c++ - 如何优化字符串复制(内存分配)？

Question

我正在制作一个程序(想想:类似 Launchy 的东西)，它或多或少地遍历一堆字符串并根据某些标准对它们进行排名。

我将结果存储在 vector<SearchSuggestion> 中，其中结构当前定义如下:

struct SearchSuggestion
{
    std::string path;
    int tag;
};

在我的程序中，由于需要操作大量文件路径等等，我复制了很多结构(以及字符串)。

虽然这会导致 Release模式出现明显但很小的延迟，但它显着 减慢了我的程序的调试速度(即，在击键之间有几秒钟的停顿)。寻找原因，我发现几乎所有时间都花在了以下堆栈跟踪上:

ntdll.dll!RtlCompareMemoryUlong()   
ntdll.dll!RtlpAllocateHeap()    
ntdll.dll!RtlAllocateHeap()     
ntdll.dll!RtlDebugAllocateHeap()    
ntdll.dll!string "Enabling heap debug options\n"()  
ntdll.dll!RtlAllocateHeap()     
msvcr90d.dll!_heap_alloc_base(unsigned __int64)     C
msvcr90d.dll!_heap_alloc_dbg_impl(unsigned __int64, int, const char *, int, int *)
msvcr90d.dll!_nh_malloc_dbg_impl(unsigned __int64, int, int, const char *, int, int *)
msvcr90d.dll!_nh_malloc_dbg(unsigned __int64, int, int, const char *, int)
msvcr90d.dll!malloc(unsigned __int64)
msvcr90d.dll!operator new(unsigned __int64)
MyProgram.exe!std::_Allocate<wchar_t>(unsigned __int64, wchar_t *)
MyProgram.exe!std::allocator<wchar_t>::allocate(unsigned __int64)
MyProgram.exe!std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::_Copy(unsigned __int64, unsigned __int64)
MyProgram.exe!std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::_Grow(unsigned __int64, bool)
MyProgram.exe!std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::assign(const std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > &, unsigned __int64, unsigned __int64)
MyProgram.exe!std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >(const std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > &)
MyProgram.exe!SearchSuggestion::SearchSuggestion(const SearchSuggestion &)
MyProgram.exe!std::_Construct<SearchSuggestion,SearchSuggestion>(SearchSuggestion *, const SearchSuggestion &)
MyProgram.exe!std::allocator<SearchSuggestion>::construct(SearchSuggestion *, const SearchSuggestion &)
MyProgram.exe!std::_Uninit_copy<SearchSuggestion * __ptr64,SearchSuggestion * __ptr64,std::allocator<SearchSuggestion> >(SearchSuggestion *, SearchSuggestion *, SearchSuggestion *, std::allocator<SearchSuggestion> &, std::_Nonscalar_ptr_iterator_tag, std::_Nonscalar_ptr_iterator_tag)
MyProgram.exe!stdext::unchecked_uninitialized_copy<SearchSuggestion * __ptr64,SearchSuggestion * __ptr64,std::allocator<SearchSuggestion> >(SearchSuggestion *, SearchSuggestion *, SearchSuggestion *, std::allocator<SearchSuggestion> &)
MyProgram.exe!std::_Uninit_move<SearchSuggestion * __ptr64,SearchSuggestion * __ptr64,std::allocator<SearchSuggestion>,std::_Undefined_move_tag>(SearchSuggestion *, SearchSuggestion *, SearchSuggestion *, std::allocator<SearchSuggestion> &, std::_Undefined_move_tag, std::_Undefined_move_tag)
MyProgram.exe!stdext::_Unchecked_uninitialized_move<SearchSuggestion * __ptr64,SearchSuggestion * __ptr64,std::allocator<SearchSuggestion> >(SearchSuggestion *, SearchSuggestion *, SearchSuggestion *, std::allocator<SearchSuggestion> &)
MyProgram.exe!std::vector<SearchSuggestion,std::allocator<SearchSuggestion> >::_Umove<SearchSuggestion * __ptr64>(SearchSuggestion *, SearchSuggestion *, SearchSuggestion *)
MyProgram.exe!std::vector<SearchSuggestion,std::allocator<SearchSuggestion> >::_Insert_n(std::_Vector_const_iterator<SearchSuggestion,std::allocator<SearchSuggestion> > *, unsigned __int64, const SearchSuggestion &)
MyProgram.exe!std::vector<SearchSuggestion,std::allocator<SearchSuggestion> >::insert(std::_Vector_const_iterator<SearchSuggestion,std::allocator<SearchSuggestion> > *, const SearchSuggestion &)
MyProgram.exe!std::vector<SearchSuggestion,std::allocator<SearchSuggestion> >::push_back(const SearchSuggestion &)
MyProgram.exe!Appender<std::vector<SearchSuggestion,std::allocator<SearchSuggestion> > >(const wchar_t *, _tfinddata *, void *)
MyProgram.exe!EnumMatches(const std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > &, const std::vector<std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >,std::allocator<std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > > > &, int (const wchar_t *, _tfinddata *, void *)*, void *, int)
...

所以很明显这是对std::string的复制这花费的时间太长，可能是由于引用位置不佳。

所以现在我的问题很简单:

如何提高分配大量小字符串的性能？

Answer 1

一种方法是停止按值将它们放入 SearchSuggestion 中结构。而是给出每个 SearchSuggestion std::string 的句柄代表路径。

struct SearchSuggestion {
  int pathId;
  int tag;
};

这将使 vector 内的复制更加有效，因为它只是围绕简单的 int 进行复制。 s 而不是复杂的 std::string值。

然后您可以使用 std::map<int, std::string>将路径 ID 映射到真实路径的结构。