path: root/runtime/stack_map.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_RUNTIME_STACK_MAP_H_
#define ART_RUNTIME_STACK_MAP_H_

#include "base/bit_vector.h"
#include "base/bit_utils.h"
#include "memory_region.h"

namespace art {

// Size of a frame slot, in bytes.  This constant is a signed value,
// to please the compiler in arithmetic operations involving int32_t
// (signed) values.
static constexpr ssize_t kFrameSlotSize = 4;

// Size of Dex virtual registers.
static constexpr size_t kVRegSize = 4;

class CodeInfo;

/**
 * Classes in the following file are wrapper on stack map information backed
 * by a MemoryRegion. As such they read and write to the region, they don't have
 * their own fields.
 */

/**
 * Inline information for a specific PC. The information is of the form:
 * [inlining_depth, [method_dex reference]+]
 */
class InlineInfo {
 public:
  explicit InlineInfo(MemoryRegion region) : region_(region) {}

  uint8_t GetDepth() const {
    return region_.LoadUnaligned<uint8_t>(kDepthOffset);
  }

  void SetDepth(uint8_t depth) {
    region_.StoreUnaligned<uint8_t>(kDepthOffset, depth);
  }

  uint32_t GetMethodReferenceIndexAtDepth(uint8_t depth) const {
    return region_.LoadUnaligned<uint32_t>(kFixedSize + depth * SingleEntrySize());
  }

  void SetMethodReferenceIndexAtDepth(uint8_t depth, uint32_t index) {
    region_.StoreUnaligned<uint32_t>(kFixedSize + depth * SingleEntrySize(), index);
  }

  static size_t SingleEntrySize() {
    return sizeof(uint32_t);
  }

 private:
  // TODO: Instead of plain types such as "uint8_t", introduce
  // typedefs (and document the memory layout of InlineInfo).
  static constexpr int kDepthOffset = 0;
  static constexpr int kFixedSize = kDepthOffset + sizeof(uint8_t);

  MemoryRegion region_;

  friend class CodeInfo;
  friend class StackMap;
  friend class StackMapStream;
};

// Dex register location container used by DexRegisterMap and StackMapStream.
class DexRegisterLocation {
 public:
  /*
   * The location kind used to populate the Dex register information in a
   * StackMapStream can either be:
   * - kNone: the register has no location yet, meaning it has not been set;
   * - kConstant: value holds the constant;
   * - kStack: value holds the stack offset;
   * - kRegister: value holds the physical register number;
   * - kFpuRegister: value holds the physical register number.
   *
   * In addition, DexRegisterMap also uses these values:
   * - kInStackLargeOffset: value holds a "large" stack offset (greater than
   *   or equal to 128 bytes);
   * - kConstantLargeValue: value holds a "large" constant (lower than 0, or
   *   or greater than or equal to 32).
   */
  enum class Kind : uint8_t {
    // Short location kinds, for entries fitting on one byte (3 bits
    // for the kind, 5 bits for the value) in a DexRegisterMap.
    kNone = 0,                // 0b000
    kInStack = 1,             // 0b001
    kInRegister = 2,          // 0b010
    kInFpuRegister = 3,       // 0b011
    kConstant = 4,            // 0b100

    // Large location kinds, requiring a 5-byte encoding (1 byte for the
    // kind, 4 bytes for the value).

    // Stack location at a large offset, meaning that the offset value
    // divided by the stack frame slot size (4 bytes) cannot fit on a
    // 5-bit unsigned integer (i.e., this offset value is greater than
    // or equal to 2^5 * 4 = 128 bytes).
    kInStackLargeOffset = 5,  // 0b101

    // Large constant, that cannot fit on a 5-bit signed integer (i.e.,
    // lower than 0, or greater than or equal to 2^5 = 32).
    kConstantLargeValue = 6,  // 0b110

    kLastLocationKind = kConstantLargeValue
  };

  static_assert(
      sizeof(Kind) == 1u,
      "art::DexRegisterLocation::Kind has a size different from one byte.");

  static const char* PrettyDescriptor(Kind kind) {
    switch (kind) {
      case Kind::kNone:
        return "none";
      case Kind::kInStack:
        return "in stack";
      case Kind::kInRegister:
        return "in register";
      case Kind::kInFpuRegister:
        return "in fpu register";
      case Kind::kConstant:
        return "as constant";
      case Kind::kInStackLargeOffset:
        return "in stack (large offset)";
      case Kind::kConstantLargeValue:
        return "as constant (large value)";
      default:
        UNREACHABLE();
    }
  }

  static bool IsShortLocationKind(Kind kind) {
    switch (kind) {
      case Kind::kNone:
      case Kind::kInStack:
      case Kind::kInRegister:
      case Kind::kInFpuRegister:
      case Kind::kConstant:
        return true;

      case Kind::kInStackLargeOffset:
      case Kind::kConstantLargeValue:
        return false;

      default:
        UNREACHABLE();
    }
  }

  // Convert `kind` to a "surface" kind, i.e. one that doesn't include
  // any value with a "large" qualifier.
  // TODO: Introduce another enum type for the surface kind?
  static Kind ConvertToSurfaceKind(Kind kind) {
    switch (kind) {
      case Kind::kNone:
      case Kind::kInStack:
      case Kind::kInRegister:
      case Kind::kInFpuRegister:
      case Kind::kConstant:
        return kind;

      case Kind::kInStackLargeOffset:
        return Kind::kInStack;

      case Kind::kConstantLargeValue:
        return Kind::kConstant;

      default:
        UNREACHABLE();
    }
  }

  // Required by art::StackMapStream::LocationCatalogEntriesIndices.
  DexRegisterLocation() : kind_(Kind::kNone), value_(0) {}

