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Low-hanging fruit optimization in string::__move_assign().

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shrink_to_fit() ends up doing a lot work to get information that we
already know since we just called clear(). This change seems concise
enough to be worth the couple extra lines and my benchmarks show that it
is indeed a pretty decent win. It looks like the same thing is going on
twice in __copy_assign_alloc(), but I didn't want to go overboard since
this is my first contribution to llvm/libc++.

Patch by Timothy VanSlyke!

Differential Revision: https://reviews.llvm.org/D41976

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@327064 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Vedant Kumar
2018-03-08 21:15:26 +00:00
parent 46b8a51b49
commit 2b588cbf15
2 changed files with 69 additions and 6 deletions
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27
include/string
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@@ -1257,6 +1257,8 @@ public:
_LIBCPP_INLINE_VISIBILITY bool __invariants() const;
_LIBCPP_INLINE_VISIBILITY void __clear_and_shrink();
_LIBCPP_INLINE_VISIBILITY
bool __is_long() const _NOEXCEPT
{return bool(__r_.first().__s.__size_ & __short_mask);}
@@ -1426,16 +1428,14 @@ private:
{
if (!__str.__is_long())
{
clear();
shrink_to_fit();
__clear_and_shrink();
__alloc() = __str.__alloc();
}
else
{
allocator_type __a = __str.__alloc();
pointer __p = __alloc_traits::allocate(__a, __str.__get_long_cap());
clear();
shrink_to_fit();
__clear_and_shrink();
__alloc() = _VSTD::move(__a);
__set_long_pointer(__p);
__set_long_cap(__str.__get_long_cap());
@@ -2125,8 +2125,7 @@ basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, tr
_NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
#endif
{
clear();
shrink_to_fit();
__clear_and_shrink();
__r_.first() = __str.__r_.first();
__move_assign_alloc(__str);
__str.__zero();
@@ -3579,6 +3578,22 @@ basic_string<_CharT, _Traits, _Allocator>::__invariants() const
return true;
}
// __clear_and_shrink
template<class _CharT, class _Traits, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
void
basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink()
{
clear();
if(__is_long())
{
__alloc_traits::deallocate(__alloc(), __get_long_pointer(), capacity() + 1);
__set_long_cap(0);
__set_short_size(0);
}
}
// operator==
template<class _CharT, class _Traits, class _Allocator>