说明

unordered_map 是关联容器，含有带唯一键的键-值对。搜索、插入和元素移除拥有平均常数时间复杂度。元素在内部不以任何特定顺序排序，而是组织进桶中。元素放进哪个桶完全依赖于其键的哈希。这允许对单独元素的快速访问，因为一旦计算哈希，则它准确指代元素所放进的桶。

问题

原系统基于GCC4.8.5，使用C++11标准开发，内部基于unordered_map存储数据，新系统先在升级GCC为7.3.0，仍然使用C++11标准开发。新旧系统都基于一份持久化文件恢复数据，并按照同一顺序插入unordered_map，并遍历unordered_map组包对外发送，通过对比新旧系统对外发包内容一致性，来验证新旧系统的正确性。

但验证的现象是新旧系统发包顺序不一致。

原因分析

• 哈希策略在不同GCC版本中有变化，插入时rehash时机不一样了（__prime_list不同）

• 初始桶的大小不一样，GCC4.8.5有默认值 10，之后的版本取消了默认值

根本原因：初始桶大小和每次插入时桶大小要保持一致

解决方案

• 替换GCC7.3.0里的哈希策略为GCC4.8.5的（标准库生成是弱符号，使用stub.cpp强符号替换）

//stub.cpp
#include 
#include 
#include 
  
namespace std 
{
namespace __detail
{
  extern const unsigned long __prime_list[] = // 256 + 1 or 256 + 48 + 1
  {
    2ul, 3ul, 5ul, 7ul, 11ul, 13ul, 17ul, 19ul, 23ul, 29ul, 31ul,
    37ul, 41ul, 43ul, 47ul, 53ul, 59ul, 61ul, 67ul, 71ul, 73ul, 79ul,
    83ul, 89ul, 97ul, 103ul, 109ul, 113ul, 127ul, 137ul, 139ul, 149ul,
    157ul, 167ul, 179ul, 193ul, 199ul, 211ul, 227ul, 241ul, 257ul,
    277ul, 293ul, 313ul, 337ul, 359ul, 383ul, 409ul, 439ul, 467ul,
    503ul, 541ul, 577ul, 619ul, 661ul, 709ul, 761ul, 823ul, 887ul,
    953ul, 1031ul, 1109ul, 1193ul, 1289ul, 1381ul, 1493ul, 1613ul,
    1741ul, 1879ul, 2029ul, 2179ul, 2357ul, 2549ul, 2753ul, 2971ul,
    3209ul, 3469ul, 3739ul, 4027ul, 4349ul, 4703ul, 5087ul, 5503ul,
    5953ul, 6427ul, 6949ul, 7517ul, 8123ul, 8783ul, 9497ul, 10273ul,
    11113ul, 12011ul, 12983ul, 14033ul, 15173ul, 16411ul, 17749ul,
    19183ul, 20753ul, 22447ul, 24281ul, 26267ul, 28411ul, 30727ul,
    33223ul, 35933ul, 38873ul, 42043ul, 45481ul, 49201ul, 53201ul,
    57557ul, 62233ul, 67307ul, 72817ul, 78779ul, 85229ul, 92203ul,
    99733ul, 107897ul, 116731ul, 126271ul, 136607ul, 147793ul,
    159871ul, 172933ul, 187091ul, 202409ul, 218971ul, 236897ul,
    256279ul, 277261ul, 299951ul, 324503ul, 351061ul, 379787ul,
    410857ul, 444487ul, 480881ul, 520241ul, 562841ul, 608903ul,
    658753ul, 712697ul, 771049ul, 834181ul, 902483ul, 976369ul,
    1056323ul, 1142821ul, 1236397ul, 1337629ul, 1447153ul, 1565659ul,
    1693859ul, 1832561ul, 1982627ul, 2144977ul, 2320627ul, 2510653ul,
    2716249ul, 2938679ul, 3179303ul, 3439651ul, 3721303ul, 4026031ul,
    4355707ul, 4712381ul, 5098259ul, 5515729ul, 5967347ul, 6456007ul,
    6984629ul, 7556579ul, 8175383ul, 8844859ul, 9569143ul, 10352717ul,
    11200489ul, 12117689ul, 13109983ul, 14183539ul, 15345007ul,
    16601593ul, 17961079ul, 19431899ul, 21023161ul, 22744717ul,
    24607243ul, 26622317ul, 28802401ul, 31160981ul, 33712729ul,
    36473443ul, 39460231ul, 42691603ul, 46187573ul, 49969847ul,
    54061849ul, 58488943ul, 63278561ul, 68460391ul, 74066549ul,
    80131819ul, 86693767ul, 93793069ul, 101473717ul, 109783337ul,
    118773397ul, 128499677ul, 139022417ul, 150406843ul, 162723577ul,
    176048909ul, 190465427ul, 206062531ul, 222936881ul, 241193053ul,