  DexRegisterLocation(Kind kind, int32_t value) : kind_(kind), value_(value) {}

  static DexRegisterLocation None() {
    return DexRegisterLocation(Kind::kNone, 0);
  }

  // Get the "surface" kind of the location, i.e., the one that doesn't
  // include any value with a "large" qualifier.
  Kind GetKind() const {
    return ConvertToSurfaceKind(kind_);
  }

  // Get the value of the location.
  int32_t GetValue() const { return value_; }

  // Get the actual kind of the location.
  Kind GetInternalKind() const { return kind_; }

  bool operator==(DexRegisterLocation other) const {
    return kind_ == other.kind_ && value_ == other.value_;
  }

  bool operator!=(DexRegisterLocation other) const {
    return !(*this == other);
  }

 private:
  Kind kind_;
  int32_t value_;

  friend class DexRegisterLocationHashFn;
};

/**
 * Store information on unique Dex register locations used in a method.
 * The information is of the form:
 * [DexRegisterLocation+].
 * DexRegisterLocations are either 1- or 5-byte wide (see art::DexRegisterLocation::Kind).
 */
class DexRegisterLocationCatalog {
 public:
  explicit DexRegisterLocationCatalog(MemoryRegion region) : region_(region) {}

  // Short (compressed) location, fitting on one byte.
  typedef uint8_t ShortLocation;

  void SetRegisterInfo(size_t offset, const DexRegisterLocation& dex_register_location) {
    DexRegisterLocation::Kind kind = ComputeCompressedKind(dex_register_location);
    int32_t value = dex_register_location.GetValue();
    if (DexRegisterLocation::IsShortLocationKind(kind)) {
      // Short location.  Compress the kind and the value as a single byte.
      if (kind == DexRegisterLocation::Kind::kInStack) {
        // Instead of storing stack offsets expressed in bytes for
        // short stack locations, store slot offsets.  A stack offset
        // is a multiple of 4 (kFrameSlotSize).  This means that by
        // dividing it by 4, we can fit values from the [0, 128)
        // interval in a short stack location, and not just values
        // from the [0, 32) interval.
        DCHECK_EQ(value % kFrameSlotSize, 0);
        value /= kFrameSlotSize;
      }
      DCHECK(IsShortValue(value)) << value;
      region_.StoreUnaligned<ShortLocation>(offset, MakeShortLocation(kind, value));
    } else {
      // Large location.  Write the location on one byte and the value
      // on 4 bytes.
      DCHECK(!IsShortValue(value)) << value;
      if (kind == DexRegisterLocation::Kind::kInStackLargeOffset) {
        // Also divide large stack offsets by 4 for the sake of consistency.
        DCHECK_EQ(value % kFrameSlotSize, 0);
        value /= kFrameSlotSize;
      }
      // Data can be unaligned as the written Dex register locations can
      // either be 1-byte or 5-byte wide.  Use
      // art::MemoryRegion::StoreUnaligned instead of
      // art::MemoryRegion::Store to prevent unligned word accesses on ARM.
      region_.StoreUnaligned<DexRegisterLocation::Kind>(offset, kind);
      region_.StoreUnaligned<int32_t>(offset + sizeof(DexRegisterLocation::Kind), value);
    }
  }

  // Find the offset of the location catalog entry number `location_catalog_entry_index`.
  size_t FindLocationOffset(size_t location_catalog_entry_index) const {
    size_t offset = kFixedSize;
    // Skip the first `location_catalog_entry_index - 1` entries.
    for (uint16_t i = 0; i < location_catalog_entry_index; ++i) {
      // Read the first next byte and inspect its first 3 bits to decide
      // whether it is a short or a large location.
      DexRegisterLocation::Kind kind = ExtractKindAtOffset(offset);
      if (DexRegisterLocation::IsShortLocationKind(kind)) {
        // Short location.  Skip the current byte.
        offset += SingleShortEntrySize();
      } else {
        // Large location.  Skip the 5 next bytes.
        offset += SingleLargeEntrySize();
      }
    }
    return offset;
  }

  // Get the internal kind of entry at `location_catalog_entry_index`.
  DexRegisterLocation::Kind GetLocationInternalKind(size_t location_catalog_entry_index) const {
    if (location_catalog_entry_index == kNoLocationEntryIndex) {
      return DexRegisterLocation::Kind::kNone;
    }
    return ExtractKindAtOffset(FindLocationOffset(location_catalog_entry_index));
  }

  // Get the (surface) kind and value of entry at `location_catalog_entry_index`.
  DexRegisterLocation GetDexRegisterLocation(size_t location_catalog_entry_index) const {
    if (location_catalog_entry_index == kNoLocationEntryIndex) {
      return DexRegisterLocation::None();
    }
    size_t offset = FindLocationOffset(location_catalog_entry_index);
    // Read the first byte and inspect its first 3 bits to get the location.
    ShortLocation first_byte = region_.LoadUnaligned<ShortLocation>(offset);
    DexRegisterLocation::Kind kind = ExtractKindFromShortLocation(first_byte);
    if (DexRegisterLocation::IsShortLocationKind(kind)) {
      // Short location.  Extract the value from the remaining 5 bits.
      int32_t value = ExtractValueFromShortLocation(first_byte);
      if (kind == DexRegisterLocation::Kind::kInStack) {
        // Convert the stack slot (short) offset to a byte offset value.
        value *= kFrameSlotSize;
      }
      return DexRegisterLocation(kind, value);
    } else {
      // Large location.  Read the four next bytes to get the value.
      int32_t value = region_.LoadUnaligned<int32_t>(offset + sizeof(DexRegisterLocation::Kind));
      if (kind == DexRegisterLocation::Kind::kInStackLargeOffset) {
        // Convert the stack slot (large) offset to a byte offset value.
        value *= kFrameSlotSize;
      }
      return DexRegisterLocation(kind, value);
    }
  }

