Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9370
GCC and newer clang, e.g. clang-12 treat `std::unique_ptr` slightly differently.
For the following code
```
#include <iostream>
#include <memory>
#include <type_traits>
struct A {
std::unique_ptr<int> m1;
};
int main()
{
std::cout << std::boolalpha;
std::cout << std::is_standard_layout<A>::value << '\n';
return 0;
}
```
GCC11(C++20) (tested on https://en.cppreference.com/w/cpp/types/is_standard_layout) will print "true", while newer clang, e.g. clang-12 will print "false". This breaks the usage of `offsetof()` on structs with non-static members of type `std::unique_ptr`.
Fixing this by replacing the builtin `offsetof` with a trick documented at https://gist.github.com/graphitemaster/494f21190bb2c63c5516.
Reviewed By: jay-zhuang
Differential Revision: D33420840
fbshipit-source-id: 02bde281dfa28809bec787ad0f7019e85dd9c607
128e36ca53