c++/src/Dispatch.hh

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 ORC_DISPATCH_HH
#define ORC_DISPATCH_HH

#include <utility>
#include <vector>

#include "CpuInfoUtil.hh"

namespace orc {
  enum class DispatchLevel : int {
    // These dispatch levels, corresponding to instruction set features,
    // are sorted in increasing order of preference.
    NONE = 0,
    AVX512,
    MAX
  };

  /**
   * A facility for dynamic dispatch according to available DispatchLevel.
   *
   * Typical use:
   *
   *   static void my_function_default(...);
   *   static void my_function_avx512(...);
   *
   *   struct MyDynamicFunction {
   *     using FunctionType = decltype(&my_function_default);
   *
   *     static std::vector<std::pair<DispatchLevel, FunctionType>> implementations() {
   *       return {
   *         { DispatchLevel::NONE, my_function_default }
   *   #if defined(ORC_HAVE_RUNTIME_AVX512)
   *         , { DispatchLevel::AVX512, my_function_avx512 }
   *   #endif
   *       };
   *     }
   *   };
   *
   *   void my_function(...) {
   *     static DynamicDispatch<MyDynamicFunction> dispatch;
   *     return dispatch.func(...);
   *   }
   */
  template <typename DynamicFunction>
  class DynamicDispatch {
   protected:
    using FunctionType = typename DynamicFunction::FunctionType;
    using Implementation = std::pair<DispatchLevel, FunctionType>;

   public:
    DynamicDispatch() {
      Resolve(DynamicFunction::implementations());
    }

    FunctionType func = {};

   protected:
    // Use the Implementation with the highest DispatchLevel
    void Resolve(const std::vector<Implementation>& implementations) {
      Implementation cur{DispatchLevel::NONE, {}};

      for (const auto& impl : implementations) {
        if (impl.first >= cur.first && levelSupported(impl.first)) {
          // Higher (or same) level than current
          cur = impl;
        }
      }

      if (!cur.second) {
        throw InvalidArgument("No appropriate implementation found");
      }
      func = cur.second;
    }

   private:
    bool levelSupported(DispatchLevel level) const {
      static const auto cpu_info = CpuInfo::getInstance();

      switch (level) {
        case DispatchLevel::NONE:
          return true;
        case DispatchLevel::AVX512:
        case DispatchLevel::MAX:
          return cpu_info->isSupported(CpuInfo::AVX512);
        default:
          return false;
      }
    }
  };
}  // namespace orc

#endif