  // Compute the compressed kind of `location`.
  static DexRegisterLocation::Kind ComputeCompressedKind(const DexRegisterLocation& location) {
    switch (location.GetInternalKind()) {
      case DexRegisterLocation::Kind::kNone:
        DCHECK_EQ(location.GetValue(), 0);
        return DexRegisterLocation::Kind::kNone;

      case DexRegisterLocation::Kind::kInRegister:
        DCHECK_GE(location.GetValue(), 0);
        DCHECK_LT(location.GetValue(), 1 << kValueBits);
        return DexRegisterLocation::Kind::kInRegister;

      case DexRegisterLocation::Kind::kInFpuRegister:
        DCHECK_GE(location.GetValue(), 0);
        DCHECK_LT(location.GetValue(), 1 << kValueBits);
        return DexRegisterLocation::Kind::kInFpuRegister;

      case DexRegisterLocation::Kind::kInStack:
        return IsShortStackOffsetValue(location.GetValue())
            ? DexRegisterLocation::Kind::kInStack
            : DexRegisterLocation::Kind::kInStackLargeOffset;

      case DexRegisterLocation::Kind::kConstant:
        return IsShortConstantValue(location.GetValue())
            ? DexRegisterLocation::Kind::kConstant
            : DexRegisterLocation::Kind::kConstantLargeValue;

      default:
        LOG(FATAL) << "Unexpected location kind"
                   << DexRegisterLocation::PrettyDescriptor(location.GetInternalKind());
        UNREACHABLE();
    }
  }

  // Can `location` be turned into a short location?
  static bool CanBeEncodedAsShortLocation(const DexRegisterLocation& location) {
    switch (location.GetInternalKind()) {
      case DexRegisterLocation::Kind::kNone:
      case DexRegisterLocation::Kind::kInRegister:
      case DexRegisterLocation::Kind::kInFpuRegister:
        return true;

      case DexRegisterLocation::Kind::kInStack:
        return IsShortStackOffsetValue(location.GetValue());

      case DexRegisterLocation::Kind::kConstant:
        return IsShortConstantValue(location.GetValue());

      default:
        UNREACHABLE();
    }
  }

  static size_t EntrySize(const DexRegisterLocation& location) {
    return CanBeEncodedAsShortLocation(location) ? SingleShortEntrySize() : SingleLargeEntrySize();
  }

  static size_t SingleShortEntrySize() {
    return sizeof(ShortLocation);
  }

  static size_t SingleLargeEntrySize() {
    return sizeof(DexRegisterLocation::Kind) + sizeof(int32_t);
  }

  size_t Size() const {
    return region_.size();
  }

  // Special (invalid) Dex register location catalog entry index meaning
  // that there is no location for a given Dex register (i.e., it is
  // mapped to a DexRegisterLocation::Kind::kNone location).
  static constexpr size_t kNoLocationEntryIndex = -1;

 private:
  static constexpr int kFixedSize = 0;

  // Width of the kind "field" in a short location, in bits.
  static constexpr size_t kKindBits = 3;
  // Width of the value "field" in a short location, in bits.
  static constexpr size_t kValueBits = 5;

  static constexpr uint8_t kKindMask = (1 << kKindBits) - 1;
  static constexpr int32_t kValueMask = (1 << kValueBits) - 1;
  static constexpr size_t kKindOffset = 0;
  static constexpr size_t kValueOffset = kKindBits;

  static bool IsShortStackOffsetValue(int32_t value) {
    DCHECK_EQ(value % kFrameSlotSize, 0);
    return IsShortValue(value / kFrameSlotSize);
  }

  static bool IsShortConstantValue(int32_t value) {
    return IsShortValue(value);
  }

  static bool IsShortValue(int32_t value) {
    return IsUint<kValueBits>(value);
  }

  static ShortLocation MakeShortLocation(DexRegisterLocation::Kind kind, int32_t value) {
    uint8_t kind_integer_value = static_cast<uint8_t>(kind);
    DCHECK(IsUint<kKindBits>(kind_integer_value)) << kind_integer_value;
    DCHECK(IsShortValue(value)) << value;
    return (kind_integer_value & kKindMask) << kKindOffset
        | (value & kValueMask) << kValueOffset;
  }

  static DexRegisterLocation::Kind ExtractKindFromShortLocation(ShortLocation location) {
    uint8_t kind = (location >> kKindOffset) & kKindMask;
    DCHECK_LE(kind, static_cast<uint8_t>(DexRegisterLocation::Kind::kLastLocationKind));
    // We do not encode kNone locations in the stack map.
    DCHECK_NE(kind, static_cast<uint8_t>(DexRegisterLocation::Kind::kNone));
    return static_cast<DexRegisterLocation::Kind>(kind);
  }

  static int32_t ExtractValueFromShortLocation(ShortLocation location) {
    return (location >> kValueOffset) & kValueMask;
  }

  // Extract a location kind from the byte at position `offset`.
  DexRegisterLocation::Kind ExtractKindAtOffset(size_t offset) const {
    ShortLocation first_byte = region_.LoadUnaligned<ShortLocation>(offset);
    return ExtractKindFromShortLocation(first_byte);
  }

  MemoryRegion region_;

  friend class CodeInfo;
  friend class StackMapStream;
};

