path: root/js/src/jit/CompactBuffer.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef jit_Compactbuffer_h
#define jit_Compactbuffer_h

#include "mozilla/Assertions.h"

#include <stddef.h>
#include <stdint.h>

#include "js/AllocPolicy.h"
#include "js/Vector.h"

namespace js {
namespace jit {

class CompactBufferWriter;

// CompactBuffers are byte streams designed for compressable integers. It has
// helper functions for writing bytes, fixed-size integers, and variable-sized
// integers. Variable sized integers are encoded in 1-5 bytes, each byte
// containing 7 bits of the integer and a bit which specifies whether the next
// byte is also part of the integer.
//
// Fixed-width integers are also available, in case the actual value will not
// be known until later.

class CompactBufferReader {
  const uint8_t* buffer_;
  const uint8_t* end_;

  uint32_t readVariableLength() {
    uint32_t val = 0;
    uint32_t shift = 0;
    uint8_t byte;
    while (true) {
      MOZ_ASSERT(shift < 32);
      byte = readByte();
      val |= (uint32_t(byte) >> 1) << shift;
      shift += 7;
      if (!(byte & 1)) {
        return val;
      }
    }
  }

  uint64_t readVariableLength64() {
    uint64_t val = 0;
    uint32_t shift = 0;
    uint8_t byte;
    while (true) {
      MOZ_ASSERT(shift < 64);
      byte = readByte();
      val |= (uint64_t(byte) >> 1) << shift;
      shift += 7;
      if (!(byte & 1)) {
        return val;
      }
    }
  }

 public:
  CompactBufferReader(const uint8_t* start, const uint8_t* end)
      : buffer_(start), end_(end) {}
  inline explicit CompactBufferReader(const CompactBufferWriter& writer);
  uint8_t readByte() {
    MOZ_ASSERT(buffer_ < end_);
    return *buffer_++;
  }
  uint32_t readFixedUint32_t() {
    uint32_t b0 = readByte();
    uint32_t b1 = readByte();
    uint32_t b2 = readByte();
    uint32_t b3 = readByte();
    return b0 | (b1 << 8) | (b2 << 16) | (b3 << 24);
  }
  uint16_t readFixedUint16_t() {
    uint32_t b0 = readByte();
    uint32_t b1 = readByte();
    return b0 | (b1 << 8);
  }
  uint32_t readNativeEndianUint32_t() {
    // Must be at 4-byte boundary
    MOZ_ASSERT(uintptr_t(buffer_) % sizeof(uint32_t) == 0);
    return *reinterpret_cast<const uint32_t*>(buffer_);
  }
  uint32_t readUnsigned() { return readVariableLength(); }
  uint64_t readUnsigned64() { return readVariableLength64(); }
  int32_t readSigned() {
    uint8_t b = readByte();
    bool isNegative = !!(b & (1 << 0));
    bool more = !!(b & (1 << 1));
    int32_t result = b >> 2;
    if (more) {
      result |= readUnsigned() << 6;
    }
    if (isNegative) {
      return -result;
    }
    return result;
  }
  // Reads a value written by writeUnsigned15Bit.
  uint32_t readUnsigned15Bit() {
    uint8_t byte = readByte();
    uint32_t val = byte >> 1;
    if (byte & 1) {
      val |= uint32_t(readByte()) << 7;
    }
    return val;
  }
  void* readRawPointer() {
    uintptr_t ptrWord = 0;
    for (unsigned i = 0; i < sizeof(uintptr_t); i++) {
      ptrWord |= static_cast<uintptr_t>(readByte()) << (i * 8);
    }
    return reinterpret_cast<void*>(ptrWord);
  }

  bool more() const {
    MOZ_ASSERT(buffer_ <= end_);
    return buffer_ < end_;
  }

  void seek(const uint8_t* start, uint32_t offset) {
    buffer_ = start + offset;
    MOZ_ASSERT(start < end_);
    MOZ_ASSERT(buffer_ <= end_);
  }

  const uint8_t* currentPosition() const { return buffer_; }
};

class CompactBufferWriter {
  js::Vector<uint8_t, 32, SystemAllocPolicy> buffer_;
  bool enoughMemory_;

