compiler/gc_map_builder.h

/*
 * Copyright (C) 2014 The Android Open Source Project
 *
 * 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 ART_COMPILER_GC_MAP_BUILDER_H_
#define ART_COMPILER_GC_MAP_BUILDER_H_

#include <vector>

#include "base/bit_utils.h"
#include "gc_map.h"

namespace art {

class GcMapBuilder {
 public:
  template <typename Vector>
  GcMapBuilder(Vector* table, size_t entries, uint32_t max_native_offset,
               size_t references_width)
      : entries_(entries), references_width_(entries != 0u ? references_width : 0u),
        native_offset_width_(entries != 0 && max_native_offset != 0
                             ? sizeof(max_native_offset) - CLZ(max_native_offset) / 8u
                             : 0u),
        in_use_(entries) {
    static_assert(std::is_same<typename Vector::value_type, uint8_t>::value, "Invalid value type");

    // Resize table and set up header.
    table->resize((EntryWidth() * entries) + sizeof(uint32_t));
    table_ = table->data();
    CHECK_LT(native_offset_width_, 1U << 3);
    (*table)[0] = native_offset_width_ & 7;
    CHECK_LT(references_width_, 1U << 13);
    (*table)[0] |= (references_width_ << 3) & 0xFF;
    (*table)[1] = (references_width_ >> 5) & 0xFF;
    CHECK_LT(entries, 1U << 16);
    (*table)[2] = entries & 0xFF;
    (*table)[3] = (entries >> 8) & 0xFF;
  }

  void AddEntry(uint32_t native_offset, const uint8_t* references) {
    size_t table_index = TableIndex(native_offset);
    while (in_use_[table_index]) {
      table_index = (table_index + 1) % entries_;
    }
    in_use_[table_index] = true;
    SetCodeOffset(table_index, native_offset);
    DCHECK_EQ(native_offset, GetCodeOffset(table_index));
    SetReferences(table_index, references);
  }

 private:
  size_t TableIndex(uint32_t native_offset) {
    return NativePcOffsetToReferenceMap::Hash(native_offset) % entries_;
  }

  uint32_t GetCodeOffset(size_t table_index) {
    uint32_t native_offset = 0;
    size_t table_offset = (table_index * EntryWidth()) + sizeof(uint32_t);
    for (size_t i = 0; i < native_offset_width_; i++) {
      native_offset |= table_[table_offset + i] << (i * 8);
    }
    return native_offset;
  }

  void SetCodeOffset(size_t table_index, uint32_t native_offset) {
    size_t table_offset = (table_index * EntryWidth()) + sizeof(uint32_t);
    for (size_t i = 0; i < native_offset_width_; i++) {
      table_[table_offset + i] = (native_offset >> (i * 8)) & 0xFF;
    }
  }

  void SetReferences(size_t table_index, const uint8_t* references) {
    size_t table_offset = (table_index * EntryWidth()) + sizeof(uint32_t);
    memcpy(&table_[table_offset + native_offset_width_], references, references_width_);
  }

  size_t EntryWidth() const {
    return native_offset_width_ + references_width_;
  }

  // Number of entries in the table.
  const size_t entries_;
  // Number of bytes used to encode the reference bitmap.
  const size_t references_width_;
  // Number of bytes used to encode a native offset.
  const size_t native_offset_width_;
  // Entries that are in use.
  std::vector<bool> in_use_;
  // The table we're building.
  uint8_t* table_;
};

}  // namespace art

#endif  // ART_COMPILER_GC_MAP_BUILDER_H_