/* Information on Dex register locations for a specific PC, mapping a
 * stack map's Dex register to a location entry in a DexRegisterLocationCatalog.
 * The information is of the form:
 * [live_bit_mask, entries*]
 * where entries are concatenated unsigned integer values encoded on a number
 * of bits (fixed per DexRegisterMap instances of a CodeInfo object) depending
 * on the number of entries in the Dex register location catalog
 * (see DexRegisterMap::SingleEntrySizeInBits).  The map is 1-byte aligned.
 */
class DexRegisterMap {
 public:
  explicit DexRegisterMap(MemoryRegion region) : region_(region) {}

  // Get the surface kind of Dex register `dex_register_number`.
  DexRegisterLocation::Kind GetLocationKind(uint16_t dex_register_number,
                                            uint16_t number_of_dex_registers,
                                            const CodeInfo& code_info) const {
    return DexRegisterLocation::ConvertToSurfaceKind(
        GetLocationInternalKind(dex_register_number, number_of_dex_registers, code_info));
  }

  // Get the internal kind of Dex register `dex_register_number`.
  DexRegisterLocation::Kind GetLocationInternalKind(uint16_t dex_register_number,
                                                    uint16_t number_of_dex_registers,
                                                    const CodeInfo& code_info) const;

  // Get the Dex register location `dex_register_number`.
  DexRegisterLocation GetDexRegisterLocation(uint16_t dex_register_number,
                                             uint16_t number_of_dex_registers,
                                             const CodeInfo& code_info) const;

  int32_t GetStackOffsetInBytes(uint16_t dex_register_number,
                                uint16_t number_of_dex_registers,
                                const CodeInfo& code_info) const {
    DexRegisterLocation location =
        GetDexRegisterLocation(dex_register_number, number_of_dex_registers, code_info);
    DCHECK(location.GetKind() == DexRegisterLocation::Kind::kInStack);
    // GetDexRegisterLocation returns the offset in bytes.
    return location.GetValue();
  }

  int32_t GetConstant(uint16_t dex_register_number,
                      uint16_t number_of_dex_registers,
                      const CodeInfo& code_info) const {
    DexRegisterLocation location =
        GetDexRegisterLocation(dex_register_number, number_of_dex_registers, code_info);
    DCHECK(location.GetKind() == DexRegisterLocation::Kind::kConstant);
    return location.GetValue();
  }

  int32_t GetMachineRegister(uint16_t dex_register_number,
                             uint16_t number_of_dex_registers,
                             const CodeInfo& code_info) const {
    DexRegisterLocation location =
        GetDexRegisterLocation(dex_register_number, number_of_dex_registers, code_info);
    DCHECK(location.GetInternalKind() == DexRegisterLocation::Kind::kInRegister
           || location.GetInternalKind() == DexRegisterLocation::Kind::kInFpuRegister)
        << DexRegisterLocation::PrettyDescriptor(location.GetInternalKind());
    return location.GetValue();
  }

  // Get the index of the entry in the Dex register location catalog
  // corresponding to `dex_register_number`.
  size_t GetLocationCatalogEntryIndex(uint16_t dex_register_number,
                                      uint16_t number_of_dex_registers,
                                      size_t number_of_location_catalog_entries) const {
    if (!IsDexRegisterLive(dex_register_number)) {
      return DexRegisterLocationCatalog::kNoLocationEntryIndex;
    }

    if (number_of_location_catalog_entries == 1) {
      // We do not allocate space for location maps in the case of a
      // single-entry location catalog, as it is useless.  The only valid
      // entry index is 0;
      return 0;
    }

    // The bit offset of the beginning of the map locations.
    size_t map_locations_offset_in_bits =
        GetLocationMappingDataOffset(number_of_dex_registers) * kBitsPerByte;
    size_t index_in_dex_register_map = GetIndexInDexRegisterMap(dex_register_number);
    DCHECK_LT(index_in_dex_register_map, GetNumberOfLiveDexRegisters(number_of_dex_registers));
    // The bit size of an entry.
    size_t map_entry_size_in_bits = SingleEntrySizeInBits(number_of_location_catalog_entries);
    // The bit offset where `index_in_dex_register_map` is located.
    size_t entry_offset_in_bits =
        map_locations_offset_in_bits + index_in_dex_register_map * map_entry_size_in_bits;
    size_t location_catalog_entry_index =
        region_.LoadBits(entry_offset_in_bits, map_entry_size_in_bits);
    DCHECK_LT(location_catalog_entry_index, number_of_location_catalog_entries);
    return location_catalog_entry_index;
  }

  // Map entry at `index_in_dex_register_map` to `location_catalog_entry_index`.
  void SetLocationCatalogEntryIndex(size_t index_in_dex_register_map,
                                    size_t location_catalog_entry_index,
                                    uint16_t number_of_dex_registers,
                                    size_t number_of_location_catalog_entries) {
    DCHECK_LT(index_in_dex_register_map, GetNumberOfLiveDexRegisters(number_of_dex_registers));
    DCHECK_LT(location_catalog_entry_index, number_of_location_catalog_entries);

    if (number_of_location_catalog_entries == 1) {
      // We do not allocate space for location maps in the case of a
      // single-entry location catalog, as it is useless.
      return;
    }

