[libc++][ASan] Stop optimizations in std::basic_string

This commit stops some optimizations when compiling with ASan.
- Short strings make std::basic_string to not be trivially relocatable, because memory has to be unpoisoned. It changes value of `__trivially_relocatable` when compiling with ASan.
- It truns off compiler stack optimizations with `__asan_volatile_wrapper`, the function is not used when compiling without ASan.
- It turns off instrumentation in a few functions.

Author: Advenam Tacet

libcxx/include/string

@@ -735,11 +735,39 @@ public:
   //
   // This string implementation doesn't contain any references into itself. It only contains a bit that says whether
   // it is in small or large string mode, so the entire structure is trivially relocatable if its members are.
+#if !defined(_LIBCPP_HAS_NO_ASAN) && defined(_LIBCPP_INSTRUMENTED_WITH_ASAN)
+  // When compiling with AddressSanitizer (ASan), basic_string cannot be trivially
+  // relocatable. Because the object's memory might be poisoned when its content
+  // is kept inside objects memory (short string optimization), instead of in allocated
+  // external memory. In such cases, the destructor is responsible for unpoisoning
+  // the memory to avoid triggering false positives.
+  // Therefore it's crucial to ensure the destructor is called
+  using __trivially_relocatable = false_type;
+#else
   using __trivially_relocatable = __conditional_t<
       __libcpp_is_trivially_relocatable<allocator_type>::value && __libcpp_is_trivially_relocatable<pointer>::value,
       basic_string,
       void>;
+#endif
+#if !defined(_LIBCPP_HAS_NO_ASAN) && defined(_LIBCPP_INSTRUMENTED_WITH_ASAN)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+  pointer __asan_volatile_wrapper(pointer __ptr) const {
+    volatile pointer __copy_ptr = __ptr;
+
+    return __copy_ptr;
+  }
 
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+  const_pointer const __asan_volatile_wrapper(const_pointer __ptr) const {
+    volatile const_pointer __copy_ptr = __ptr;
+
+    return __copy_ptr;
+  }
+
+#define _LIBCPP_ASAN_VOLATILE_WRAPPER(ptr) __asan_volatile_wrapper(ptr)
+#else
+#define _LIBCPP_ASAN_VOLATILE_WRAPPER(ptr) ptr
+#endif
   static_assert((!is_array<value_type>::value), "Character type of basic_string must not be an array");
   static_assert((is_standard_layout<value_type>::value), "Character type of basic_string must be standard-layout");
   static_assert((is_trivial<value_type>::value), "Character type of basic_string must be trivial");
@@ -1885,16 +1913,16 @@ private:
     __r_.first().__l.__data_ = __p;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pointer __get_long_pointer() _NOEXCEPT {
-    return __r_.first().__l.__data_;
+    return _LIBCPP_ASAN_VOLATILE_WRAPPER(__r_.first().__l.__data_);
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_pointer __get_long_pointer() const _NOEXCEPT {
-    return __r_.first().__l.__data_;
+    return _LIBCPP_ASAN_VOLATILE_WRAPPER(__r_.first().__l.__data_);
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pointer __get_short_pointer() _NOEXCEPT {
-    return pointer_traits<pointer>::pointer_to(__r_.first().__s.__data_[0]);
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS pointer __get_short_pointer() _NOEXCEPT {
+    return _LIBCPP_ASAN_VOLATILE_WRAPPER(pointer_traits<pointer>::pointer_to(__r_.first().__s.__data_[0]));
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_pointer __get_short_pointer() const _NOEXCEPT {
-    return pointer_traits<const_pointer>::pointer_to(__r_.first().__s.__data_[0]);
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS const_pointer __get_short_pointer() const _NOEXCEPT {
+    return _LIBCPP_ASAN_VOLATILE_WRAPPER(pointer_traits<const_pointer>::pointer_to(__r_.first().__s.__data_[0]));
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pointer __get_pointer() _NOEXCEPT {
     return __is_long() ? __get_long_pointer() : __get_short_pointer();