runtime/memory_region.h

/*
 * Copyright (C) 2011 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_RUNTIME_MEMORY_REGION_H_
#define ART_RUNTIME_MEMORY_REGION_H_

#include <stdint.h>

#include "base/logging.h"
#include "base/macros.h"
#include "base/value_object.h"
#include "globals.h"
#include "utils.h"

namespace art {

// Memory regions are useful for accessing memory with bounds check in
// debug mode. They can be safely passed by value and do not assume ownership
// of the region.
class MemoryRegion FINAL : public ValueObject {
 public:
  MemoryRegion() : pointer_(nullptr), size_(0) {}
  MemoryRegion(void* pointer_in, uintptr_t size_in) : pointer_(pointer_in), size_(size_in) {}

  void* pointer() const { return pointer_; }
  size_t size() const { return size_; }
  size_t size_in_bits() const { return size_ * kBitsPerByte; }

  static size_t pointer_offset() {
    return OFFSETOF_MEMBER(MemoryRegion, pointer_);
  }

  uint8_t* start() const { return reinterpret_cast<uint8_t*>(pointer_); }
  uint8_t* end() const { return start() + size_; }

  // Load value of type `T` at `offset`.  The memory address corresponding
  // to `offset` should be word-aligned.
  template<typename T> T Load(uintptr_t offset) const {
    // TODO: DCHECK that the address is word-aligned.
    return *ComputeInternalPointer<T>(offset);
  }

  // Store `value` (of type `T`) at `offset`.  The memory address
  // corresponding to `offset` should be word-aligned.
  template<typename T> void Store(uintptr_t offset, T value) const {
    // TODO: DCHECK that the address is word-aligned.
    *ComputeInternalPointer<T>(offset) = value;
  }

  // TODO: Local hack to prevent name clashes between two conflicting
  // implementations of bit_cast:
  // - art::bit_cast<Destination, Source> runtime/base/casts.h, and
  // - art::bit_cast<Source, Destination> from runtime/utils.h.
  // Remove this when these routines have been merged.
  template<typename Source, typename Destination>
  static Destination local_bit_cast(Source in) {
    static_assert(sizeof(Source) <= sizeof(Destination),
                  "Size of Source not <= size of Destination");
    union {
      Source u;
      Destination v;
    } tmp;
    tmp.u = in;
    return tmp.v;
  }

  // Load value of type `T` at `offset`.  The memory address corresponding
  // to `offset` does not need to be word-aligned.
  template<typename T> T LoadUnaligned(uintptr_t offset) const {
    // Equivalent unsigned integer type corresponding to T.
    typedef typename UnsignedIntegerType<sizeof(T)>::type U;
    U equivalent_unsigned_integer_value = 0;
    // Read the value byte by byte in a little-endian fashion.
    for (size_t i = 0; i < sizeof(U); ++i) {
      equivalent_unsigned_integer_value +=
          *ComputeInternalPointer<uint8_t>(offset + i) << (i * kBitsPerByte);
    }
    return local_bit_cast<U, T>(equivalent_unsigned_integer_value);
  }

  // Store `value` (of type `T`) at `offset`.  The memory address
  // corresponding to `offset` does not need to be word-aligned.
  template<typename T> void StoreUnaligned(uintptr_t offset, T value) const {
    // Equivalent unsigned integer type corresponding to T.
    typedef typename UnsignedIntegerType<sizeof(T)>::type U;
    U equivalent_unsigned_integer_value = local_bit_cast<T, U>(value);
    // Write the value byte by byte in a little-endian fashion.
    for (size_t i = 0; i < sizeof(U); ++i) {
      *ComputeInternalPointer<uint8_t>(offset + i) =
          (equivalent_unsigned_integer_value >> (i * kBitsPerByte)) & 0xFF;
    }
  }

  template<typename T> T* PointerTo(uintptr_t offset) const {
    return ComputeInternalPointer<T>(offset);
  }

  // Load a single bit in the region. The bit at offset 0 is the least
  // significant bit in the first byte.
  bool LoadBit(uintptr_t bit_offset) const {
    uint8_t bit_mask;
    uint8_t byte = *ComputeBitPointer(bit_offset, &bit_mask);
    return byte & bit_mask;
  }

  void StoreBit(uintptr_t bit_offset, bool value) const {
    uint8_t bit_mask;
    uint8_t* byte = ComputeBitPointer(bit_offset, &bit_mask);
    if (value) {
      *byte |= bit_mask;
    } else {
      *byte &= ~bit_mask;
    }
  }

  void CopyFrom(size_t offset, const MemoryRegion& from) const;

  // Compute a sub memory region based on an existing one.
  MemoryRegion Subregion(uintptr_t offset, uintptr_t size_in) const {
    CHECK_GE(this->size(), size_in);
    CHECK_LE(offset,  this->size() - size_in);
    return MemoryRegion(reinterpret_cast<void*>(start() + offset), size_in);
  }

  // Compute an extended memory region based on an existing one.
  void Extend(const MemoryRegion& region, uintptr_t extra) {
    pointer_ = region.pointer();
    size_ = (region.size() + extra);
  }

 private:
  template<typename T> T* ComputeInternalPointer(size_t offset) const {
    CHECK_GE(size(), sizeof(T));
    CHECK_LE(offset, size() - sizeof(T));
    return reinterpret_cast<T*>(start() + offset);
  }

  // Locate the bit with the given offset. Returns a pointer to the byte
  // containing the bit, and sets bit_mask to the bit within that byte.
  uint8_t* ComputeBitPointer(uintptr_t bit_offset, uint8_t* bit_mask) const {
    uintptr_t bit_remainder = (bit_offset & (kBitsPerByte - 1));
    *bit_mask = (1U << bit_remainder);
    uintptr_t byte_offset = (bit_offset >> kBitsPerByteLog2);
    return ComputeInternalPointer<uint8_t>(byte_offset);
  }

  void* pointer_;
  size_t size_;
};

}  // namespace art

#endif  // ART_RUNTIME_MEMORY_REGION_H_