    // The bit offset of the beginning of the map locations.
    size_t map_locations_offset_in_bits =
        GetLocationMappingDataOffset(number_of_dex_registers) * kBitsPerByte;
    // The bit size of an entry.
    size_t map_entry_size_in_bits = SingleEntrySizeInBits(number_of_location_catalog_entries);
    // The bit offset where `index_in_dex_register_map` is located.
    size_t entry_offset_in_bits =
        map_locations_offset_in_bits + index_in_dex_register_map * map_entry_size_in_bits;
    region_.StoreBits(entry_offset_in_bits, location_catalog_entry_index, map_entry_size_in_bits);
  }

  void SetLiveBitMask(uint16_t number_of_dex_registers,
                      const BitVector& live_dex_registers_mask) {
    size_t live_bit_mask_offset_in_bits = GetLiveBitMaskOffset() * kBitsPerByte;
    for (uint16_t i = 0; i < number_of_dex_registers; ++i) {
      region_.StoreBit(live_bit_mask_offset_in_bits + i, live_dex_registers_mask.IsBitSet(i));
    }
  }

  bool IsDexRegisterLive(uint16_t dex_register_number) const {
    size_t live_bit_mask_offset_in_bits = GetLiveBitMaskOffset() * kBitsPerByte;
    return region_.LoadBit(live_bit_mask_offset_in_bits + dex_register_number);
  }

  size_t GetNumberOfLiveDexRegisters(uint16_t number_of_dex_registers) const {
    size_t number_of_live_dex_registers = 0;
    for (size_t i = 0; i < number_of_dex_registers; ++i) {
      if (IsDexRegisterLive(i)) {
        ++number_of_live_dex_registers;
      }
    }
    return number_of_live_dex_registers;
  }

  static size_t GetLiveBitMaskOffset() {
    return kFixedSize;
  }

  // Compute the size of the live register bit mask (in bytes), for a
  // method having `number_of_dex_registers` Dex registers.
  static size_t GetLiveBitMaskSize(uint16_t number_of_dex_registers) {
    return RoundUp(number_of_dex_registers, kBitsPerByte) / kBitsPerByte;
  }

  static size_t GetLocationMappingDataOffset(uint16_t number_of_dex_registers) {
    return GetLiveBitMaskOffset() + GetLiveBitMaskSize(number_of_dex_registers);
  }

  size_t GetLocationMappingDataSize(uint16_t number_of_dex_registers,
                                    size_t number_of_location_catalog_entries) const {
    size_t location_mapping_data_size_in_bits =
        GetNumberOfLiveDexRegisters(number_of_dex_registers)
        * SingleEntrySizeInBits(number_of_location_catalog_entries);
    return RoundUp(location_mapping_data_size_in_bits, kBitsPerByte) / kBitsPerByte;
  }

  // Return the size of a map entry in bits.  Note that if
  // `number_of_location_catalog_entries` equals 1, this function returns 0,
  // which is fine, as there is no need to allocate a map for a
  // single-entry location catalog; the only valid location catalog entry index
  // for a live register in this case is 0 and there is no need to
  // store it.
  static size_t SingleEntrySizeInBits(size_t number_of_location_catalog_entries) {
    // Handle the case of 0, as we cannot pass 0 to art::WhichPowerOf2.
    return number_of_location_catalog_entries == 0
        ? 0u
        : WhichPowerOf2(RoundUpToPowerOfTwo(number_of_location_catalog_entries));
  }

  // Return the size of the DexRegisterMap object, in bytes.
  size_t Size() const {
    return region_.size();
  }

 private:
  // Return the index in the Dex register map corresponding to the Dex
  // register number `dex_register_number`.
  size_t GetIndexInDexRegisterMap(uint16_t dex_register_number) const {
    if (!IsDexRegisterLive(dex_register_number)) {
      return kInvalidIndexInDexRegisterMap;
    }
    return GetNumberOfLiveDexRegisters(dex_register_number);
  }

  // Special (invalid) Dex register map entry index meaning that there
  // is no index in the map for a given Dex register (i.e., it must
  // have been mapped to a DexRegisterLocation::Kind::kNone location).
  static constexpr size_t kInvalidIndexInDexRegisterMap = -1;

  static constexpr int kFixedSize = 0;

  MemoryRegion region_;

  friend class CodeInfo;
  friend class StackMapStream;
};

/**
 * A Stack Map holds compilation information for a specific PC necessary for:
 * - Mapping it to a dex PC,
 * - Knowing which stack entries are objects,
 * - Knowing which registers hold objects,
 * - Knowing the inlining information,
 * - Knowing the values of dex registers.
 *
 * The information is of the form:
 * [dex_pc, native_pc_offset, dex_register_map_offset, inlining_info_offset, register_mask,
 * stack_mask].
 *
 * Note that register_mask is fixed size, but stack_mask is variable size, depending on the
 * stack size of a method.
 */
class StackMap {
 public:
  StackMap() {}

  explicit StackMap(MemoryRegion region) : region_(region) {}

  bool IsValid() const { return region_.pointer() != nullptr; }

  uint32_t GetDexPc(const CodeInfo& info) const;

  void SetDexPc(const CodeInfo& info, uint32_t dex_pc);

  uint32_t GetNativePcOffset(const CodeInfo& info) const;

  void SetNativePcOffset(const CodeInfo& info, uint32_t native_pc_offset);

  uint32_t GetDexRegisterMapOffset(const CodeInfo& info) const;

  void SetDexRegisterMapOffset(const CodeInfo& info, uint32_t offset);

  uint32_t GetInlineDescriptorOffset(const CodeInfo& info) const;

  void SetInlineDescriptorOffset(const CodeInfo& info, uint32_t offset);

  uint32_t GetRegisterMask(const CodeInfo& info) const;

  void SetRegisterMask(const CodeInfo& info, uint32_t mask);

