Intel introduced a new instruction “serialize” which ensures that all modifications to flags, registers, and memory by previous instructions are completed and all buffered writes are drained to memory before the next instruction is fetched and executed. It is a serializing instruction and can be used to implement cross modify fence (OrderAccess::cross_modify_fence_impl) more efficiently instead of “cpuid” on supported 32-bit and 64-bit x86 platforms.

https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html