    260944219ul, 282312799ul, 305431229ul, 330442829ul, 357502601ul,
    386778277ul, 418451333ul, 452718089ul, 489790921ul, 529899637ul,
    573292817ul, 620239453ul, 671030513ul, 725980837ul, 785430967ul,
    849749479ul, 919334987ul, 994618837ul, 1076067617ul, 1164186217ul,
    1259520799ul, 1362662261ul, 1474249943ul, 1594975441ul, 1725587117ul,
    1866894511ul, 2019773507ul, 2185171673ul, 2364114217ul, 2557710269ul,
    2767159799ul, 2993761039ul, 3238918481ul, 3504151727ul, 3791104843ul,
    4101556399ul, 4294967291ul,
    // Sentinel, so we don't have to test the result of lower_bound,
    // or, on 64-bit machines, rest of the table.
#if __SIZEOF_LONG__ != 8
    4294967291ul
#else
    6442450933ul, 8589934583ul, 12884901857ul, 17179869143ul,
    25769803693ul, 34359738337ul, 51539607367ul, 68719476731ul,
    103079215087ul, 137438953447ul, 206158430123ul, 274877906899ul,
    412316860387ul, 549755813881ul, 824633720731ul, 1099511627689ul,
    1649267441579ul, 2199023255531ul, 3298534883309ul, 4398046511093ul,
    6597069766607ul, 8796093022151ul, 13194139533241ul, 17592186044399ul,
    26388279066581ul, 35184372088777ul, 52776558133177ul, 70368744177643ul,
    105553116266399ul, 140737488355213ul, 211106232532861ul, 281474976710597ul,
    562949953421231ul, 1125899906842597ul, 2251799813685119ul,
    4503599627370449ul, 9007199254740881ul, 18014398509481951ul,
    36028797018963913ul, 72057594037927931ul, 144115188075855859ul,
    288230376151711717ul, 576460752303423433ul,
    1152921504606846883ul, 2305843009213693951ul,
    4611686018427387847ul, 9223372036854775783ul,
    18446744073709551557ul, 18446744073709551557ul
#endif
  };
  /// Default value for rehash policy.  Bucket size is (usually) the
  /// smallest prime that keeps the load factor small enough.
  struct _Prime_rehash_policy
  {
    _Prime_rehash_policy(float __z = 1.0);

    float
    max_load_factor() const noexcept;


    // Return a bucket size no smaller than n.
    std::size_t
    _M_next_bkt(std::size_t __n) const;

    // Return a bucket count appropriate for n elements
    std::size_t
    _M_bkt_for_elements(std::size_t __n) const;

    // __n_bkt is current bucket count, __n_elt is current element count,
    // and __n_ins is number of elements to be inserted.  Do we need to
    // increase bucket count?  If so, return make_pair(true, n), where n
    // is the new bucket count.  If not, return make_pair(false, 0).
    std::pair
    _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
		   std::size_t __n_ins) const;

    typedef std::size_t _State;

    _State
    _M_state() const;

    void
    _M_reset(_State __state);


    enum { _S_n_primes = sizeof(unsigned long) != 8 ? 256 : 256 + 48 };

    static const std::size_t _S_growth_factor = 2;

    float                _M_max_load_factor;
    mutable std::size_t  _M_next_resize;
  };

