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[libc++] Simplify the implementation of reserve() and shrink_to_fit() #113453

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Nov 28, 2024
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[libc++] Simplify the implementation of reserve() and shrink_to_fit() #113453

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119 changes: 59 additions & 60 deletions libcxx/include/string
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Original file line number Diff line number Diff line change
Expand Up @@ -1884,8 +1884,6 @@ private:
operator==(const basic_string<char, char_traits<char>, _Alloc>& __lhs,
const basic_string<char, char_traits<char>, _Alloc>& __rhs) _NOEXCEPT;

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __shrink_or_extend(size_type __target_capacity);

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS bool
__is_long() const _NOEXCEPT {
if (__libcpp_is_constant_evaluated() && __builtin_constant_p(__rep_.__l.__is_long_)) {
Expand Down Expand Up @@ -2080,6 +2078,21 @@ private:
#endif
}

// Disable ASan annotations and enable them again when going out of scope. It is assumed that the string is in a valid
// state at that point, so `size()` can be called safely.
struct [[__nodiscard__]] __annotation_guard {
__annotation_guard(const __annotation_guard&) = delete;
__annotation_guard& operator=(const __annotation_guard&) = delete;

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __annotation_guard(basic_string& __str) : __str_(__str) {
__str_.__annotate_delete();
}

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__annotation_guard() { __str_.__annotate_new(__str_.size()); }

basic_string& __str_;
};

template <size_type __a>
static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type __align_it(size_type __s) _NOEXCEPT {
return (__s + (__a - 1)) & ~(__a - 1);
Expand Down Expand Up @@ -3358,7 +3371,16 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::re
if (__requested_capacity <= capacity())
return;

__shrink_or_extend(__recommend(__requested_capacity));
__annotation_guard __g(*this);
auto __allocation = std::__allocate_at_least(__alloc_, __recommend(__requested_capacity) + 1);
auto __size = size();
__begin_lifetime(__allocation.ptr, __allocation.count);
traits_type::copy(std::__to_address(__allocation.ptr), data(), __size + 1);
if (__is_long())
__alloc_traits::deallocate(__alloc_, __get_long_pointer(), __get_long_size() + 1);
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This should pass capacity, not size.

__set_long_cap(__allocation.count);
__set_long_size(__size);
__set_long_pointer(__allocation.ptr);
}

template <class _CharT, class _Traits, class _Allocator>
Expand All @@ -3367,69 +3389,46 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocat
if (__target_capacity == capacity())
return;

__shrink_or_extend(__target_capacity);
}

template <class _CharT, class _Traits, class _Allocator>
inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void
basic_string<_CharT, _Traits, _Allocator>::__shrink_or_extend(size_type __target_capacity) {
__annotate_delete();
auto __guard = std::__make_scope_guard(__annotate_new_size(*this));
size_type __cap = capacity();
size_type __sz = size();
_LIBCPP_ASSERT_INTERNAL(__is_long(), "Trying to shrink small string");

pointer __new_data, __p;
bool __was_long, __now_long;
// We're a long string and we're shrinking into the small buffer.
if (__fits_in_sso(__target_capacity)) {
__was_long = true;
__now_long = false;
__new_data = __get_short_pointer();
__p = __get_long_pointer();
} else {
if (__target_capacity > __cap) {
// Extend
// - called from reserve should propagate the exception thrown.
auto __allocation = std::__allocate_at_least(__alloc_, __target_capacity + 1);
__new_data = __allocation.ptr;
__target_capacity = __allocation.count - 1;
} else {
// Shrink
// - called from shrink_to_fit should not throw.
// - called from reserve may throw but is not required to.
__annotation_guard __g(*this);
auto __ptr = __get_long_pointer();
auto __size = __get_long_size();
auto __cap = __get_long_cap();
traits_type::copy(std::__to_address(__get_short_pointer()), std::__to_address(__ptr), __size + 1);
__set_short_size(__size);
__alloc_traits::deallocate(__alloc_, __ptr, __cap);
return;
}

#if _LIBCPP_HAS_EXCEPTIONS
try {
try {
#endif // _LIBCPP_HAS_EXCEPTIONS
auto __allocation = std::__allocate_at_least(__alloc_, __target_capacity + 1);

// The Standard mandates shrink_to_fit() does not increase the capacity.
// With equal capacity keep the existing buffer. This avoids extra work
// due to swapping the elements.
if (__allocation.count - 1 > capacity()) {
__alloc_traits::deallocate(__alloc_, __allocation.ptr, __allocation.count);
return;
}
__new_data = __allocation.ptr;
__target_capacity = __allocation.count - 1;
__annotation_guard __g(*this);
auto __size = size();
auto __allocation = std::__allocate_at_least(__alloc_, __target_capacity + 1);

// The Standard mandates shrink_to_fit() does not increase the capacity.
// With equal capacity keep the existing buffer. This avoids extra work
// due to swapping the elements.
if (__allocation.count - 1 > capacity()) {
__alloc_traits::deallocate(__alloc_, __allocation.ptr, __allocation.count);
return;
}

__begin_lifetime(__allocation.ptr, __allocation.count);
auto __ptr = __get_long_pointer();
traits_type::copy(std::__to_address(__allocation.ptr), std::__to_address(__ptr), __size + 1);
__alloc_traits::deallocate(__alloc_, __ptr, __get_long_cap());
__set_long_cap(__allocation.count);
__set_long_pointer(__allocation.ptr);
#if _LIBCPP_HAS_EXCEPTIONS
} catch (...) {
return;
}
} catch (...) {
return;
}
#endif // _LIBCPP_HAS_EXCEPTIONS
}
__begin_lifetime(__new_data, __target_capacity + 1);
__now_long = true;
__was_long = __is_long();
__p = __get_pointer();
}
traits_type::copy(std::__to_address(__new_data), std::__to_address(__p), size() + 1);
if (__was_long)
__alloc_traits::deallocate(__alloc_, __p, __cap + 1);
if (__now_long) {
__set_long_cap(__target_capacity + 1);
__set_long_size(__sz);
__set_long_pointer(__new_data);
} else
__set_short_size(__sz);
}

template <class _CharT, class _Traits, class _Allocator>
Expand Down
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