chrome中tcmalloc的使用

chrome中内存分配采用了第三方库tcmalloc，这个库主要提供给应用程序内存管理方面的优化，按资料说内存存取速度会从300ns降到50ns。更具体的关于这个tcmalloc的信息大家可以查网上的资料看看， 本人对tcmalloc的实现不是很熟，这里主要向熟悉一下chrome的代码，主要说说 tcmalloc如何集成到了chrome中，通过研究这个，一是看看如何使用tcmalloc，另外可以对window下VC的内存C runtime的库有更多的了解。

tcmalloc主要提供了自有的一套内存分配的函数，来替换我们常使用的new delete等VC++默认提供的内存分配实现。

chrome中的tcmalloc的源码位于third_party下，源代码通过GYP引入到chrome中的allocator项目中，我们重点可以看下 allocator_shim.cc中的代码。该文件中对外提供了内存分配的malloc, free和reallocator等函数，用来接管系统默认提供的内存非配函数。

extern "C" {
void* malloc(size_t size) __THROW {
  void* ptr;
  for (;;) {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
    switch (allocator) {
      case JEMALLOC:
        ptr = je_malloc(size);
        break;
      case WINHEAP:
      case WINLFH:
        ptr = win_heap_malloc(size);
        break;
      case TCMALLOC:
      default:
        ptr = do_malloc(size);
        break;
    }
#else
    // TCMalloc case.
    ptr = do_malloc(size);
#endif
    if (ptr)
      return ptr;

    if (!new_mode || !call_new_handler(true))
      break;
  }
  return ptr;
}

void free(void* p) __THROW {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
  switch (allocator) {
    case JEMALLOC:
      je_free(p);
      return;
    case WINHEAP:
    case WINLFH:
      win_heap_free(p);
      return;
  }
#endif
  // TCMalloc case.
  do_free(p);
}

void* realloc(void* ptr, size_t size) __THROW {
  // Webkit is brittle for allocators that return NULL for malloc(0).  The
  // realloc(0, 0) code path does not guarantee a non-NULL return, so be sure
  // to call malloc for this case.
  if (!ptr)
    return malloc(size);

  void* new_ptr;
  for (;;) {
#ifdef ENABLE_DYNAMIC_ALLOCATOR_SWITCHING
    switch (allocator) {
      case JEMALLOC:
        new_ptr = je_realloc(ptr, size);
        break;
      case WINHEAP:
      case WINLFH:
        new_ptr = win_heap_realloc(ptr, size);
        break;
      case TCMALLOC:
      default:
        new_ptr = do_realloc(ptr, size);
        break;
    }
#else
    // TCMalloc case.
    new_ptr = do_realloc(ptr, size);
#endif

    // Subtle warning:  NULL return does not alwas indicate out-of-memory.  If
    // the requested new size is zero, realloc should free the ptr and return
    // NULL.
    if (new_ptr || !size)
      return new_ptr;
    if (!new_mode || !call_new_handler(true))
      break;
  }
  return new_ptr;
}

　　通过这三个函数的代码我们可以看出，chrome提供了进行内存库切换的枚举， 不同的枚举值对应不同的具体malloc的实现转发，具体由JEMalloc, tcmalloc和默认等几种不同的内存库可以使用。

按照我们的经验，如果直接编译的话，那么就会有两份的malloc的实现，此时链接是不会通过的，chrome中如何解决？

chrome下的base中allocator相关的项目有4个，其中有一个名为libcmt的项目，其中只有一个python的脚本， 代码如下：

#!/usr/bin/env python

# Copyright (c) 2012 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
#
# This script takes libcmt.lib for VS2005/08/10/12/13 and removes the allocation
# related functions from it.
#
# Usage: prep_libc.py <VCLibDir> <OutputDir> <arch>
#
# VCLibDir is the path where VC is installed, something like:
#    C:\Program Files\Microsoft Visual Studio 8\VC\lib
# OutputDir is the directory where the modified libcmt file should be stored.
# arch is either 'ia32' or 'x64'

import os
import shutil
import subprocess
import sys

def run(command, filter=None):
  """Run |command|, removing any lines that match |filter|. The filter is
  to remove the echoing of input filename that 'lib' does."""
  popen = subprocess.Popen(
      command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
  out, _ = popen.communicate()
  for line in out.splitlines():
    if filter and line.strip() != filter:
      print line
  return popen.returncode

def main():
  bindir = 'SELF_X86'
  objdir = 'INTEL'
  vs_install_dir = sys.argv[1]
  outdir = sys.argv[2]
  if "x64" in sys.argv[3]:
    bindir = 'SELF_64_amd64'
    objdir = 'amd64'
    vs_install_dir = os.path.join(vs_install_dir, 'amd64')
  output_lib = os.path.join(outdir, 'libcmt.lib')
  shutil.copyfile(os.path.join(vs_install_dir, 'libcmt.lib'), output_lib)
  shutil.copyfile(os.path.join(vs_install_dir, 'libcmt.pdb'),
                  os.path.join(outdir, 'libcmt.pdb'))

  vspaths = [
    'build\\intel\\mt_obj\\',
    'f:\\dd\\vctools\\crt_bld\\' + bindir + \
      '\\crt\\src\\build\\' + objdir + '\\mt_obj\\',
    'F:\\dd\\vctools\\crt_bld\\' + bindir + \
      '\\crt\\src\\build\\' + objdir + '\\mt_obj\\nativec\\\\',
    'F:\\dd\\vctools\\crt_bld\\' + bindir + \
      '\\crt\\src\\build\\' + objdir + '\\mt_obj\\nativecpp\\\\',
    'f:\\binaries\\Intermediate\\vctools\\crt_bld\\' + bindir + \
      '\\crt\\prebuild\\build\\INTEL\\mt_obj\\cpp_obj\\\\',
    ]

  objfiles = ['malloc', 'free', 'realloc', 'new', 'delete', 'new2', 'delete2',
              'align', 'msize', 'heapinit', 'expand', 'heapchk', 'heapwalk',
              'heapmin', 'sbheap', 'calloc', 'recalloc', 'calloc_impl',
              'new_mode', 'newopnt', 'newaopnt']
  for obj in objfiles:
    for vspath in vspaths:
      cmd = ('lib /nologo /ignore:4006,4014,4221 /remove:%s%s.obj %s' %
             (vspath, obj, output_lib))
      run(cmd, obj + '.obj')

if __name__ == "__main__":
  sys.exit(main())

　　这段代码主要 是从libcmt.lib中移出 malloc, free, new 等 的obj的默认实现， 就可以使用到我们所提供的malloc了。