    _Prime_rehash_policy::_Prime_rehash_policy(float __z)
    : _M_max_load_factor(__z), _M_next_resize(0) { }
    
        float
    _Prime_rehash_policy::max_load_factor() const noexcept
    { return _M_max_load_factor; }
    
        std::size_t
    _Prime_rehash_policy::_M_bkt_for_elements(std::size_t __n) const
    { return __builtin_ceil(__n / (long double)_M_max_load_factor); }
    
        _Prime_rehash_policy::_State
    _Prime_rehash_policy::_M_state() const
    { return _M_next_resize; }

    void
    _Prime_rehash_policy::_M_reset(_State __state)
    { _M_next_resize = __state; }

  // Return a prime no smaller than n.
  std::size_t
  _Prime_rehash_policy::_M_next_bkt(std::size_t __n) const
  {
    // Optimize lookups involving the first elements of __prime_list.
    // (useful to speed-up, eg, constructors)
    static const unsigned char __fast_bkt[12]
      = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11 };

    if (__n <= 11)
      {
	_M_next_resize =
	  __builtin_ceil(__fast_bkt[__n] * (long double)_M_max_load_factor);
	return __fast_bkt[__n];
      }

    const unsigned long* __next_bkt =
      std::lower_bound(__prime_list + 5, __prime_list + _S_n_primes, __n);
    _M_next_resize =
      __builtin_ceil(*__next_bkt * (long double)_M_max_load_factor);
    return *__next_bkt;
  }

  // Finds the smallest prime p such that alpha p > __n_elt + __n_ins.
  // If p > __n_bkt, return make_pair(true, p); otherwise return
  // make_pair(false, 0).  In principle this isn't very different from
  // _M_bkt_for_elements.

  // The only tricky part is that we're caching the element count at
  // which we need to rehash, so we don't have to do a floating-point
  // multiply for every insertion.

  std::pair
  _Prime_rehash_policy::
  _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
		 std::size_t __n_ins) const
  {
    if (__n_elt + __n_ins >= _M_next_resize)
      {
	long double __min_bkts = (__n_elt + __n_ins)
				   / (long double)_M_max_load_factor;
	if (__min_bkts >= __n_bkt)
	  return std::make_pair(true,
	    _M_next_bkt(std::max(__builtin_floor(__min_bkts) + 1,
					      __n_bkt * _S_growth_factor)));

	_M_next_resize
	  = __builtin_floor(__n_bkt * (long double)_M_max_load_factor);
	return std::make_pair(false, 0);
      }
    else
      return std::make_pair(false, 0);
  }
} // namespace __detail
} // namespace std 

• 设置GCC7.3.0初始桶大小为10

#include 
#include 
#include 
  
int main()
{
    // 创建三个 string 的 unordered_map （映射到 string ）
    std::unordered_map u;
    u.reserve(10);
  
    std::cout << "load_factor:" << u.load_factor() << std::endl;
    std::cout << "max_load_factor:" << u.max_load_factor() << std::endl;
    std::cout << "bucket_count:" << u.bucket_count() << std::endl;
    std::cout << "size:" << u.size() << std::endl;
    bool will_rehash = (u.max_load_factor()*u.bucket_count()) < (u.size()+1);
    std::cout << "will_rehash:" << will_rehash << std::endl;

    for(int i = 0; i < 10; i++)
    {
        u.insert({std::to_string(i), std::to_string(i)});
    }

    // 迭代并打印 unordered_map 的关键和值
    for( const auto& n : u ) {
        std::cout << "Key:[" << n.first << "] Value:[" << n.second << "]
";
    }
 
    std::cout << "load_factor:" << u.load_factor() << std::endl;
    std::cout << "max_load_factor:" << u.max_load_factor() << std::endl;
    std::cout << "bucket_count:" << u.bucket_count() << std::endl;
    std::cout << "size:" << u.size() << std::endl;
    will_rehash = (u.max_load_factor()*u.bucket_count()) > (u.size()+1);
    std::cout << "will_rehash:" << will_rehash << std::endl;
 
    return 0;
}

• CMakeLists.txt

project(unordered_map)

cmake_minimum_required(VERSION 3.5)

add_executable(the_executable
    stub.cpp
    main.cpp)

target_link_libraries(the_executable
    um)

验证

在线验证

https://godbolt.org/z/x3v36YaKc