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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@180811 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index fa44f68..85eb463 100644
--- a/include/string
+++ b/include/string
@@ -1036,6 +1036,21 @@
#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 @@
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 @@
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 @@
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 @@
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 @@
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>