  MemoryRegion GetStackMask(const CodeInfo& info) const;

  void SetStackMask(const CodeInfo& info, const BitVector& sp_map) {
    MemoryRegion region = GetStackMask(info);
    for (size_t i = 0; i < region.size_in_bits(); i++) {
      region.StoreBit(i, sp_map.IsBitSet(i));
    }
  }

  bool HasDexRegisterMap(const CodeInfo& info) const {
    return GetDexRegisterMapOffset(info) != kNoDexRegisterMap;
  }

  bool HasInlineInfo(const CodeInfo& info) const {
    return GetInlineDescriptorOffset(info) != kNoInlineInfo;
  }

  bool Equals(const StackMap& other) const {
    return region_.pointer() == other.region_.pointer()
       && region_.size() == other.region_.size();
  }

  static size_t ComputeStackMapSize(size_t stack_mask_size,
                                    size_t inline_info_size,
                                    size_t dex_register_map_size,
                                    size_t dex_pc_max,
                                    size_t native_pc_max,
                                    size_t register_mask_max);

  // Special (invalid) offset for the DexRegisterMapOffset field meaning
  // that there is no Dex register map for this stack map.
  static constexpr uint32_t kNoDexRegisterMap = -1;

  // Special (invalid) offset for the InlineDescriptorOffset field meaning
  // that there is no inline info for this stack map.
  static constexpr uint32_t kNoInlineInfo = -1;

 private:
  static size_t ComputeStackMapSizeInternal(size_t stack_mask_size,
                                            size_t number_of_bytes_for_inline_info,
                                            size_t number_of_bytes_for_dex_map,
                                            size_t number_of_bytes_for_dex_pc,
                                            size_t number_of_bytes_for_native_pc,
                                            size_t number_of_bytes_for_register_mask);

  // TODO: Instead of plain types such as "uint32_t", introduce
  // typedefs (and document the memory layout of StackMap).
  static constexpr int kRegisterMaskOffset = 0;
  static constexpr int kFixedSize = 0;

  MemoryRegion region_;

  friend class CodeInfo;
  friend class StackMapStream;
};


/**
 * Wrapper around all compiler information collected for a method.
 * The information is of the form:
 * [overall_size, number_of_location_catalog_entries, number_of_stack_maps, stack_mask_size,
 * DexRegisterLocationCatalog+, StackMap+, DexRegisterMap+, InlineInfo*].
 */
class CodeInfo {
 public:
  explicit CodeInfo(MemoryRegion region) : region_(region) {}

  explicit CodeInfo(const void* data) {
    uint32_t size = reinterpret_cast<const uint32_t*>(data)[0];
    region_ = MemoryRegion(const_cast<void*>(data), size);
  }

  static size_t EncodingSizeInBytes(size_t max_element) {
    DCHECK(IsUint<32>(max_element));
    return (max_element == 0) ? 0
         : IsUint<8>(max_element) ? 1
         : IsUint<16>(max_element) ? 2
         : IsUint<24>(max_element) ? 3
         : 4;
  }

  void SetEncoding(size_t inline_info_size,
                   size_t dex_register_map_size,
                   size_t dex_pc_max,
                   size_t native_pc_max,
                   size_t register_mask_max) {
    if (inline_info_size != 0) {
      region_.StoreBit(kHasInlineInfoBitOffset, 1);
      // + 1 to also encode kNoInlineInfo: if an inline info offset
      // is at 0xFF, we want to overflow to a larger encoding, because it will
      // conflict with kNoInlineInfo.
      // The offset is relative to the dex register map. TODO: Change this.
      SetEncodingAt(kInlineInfoBitOffset,
                    EncodingSizeInBytes(dex_register_map_size + inline_info_size + 1));
    } else {
      region_.StoreBit(kHasInlineInfoBitOffset, 0);
      SetEncodingAt(kInlineInfoBitOffset, 0);
    }
    // + 1 to also encode kNoDexRegisterMap: if a dex register map offset
    // is at 0xFF, we want to overflow to a larger encoding, because it will
    // conflict with kNoDexRegisterMap.
    SetEncodingAt(kDexRegisterMapBitOffset, EncodingSizeInBytes(dex_register_map_size + 1));
    SetEncodingAt(kDexPcBitOffset, EncodingSizeInBytes(dex_pc_max));
    SetEncodingAt(kNativePcBitOffset, EncodingSizeInBytes(native_pc_max));
    SetEncodingAt(kRegisterMaskBitOffset, EncodingSizeInBytes(register_mask_max));
  }

  void SetEncodingAt(size_t bit_offset, size_t number_of_bytes) {
    // We encode the number of bytes needed for writing a value on 3 bits,
    // for values that we know are maximum 32bits.
    region_.StoreBit(bit_offset, (number_of_bytes & 1));
    region_.StoreBit(bit_offset + 1, (number_of_bytes & 2));
    region_.StoreBit(bit_offset + 2, (number_of_bytes & 4));
  }

  size_t GetNumberOfBytesForEncoding(size_t bit_offset) const {
    return region_.LoadBit(bit_offset)
        + (region_.LoadBit(bit_offset + 1) << 1)
        + (region_.LoadBit(bit_offset + 2) << 2);
  }

  bool HasInlineInfo() const {
    return region_.LoadBit(kHasInlineInfoBitOffset);
  }

  size_t NumberOfBytesForInlineInfo() const {
    return GetNumberOfBytesForEncoding(kInlineInfoBitOffset);
  }

  size_t NumberOfBytesForDexRegisterMap() const {
    return GetNumberOfBytesForEncoding(kDexRegisterMapBitOffset);
  }

  size_t NumberOfBytesForRegisterMask() const {
    return GetNumberOfBytesForEncoding(kRegisterMaskBitOffset);
  }

