[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

@@ -735,10 +735,34 @@ 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
+  template<class PtrT>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+  PtrT __asan_volatile_wrapper(PtrT const &__ptr) const {
+#if !defined(_LIBCPP_HAS_NO_ASAN) && defined(_LIBCPP_INSTRUMENTED_WITH_ASAN)
+    if (__libcpp_is_constant_evaluated())
+      return __ptr;
+
+    PtrT volatile __copy_ptr = __ptr;
+
+    return const_cast<PtrT &>(__copy_ptr);
+#else
+    return __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");
@@ -1885,16 +1909,16 @@ 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 __asan_volatile_wrapper<pointer>(__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 __asan_volatile_wrapper<const_pointer>(__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 __asan_volatile_wrapper<pointer>(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 __asan_volatile_wrapper<const_pointer>(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();

libcxx/test/libcxx/type_traits/is_trivially_relocatable.compile.pass.cpp

@@ -48,6 +48,7 @@ static_assert(!std::__libcpp_is_trivially_relocatable<MoveOnlyTriviallyCopyable>
 // ----------------------
 
 // basic_string
+#if defined(_LIBCPP_HAS_NO_ASAN) || !defined(_LIBCPP_INSTRUMENTED_WITH_ASAN)
 struct MyChar {
   char c;
 };
@@ -78,7 +79,7 @@ static_assert(
     !std::__libcpp_is_trivially_relocatable<
         std::basic_string<MyChar, NotTriviallyRelocatableCharTraits<MyChar>, test_allocator<MyChar> > >::value,
     "");
-
+#endif
 // unique_ptr
 struct NotTriviallyRelocatableDeleter {
   NotTriviallyRelocatableDeleter(const NotTriviallyRelocatableDeleter&);