, Function* f) : context_(context) {
    Analyze(f);
  }

  // Returns liveness and register information for the basic block |bb|. If no
  // entry exist for the basic block, the function returns null.
  const RegionRegisterLiveness* Get(const BasicBlock* bb) const {
    return Get(bb->id());
  }

  // Returns liveness and register information for the basic block id |bb_id|.
  // If no entry exist for the basic block, the function returns null.
  const RegionRegisterLiveness* Get(uint32_t bb_id) const {
    RegionRegisterLivenessMap::const_iterator it = block_pressure_.find(bb_id);
    if (it != block_pressure_.end()) {
      return &it->second;
    }
    return nullptr;
  }

  IRContext* GetContext() const { return context_; }

  // Returns liveness and register information for the basic block |bb|. If no
  // entry exist for the basic block, the function returns null.
  RegionRegisterLiveness* Get(const BasicBlock* bb) { return Get(bb->id()); }

  // Returns liveness and register information for the basic block id |bb_id|.
  // If no entry exist for the basic block, the function returns null.
  RegionRegisterLiveness* Get(uint32_t bb_id) {
    RegionRegisterLivenessMap::iterator it = block_pressure_.find(bb_id);
    if (it != block_pressure_.end()) {
      return &it->second;
    }
    return nullptr;
  }

  // Returns liveness and register information for the basic block id |bb_id| or
  // create a new empty entry if no entry already existed.
  RegionRegisterLiveness* GetOrInsert(uint32_t bb_id) {
    return &block_pressure_[bb_id];
  }

  // Compute the register pressure for the |loop| and store the result into
  // |reg_pressure|. The live-in set corresponds to the live-in set of the
  // header block, the live-out set of the loop corresponds to the union of the
  // live-in sets of each exit basic block.
  void ComputeLoopRegisterPressure(const Loop& loop,
                                   RegionRegisterLiveness* reg_pressure) const;

  // Estimate the register pressure for the |l1| and |l2| as if they were making
  // one unique loop. The result is stored into |simulation_result|.
  void SimulateFusion(const Loop& l1, const Loop& l2,
                      RegionRegisterLiveness* simulation_result) const;

  // Estimate the register pressure of |loop| after it has been fissioned
  // according to |moved_instructions| and |copied_instructions|. The function
  // assumes that the fission creates a new loop before |loop|, moves any
  // instructions present inside |moved_instructions| and copies any
  // instructions present inside |copied_instructions| into this new loop.
  // The set |loop1_sim_result| store the simulation result of the loop with the
  // moved instructions. The set |loop2_sim_result| store the simulation result
  // of the loop with the removed instructions.
  void SimulateFission(
      const Loop& loop,
      const std::unordered_set<Instruction*>& moved_instructions,
      const std::unordered_set<Instruction*>& copied_instructions,
      RegionRegisterLiveness* loop1_sim_result,
      RegionRegisterLiveness* loop2_sim_result) const;

 private:
  using RegionRegisterLivenessMap =
      std::unordered_map<uint32_t, RegionRegisterLiveness>;

  IRContext* context_;
  RegionRegisterLivenessMap block_pressure_;

  void Analyze(Function* f);
};

// Handles the register pressure of a function for different regions (function,
// loop, basic block). It also contains some utilities to foresee the register
// pressure following code transformations.
class LivenessAnalysis {
  using LivenessAnalysisMap =
      std::unordered_map<const Function*, RegisterLiveness>;

 public:
  LivenessAnalysis(IRContext* context) : context_(context) {}

  // Computes the liveness analysis for the function |f| and cache the result.
  // If the analysis was performed for this function, then the cached analysis
  // is returned.
  const RegisterLiveness* Get(Function* f) {
    LivenessAnalysisMap::iterator it = analysis_cache_.find(f);
    if (it != analysis_cache_.end()) {
      return &it->second;
    }
    return &analysis_cache_.emplace(f, RegisterLiveness{context_, f})
                .first->second;
  }

 private:
  IRContext* context_;
  LivenessAnalysisMap analysis_cache_;
};

}  // namespace opt
}  // namespace spvtools

#endif  // SOURCE_OPT_REGISTER_PRESSURE_H_