text: // 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 LIBSPIRV_OPT_CONST_FOLDING_RULES_H_
  #define LIBSPIRV_OPT_CONST_FOLDING_RULES_H_
  
  #include <vector>
  
  #include "constants.h"
  
  namespace spvtools {
  namespace opt {
  
  // Constant Folding Rules:
  //
  // The folding mechanism is built around the concept of a |ConstantFoldingRule|.
  // A constant folding rule is a function that implements a method of simplifying
  // an instruction to a constant.
  //
  // The inputs to a folding rule are:
  //     |inst| - the instruction to be simplified.
  //     |constants| - if an in-operands is an id of a constant, then the
  //                   corresponding value in |constants| contains that
  //                   constant value.  Otherwise, the corresponding entry in
  //                   |constants| is |nullptr|.
  //
  // A constant folding rule returns a pointer to an Constant if |inst| can be
  // simplified using this rule. Otherwise, it returns |nullptr|.
  //
  // See const_folding_rules.cpp for examples on how to write a constant folding
  // rule.
  //
  // Be sure to add new constant folding rules to the table of constant folding
  // rules in the constructor for ConstantFoldingRules.  The new rule should be
  // added to the list for every opcode that it applies to.  Note that earlier
  // rules in the list are given priority.  That is, if an earlier rule is able to
  // fold an instruction, the later rules will not be attempted.
  
  using ConstantFoldingRule = std::function<const analysis::Constant*(
      opt::Instruction* inst,
      const std::vector<const analysis::Constant*>& constants)>;
  
  class ConstantFoldingRules {
   public:
    ConstantFoldingRules();
  
    // Returns true if there is at least 1 folding rule for |opcode|.
    bool HasFoldingRule(SpvOp opcode) const { return rules_.count(opcode); }
  
    // Returns an vector of constant folding rules for |opcode|.
    const std::vector<ConstantFoldingRule>& GetRulesForOpcode(
        SpvOp opcode) const {
      auto it = rules_.find(opcode);
      if (it != rules_.end()) {
        return it->second;
      }
      return empty_vector_;
    }
  
   private:
    std::unordered_map<uint32_t, std::vector<ConstantFoldingRule>> rules_;
    std::vector<ConstantFoldingRule> empty_vector_;
  };
  
  }  // namespace opt
  }  // namespace spvtools
  
  #endif  // LIBSPIRV_OPT_CONST_FOLDING_RULES_H_
  