  size_t NumberOfBytesForNativePc() const {
    return GetNumberOfBytesForEncoding(kNativePcBitOffset);
  }

  size_t NumberOfBytesForDexPc() const {
    return GetNumberOfBytesForEncoding(kDexPcBitOffset);
  }

  size_t ComputeStackMapRegisterMaskOffset() const {
    return StackMap::kRegisterMaskOffset;
  }

  size_t ComputeStackMapStackMaskOffset() const {
    return ComputeStackMapRegisterMaskOffset()
        + (NumberOfBytesForRegisterMask() * sizeof(uint8_t));
  }

  size_t ComputeStackMapDexPcOffset() const {
    return ComputeStackMapStackMaskOffset() + GetStackMaskSize();
  }

  size_t ComputeStackMapNativePcOffset() const {
    return ComputeStackMapDexPcOffset()
        + (NumberOfBytesForDexPc() * sizeof(uint8_t));
  }

  size_t ComputeStackMapDexRegisterMapOffset() const {
    return ComputeStackMapNativePcOffset()
        + (NumberOfBytesForNativePc() * sizeof(uint8_t));
  }

  size_t ComputeStackMapInlineInfoOffset() const {
    CHECK(HasInlineInfo());
    return ComputeStackMapDexRegisterMapOffset()
        + (NumberOfBytesForDexRegisterMap() * sizeof(uint8_t));
  }

  uint32_t GetDexRegisterLocationCatalogOffset() const {
    return kFixedSize;
  }

  DexRegisterLocationCatalog GetDexRegisterLocationCatalog() const {
    return DexRegisterLocationCatalog(region_.Subregion(
        GetDexRegisterLocationCatalogOffset(),
        GetDexRegisterLocationCatalogSize()));
  }

  StackMap GetStackMapAt(size_t i) const {
    size_t size = StackMapSize();
    return StackMap(GetStackMaps().Subregion(i * size, size));
  }

  uint32_t GetOverallSize() const {
    return region_.LoadUnaligned<uint32_t>(kOverallSizeOffset);
  }

  void SetOverallSize(uint32_t size) {
    region_.StoreUnaligned<uint32_t>(kOverallSizeOffset, size);
  }

  uint32_t GetNumberOfDexRegisterLocationCatalogEntries() const {
    return region_.LoadUnaligned<uint32_t>(kNumberOfDexRegisterLocationCatalogEntriesOffset);
  }

  void SetNumberOfDexRegisterLocationCatalogEntries(uint32_t num_entries) {
    region_.StoreUnaligned<uint32_t>(kNumberOfDexRegisterLocationCatalogEntriesOffset, num_entries);
  }

  uint32_t GetDexRegisterLocationCatalogSize() const {
    return ComputeDexRegisterLocationCatalogSize(GetDexRegisterLocationCatalogOffset(),
                                                 GetNumberOfDexRegisterLocationCatalogEntries());
  }

  uint32_t GetStackMaskSize() const {
    return region_.LoadUnaligned<uint32_t>(kStackMaskSizeOffset);
  }

  void SetStackMaskSize(uint32_t size) {
    region_.StoreUnaligned<uint32_t>(kStackMaskSizeOffset, size);
  }

  size_t GetNumberOfStackMaps() const {
    return region_.LoadUnaligned<uint32_t>(kNumberOfStackMapsOffset);
  }

  void SetNumberOfStackMaps(uint32_t number_of_stack_maps) {
    region_.StoreUnaligned<uint32_t>(kNumberOfStackMapsOffset, number_of_stack_maps);
  }

  // Get the size of one stack map of this CodeInfo object, in bytes.
  // All stack maps of a CodeInfo have the same size.
  size_t StackMapSize() const {
    return StackMap::ComputeStackMapSizeInternal(GetStackMaskSize(),
                                                 NumberOfBytesForInlineInfo(),
                                                 NumberOfBytesForDexRegisterMap(),
                                                 NumberOfBytesForDexPc(),
                                                 NumberOfBytesForNativePc(),
                                                 NumberOfBytesForRegisterMask());
  }

  // Get the size all the stack maps of this CodeInfo object, in bytes.
  size_t GetStackMapsSize() const {
    return StackMapSize() * GetNumberOfStackMaps();
  }

  size_t GetDexRegisterMapsOffset() const {
    return GetStackMapsOffset() + GetStackMapsSize();
  }

  uint32_t GetStackMapsOffset() const {
    return GetDexRegisterLocationCatalogOffset() + GetDexRegisterLocationCatalogSize();
  }

  DexRegisterMap GetDexRegisterMapOf(StackMap stack_map, uint32_t number_of_dex_registers) const {
    DCHECK(stack_map.HasDexRegisterMap(*this));
    uint32_t offset = GetDexRegisterMapsOffset() + stack_map.GetDexRegisterMapOffset(*this);
    size_t size = ComputeDexRegisterMapSizeOf(offset, number_of_dex_registers);
    return DexRegisterMap(region_.Subregion(offset, size));
  }

  InlineInfo GetInlineInfoOf(StackMap stack_map) const {
    DCHECK(stack_map.HasInlineInfo(*this));
    uint32_t offset = stack_map.GetInlineDescriptorOffset(*this) + GetDexRegisterMapsOffset();
    uint8_t depth = region_.LoadUnaligned<uint8_t>(offset);
    return InlineInfo(region_.Subregion(offset,
        InlineInfo::kFixedSize + depth * InlineInfo::SingleEntrySize()));
  }

