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

Author: philnik777

libcxx/include/string

@@ -1874,8 +1874,6 @@ 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_)) {
@@ -2070,6 +2068,21 @@ 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);
@@ -3348,7 +3361,16 @@ _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);
+  __set_long_cap(__allocation.count);
+  __set_long_size(__size);
+  __set_long_pointer(__allocation.ptr);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
@@ -3357,70 +3379,46 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocat
   if (__target_capacity == capacity())
     return;
 
-  __shrink_or_extend(__target_capacity);
-}
+  _LIBCPP_ASSERT_INTERNAL(__is_long(), "Trying to shrink small string");
 
-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();
-  size_type __cap = capacity();
-  size_type __sz  = size();
-
-  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 > __target_capacity) {
-          __alloc_traits::deallocate(__alloc_, __allocation.ptr, __allocation.count);
-          __annotate_new(__sz); // Undoes the __annotate_delete()
-          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 > __target_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);
-  __annotate_new(__sz);
 }
 
 template <class _CharT, class _Traits, class _Allocator>