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This patch introduces an alternative layout for basic_string which when the string is short, the data pointer will be word-aligned. It can be activated with -D_LIBCPP_ALTERNATE_STRING_LAYOUT. These two different layouts (the default and _LIBCPP_ALTERNATE_STRING_LAYOUT) are not ABI compatible with each other. Once one is chosen for a given platform, it is disruptive to change it. 

llvm-svn: 180811
This commit is contained in:
Howard Hinnant 2013-04-30 21:44:48 +00:00
parent 9b7005bf42
commit 35abaab7d3
1 changed files with 109 additions and 14 deletions
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123
libcxx/include/string
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@ -1036,6 +1036,21 @@ _LIBCPP_EXTERN_TEMPLATE(class __basic_string_common<true>)
#pragma warning( pop )
#endif // _MSC_VER
#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
template <class _CharT, size_t = sizeof(_CharT)>
struct __padding
{
unsigned char __xx[sizeof(_CharT)-1];
};
template <class _CharT>
struct __padding<_CharT, 1>
{
};
#endif // _LIBCPP_ALTERNATE_STRING_LAYOUT
template<class _CharT, class _Traits, class _Allocator>
class _LIBCPP_TYPE_VIS basic_string
: private __basic_string_common<true>
@ -1069,6 +1084,39 @@ public:
typedef _VSTD::reverse_iterator<const_iterator> const_reverse_iterator;
private:
#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
struct __long
{
pointer __data_;
size_type __size_;
size_type __cap_;
};
#if _LIBCPP_BIG_ENDIAN
enum {__short_mask = 0x01};
enum {__long_mask = 0x1ul};
#else // _LIBCPP_BIG_ENDIAN
enum {__short_mask = 0x80};
enum {__long_mask = ~(size_type(~0) >> 1)};
#endif // _LIBCPP_BIG_ENDIAN
enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
(sizeof(__long) - 1)/sizeof(value_type) : 2};
struct __short
{
value_type __data_[__min_cap];
struct
: __padding<value_type>
{
unsigned char __size_;
};
};
#else
struct __long
{
size_type __cap_;
@ -1084,8 +1132,6 @@ private:
enum {__long_mask = 0x1ul};
#endif // _LIBCPP_BIG_ENDIAN
enum {__mask = size_type(~0) >> 1};
enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
(sizeof(__long) - 1)/sizeof(value_type) : 2};
@ -1099,6 +1145,8 @@ private:
value_type __data_[__min_cap];
};
#endif // _LIBCPP_ALTERNATE_STRING_LAYOUT
union __lx{__long __lx; __short __lxx;};
enum {__n_words = sizeof(__lx) / sizeof(size_type)};
@ -1475,20 +1523,44 @@ private:
const allocator_type& __alloc() const _NOEXCEPT
{return __r_.second();}
#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
_LIBCPP_INLINE_VISIBILITY
void __set_short_size(size_type __s) _NOEXCEPT
#if _LIBCPP_BIG_ENDIAN
{__r_.first().__s.__size_ = (unsigned char)(__s);}
#else
# if _LIBCPP_BIG_ENDIAN
{__r_.first().__s.__size_ = (unsigned char)(__s << 1);}
#endif
# else
{__r_.first().__s.__size_ = (unsigned char)(__s);}
# endif
_LIBCPP_INLINE_VISIBILITY
size_type __get_short_size() const _NOEXCEPT
#if _LIBCPP_BIG_ENDIAN
{return __r_.first().__s.__size_;}
#else
# if _LIBCPP_BIG_ENDIAN
{return __r_.first().__s.__size_ >> 1;}
#endif
# else
{return __r_.first().__s.__size_;}
# endif
#else // _LIBCPP_ALTERNATE_STRING_LAYOUT
_LIBCPP_INLINE_VISIBILITY
void __set_short_size(size_type __s) _NOEXCEPT
# if _LIBCPP_BIG_ENDIAN
{__r_.first().__s.__size_ = (unsigned char)(__s);}
# else
{__r_.first().__s.__size_ = (unsigned char)(__s << 1);}
# endif
_LIBCPP_INLINE_VISIBILITY
size_type __get_short_size() const _NOEXCEPT
# if _LIBCPP_BIG_ENDIAN
{return __r_.first().__s.__size_;}
# else
{return __r_.first().__s.__size_ >> 1;}
# endif
#endif // _LIBCPP_ALTERNATE_STRING_LAYOUT
_LIBCPP_INLINE_VISIBILITY
void __set_long_size(size_type __s) _NOEXCEPT
{__r_.first().__l.__size_ = __s;}
@ -2316,14 +2388,37 @@ template <class _CharT, class _Traits, class _Allocator>
void
basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c)
{
size_type __cap = capacity();
size_type __sz = size();
bool __is_short = !__is_long();
size_type __cap;
size_type __sz;
if (__is_short)
{
__cap = __min_cap - 1;
__sz = __get_short_size();
}
else
{
__cap = __get_long_cap() - 1;
__sz = __get_long_size();
}
if (__sz == __cap)
{
__grow_by(__cap, 1, __sz, __sz, 0);
pointer __p = __get_pointer() + __sz;
__is_short = !__is_long();
}
pointer __p;
if (__is_short)
{
__p = __get_short_pointer() + __sz;
__set_short_size(__sz+1);
}
else
{
__p = __get_long_pointer() + __sz;
__set_long_size(__sz+1);
}
traits_type::assign(*__p, __c);
traits_type::assign(*++__p, value_type());
__set_size(__sz+1);
}
template <class _CharT, class _Traits, class _Allocator>