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diff --git a/js/src/jit/CompactBuffer.h b/js/src/jit/CompactBuffer.h
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+++ b/js/src/jit/CompactBuffer.h
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+/* -*- 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;
+      }
+    }
+  }
+
+ 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(); }
+  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 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 */