Low-hanging fruit optimization in string::__move_assign().
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
llvm-svn: 327064
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: fb36d079e57cf56819bfbc1a1c2c8282accb58d5
diff --git a/include/string b/include/string
index e8ec709..2630799 100644
--- a/include/string
+++ b/include/string
@@ -1257,6 +1257,8 @@
_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 @@
{
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 @@
_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 @@
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>