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

[Douglas Gregor]

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?

> +    return true;
> }
> 
> template<class _CharT, class _Traits, class _Allocator>
> 
> 
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