 public:
  CompactBufferWriter() : enoughMemory_(true) {}

  void setOOM() { enoughMemory_ = false; }

  // Note: writeByte() takes uint32 to catch implicit casts with a runtime
  // assert.
  void writeByte(uint32_t byte) {
    MOZ_ASSERT(byte <= 0xFF);
    if (!buffer_.append(byte)) {
      enoughMemory_ = false;
    }
  }
  void writeByteAt(uint32_t pos, uint32_t byte) {
    MOZ_ASSERT(byte <= 0xFF);
    if (!oom()) {
      buffer_[pos] = byte;
    }
  }
  // Writes a variable-length value similar to writeUnsigned, but optimized for
  // small 15-bit values that fit in one or two variable-length-encoded bytes.
  // Must be read using readUnsigned15Bit.
  void writeUnsigned15Bit(uint32_t value) {
    uint8_t byte1 = ((value & 0x7F) << 1) | (value > 0x7F);
    writeByte(byte1);
    value >>= 7;
    if (value) {
      MOZ_ASSERT(value <= 0xFF);
      writeByte(value);
    }
  }
  void writeUnsigned(uint32_t value) {
    do {
      uint8_t byte = ((value & 0x7F) << 1) | (value > 0x7F);
      writeByte(byte);
      value >>= 7;
    } while (value);
  }
  void writeUnsignedAt(uint32_t pos, uint32_t value, uint32_t original) {
    MOZ_ASSERT(value <= original);
    do {
      uint8_t byte = ((value & 0x7F) << 1) | (original > 0x7F);
      writeByteAt(pos++, byte);
      value >>= 7;
      original >>= 7;
    } while (original);
  }
  void writeUnsigned64(uint64_t value) {
    do {
      uint8_t byte = ((value & 0x7F) << 1) | (value > 0x7F);
      writeByte(byte);
      value >>= 7;
    } while (value);
  }
  void writeSigned(int32_t v) {
    bool isNegative = v < 0;
    uint32_t value = isNegative ? -v : v;
    uint8_t byte =
        ((value & 0x3F) << 2) | ((value > 0x3F) << 1) | uint32_t(isNegative);
    writeByte(byte);

    // Write out the rest of the bytes, if needed.
    value >>= 6;
    if (value == 0) {
      return;
    }
    writeUnsigned(value);
  }
  void writeFixedUint32_t(uint32_t value) {
    writeByte(value & 0xFF);
    writeByte((value >> 8) & 0xFF);
    writeByte((value >> 16) & 0xFF);
    writeByte((value >> 24) & 0xFF);
  }
  void writeFixedUint16_t(uint16_t value) {
    writeByte(value & 0xFF);
    writeByte(value >> 8);
  }
  void writeNativeEndianUint32_t(uint32_t value) {
    // Must be at 4-byte boundary
    MOZ_ASSERT_IF(!oom(), length() % sizeof(uint32_t) == 0);
    writeFixedUint32_t(0);
    if (oom()) {
      return;
    }
    uint8_t* endPtr = buffer() + length();
    reinterpret_cast<uint32_t*>(endPtr)[-1] = value;
  }
  void writeRawPointer(const void* ptr) {
    uintptr_t ptrWord = reinterpret_cast<uintptr_t>(ptr);
    for (unsigned i = 0; i < sizeof(uintptr_t); i++) {
      writeByte((ptrWord >> (i * 8)) & 0xFF);
    }
  }
  size_t length() const { return buffer_.length(); }
  uint8_t* buffer() {
    MOZ_ASSERT(!oom());
    return &buffer_[0];
  }
  const uint8_t* buffer() const {
    MOZ_ASSERT(!oom());
    return &buffer_[0];
  }
  bool oom() const { return !enoughMemory_; }
  void propagateOOM(bool success) { enoughMemory_ &= success; }
};

CompactBufferReader::CompactBufferReader(const CompactBufferWriter& writer)
    : buffer_(writer.buffer()), end_(writer.buffer() + writer.length()) {}

}  // namespace jit
}  // namespace js

#endif /* jit_Compactbuffer_h */