  StackMap GetStackMapForDexPc(uint32_t dex_pc) const {
    for (size_t i = 0, e = GetNumberOfStackMaps(); i < e; ++i) {
      StackMap stack_map = GetStackMapAt(i);
      if (stack_map.GetDexPc(*this) == dex_pc) {
        return stack_map;
      }
    }
    return StackMap();
  }

  StackMap GetStackMapForNativePcOffset(uint32_t native_pc_offset) const {
    // TODO: stack maps are sorted by native pc, we can do a binary search.
    for (size_t i = 0, e = GetNumberOfStackMaps(); i < e; ++i) {
      StackMap stack_map = GetStackMapAt(i);
      if (stack_map.GetNativePcOffset(*this) == native_pc_offset) {
        return stack_map;
      }
    }
    return StackMap();
  }

  void Dump(std::ostream& os, uint16_t number_of_dex_registers) const;
  void DumpStackMapHeader(std::ostream& os, size_t stack_map_num) const;

 private:
  // TODO: Instead of plain types such as "uint32_t", introduce
  // typedefs (and document the memory layout of CodeInfo).
  static constexpr int kOverallSizeOffset = 0;
  static constexpr int kEncodingInfoOffset = kOverallSizeOffset + sizeof(uint32_t);
  static constexpr int kNumberOfDexRegisterLocationCatalogEntriesOffset =
      kEncodingInfoOffset + sizeof(uint16_t);
  static constexpr int kNumberOfStackMapsOffset =
      kNumberOfDexRegisterLocationCatalogEntriesOffset + sizeof(uint32_t);
  static constexpr int kStackMaskSizeOffset = kNumberOfStackMapsOffset + sizeof(uint32_t);
  static constexpr int kFixedSize = kStackMaskSizeOffset + sizeof(uint32_t);

  static constexpr int kHasInlineInfoBitOffset = (kEncodingInfoOffset * kBitsPerByte);
  static constexpr int kInlineInfoBitOffset = kHasInlineInfoBitOffset + 1;
  static constexpr int kDexRegisterMapBitOffset = kInlineInfoBitOffset + 3;
  static constexpr int kDexPcBitOffset = kDexRegisterMapBitOffset + 3;
  static constexpr int kNativePcBitOffset = kDexPcBitOffset + 3;
  static constexpr int kRegisterMaskBitOffset = kNativePcBitOffset + 3;

  MemoryRegion GetStackMaps() const {
    return region_.size() == 0
        ? MemoryRegion()
        : region_.Subregion(GetStackMapsOffset(), GetStackMapsSize());
  }

  // Compute the size of the Dex register map associated to the stack map at
  // `dex_register_map_offset_in_code_info`.
  size_t ComputeDexRegisterMapSizeOf(uint32_t dex_register_map_offset_in_code_info,
                                     uint16_t number_of_dex_registers) const {
    // Offset where the actual mapping data starts within art::DexRegisterMap.
    size_t location_mapping_data_offset_in_dex_register_map =
        DexRegisterMap::GetLocationMappingDataOffset(number_of_dex_registers);
    // Create a temporary art::DexRegisterMap to be able to call
    // art::DexRegisterMap::GetNumberOfLiveDexRegisters and
    DexRegisterMap dex_register_map_without_locations(
        MemoryRegion(region_.Subregion(dex_register_map_offset_in_code_info,
                                       location_mapping_data_offset_in_dex_register_map)));
    size_t number_of_live_dex_registers =
        dex_register_map_without_locations.GetNumberOfLiveDexRegisters(number_of_dex_registers);
    size_t location_mapping_data_size_in_bits =
        DexRegisterMap::SingleEntrySizeInBits(GetNumberOfDexRegisterLocationCatalogEntries())
        * number_of_live_dex_registers;
    size_t location_mapping_data_size_in_bytes =
        RoundUp(location_mapping_data_size_in_bits, kBitsPerByte) / kBitsPerByte;
    size_t dex_register_map_size =
        location_mapping_data_offset_in_dex_register_map + location_mapping_data_size_in_bytes;
    return dex_register_map_size;
  }

  // Compute the size of a Dex register location catalog starting at offset `origin`
  // in `region_` and containing `number_of_dex_locations` entries.
  size_t ComputeDexRegisterLocationCatalogSize(uint32_t origin,
                                               uint32_t number_of_dex_locations) const {
    // TODO: Ideally, we would like to use art::DexRegisterLocationCatalog::Size or
    // art::DexRegisterLocationCatalog::FindLocationOffset, but the
    // DexRegisterLocationCatalog is not yet built.  Try to factor common code.
    size_t offset = origin + DexRegisterLocationCatalog::kFixedSize;

    // Skip the first `number_of_dex_locations - 1` entries.
    for (uint16_t i = 0; i < number_of_dex_locations; ++i) {
      // Read the first next byte and inspect its first 3 bits to decide
      // whether it is a short or a large location.
      DexRegisterLocationCatalog::ShortLocation first_byte =
          region_.LoadUnaligned<DexRegisterLocationCatalog::ShortLocation>(offset);
      DexRegisterLocation::Kind kind =
          DexRegisterLocationCatalog::ExtractKindFromShortLocation(first_byte);
      if (DexRegisterLocation::IsShortLocationKind(kind)) {
        // Short location.  Skip the current byte.
        offset += DexRegisterLocationCatalog::SingleShortEntrySize();
      } else {
        // Large location.  Skip the 5 next bytes.
        offset += DexRegisterLocationCatalog::SingleLargeEntrySize();
      }
    }
    size_t size = offset - origin;
    return size;
  }

  MemoryRegion region_;
  friend class StackMapStream;
};

}  // namespace art

#endif  // ART_RUNTIME_STACK_MAP_H_