[libcxx] r180072 - Modest performance improvement for std::string's operator==.

[Douglas Gregor]

On Apr 23, 2013, at 9:50 AM, Howard Hinnant <hhinnant at apple.com> wrote:

> On Apr 23, 2013, at 2:10 AM, Douglas Gregor <dgregor at apple.com> wrote:
> 
>> 
>> 
>> Sent from my iPhone
>> 
>> On Apr 22, 2013, at 4:55 PM, Howard Hinnant <hhinnant at apple.com> wrote:
>> 
>>> Author: hhinnant
>>> Date: Mon Apr 22 18:55:13 2013
>>> New Revision: 180072
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=180072&view=rev
>>> Log:
>>> Modest performance improvement for std::string's operator==.
>>> 
>>> Modified:
>>>  libcxx/trunk/include/string
>>> 
>>> Modified: libcxx/trunk/include/string
>>> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/string?rev=180072&r1=180071&r2=180072&view=diff
>>> ==============================================================================
>>> --- libcxx/trunk/include/string (original)
>>> +++ libcxx/trunk/include/string Mon Apr 22 18:55:13 2013
>>> @@ -1462,6 +1462,11 @@ public:
>>>   int compare(size_type __pos1, size_type __n1, const_pointer __s, size_type __n2) const;
>>> 
>>>   _LIBCPP_INLINE_VISIBILITY bool __invariants() const;
>>> +
>>> +    _LIBCPP_INLINE_VISIBILITY
>>> +    bool __is_long() const _NOEXCEPT
>>> +        {return bool(__r_.first().__s.__size_ & __short_mask);}
>>> +
>>> private:
>>>   _LIBCPP_INLINE_VISIBILITY
>>>   allocator_type& __alloc() _NOEXCEPT
>>> @@ -1471,10 +1476,6 @@ private:
>>>       {return __r_.second();}
>>> 
>>>   _LIBCPP_INLINE_VISIBILITY
>>> -    bool __is_long() const _NOEXCEPT
>>> -        {return bool(__r_.first().__s.__size_ & __short_mask);}
>>> -
>>> -    _LIBCPP_INLINE_VISIBILITY
>>>   void __set_short_size(size_type __s) _NOEXCEPT
>>> #if _LIBCPP_BIG_ENDIAN
>>>       {__r_.first().__s.__size_ = (unsigned char)(__s);}
>>> @@ -3561,9 +3562,29 @@ bool
>>> operator==(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
>>>          const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
>>> {
>>> -    return __lhs.size() == __rhs.size() && _Traits::compare(__lhs.data(),
>>> -                                                            __rhs.data(),
>>> -                                                            __lhs.size()) == 0;
>>> +    size_t __lhs_sz = __lhs.size();
>>> +    return __lhs_sz == __rhs.size() && _Traits::compare(__lhs.data(),
>>> +                                                        __rhs.data(),
>>> +                                                        __lhs_sz) == 0;
>>> +}
>>> +
>>> +template<class _Allocator>
>>> +_LIBCPP_INLINE_VISIBILITY inline
>>> +bool
>>> +operator==(const basic_string<char, char_traits<char>, _Allocator>& __lhs,
>>> +           const basic_string<char, char_traits<char>, _Allocator>& __rhs) _NOEXCEPT
>>> +{
>>> +    size_t __lhs_sz = __lhs.size();
>>> +    if (__lhs_sz != __rhs.size())
>>> +        return false;
>>> +    const char* __lp = __lhs.data();
>>> +    const char* __rp = __rhs.data();
>>> +    if (__lhs.__is_long())
>>> +        return char_traits<char>::compare(__lp, __rp, __lhs_sz) == 0;
>>> +    for (; __lhs_sz != 0; --__lhs_sz, ++__lp, ++__rp)
>>> +        if (*__lp != *__rp)
>>> +            return false;
>> 
>> Shouldn't this go through traits for the character comparison?
> 
> <shrug> It could.  There's no way to tell that it didn't.  This is exactly what char_traits<char> would do, this function only uses char_traits<char>, and the client is not allowed to replace char_traits<char>. And if the client could detect that I wasn't calling char_traits<char>::eq, changing to call char_traits<char>::eq wouldn't make it conforming.  The general form of this function is required to call Traits::compare, not Traits::eq.

I missed the fact that this was a specialization for 'char'. Sorry for the noise.

	- Doug

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