difference in mfence and asm volatile (“” : : : “memory”)

Question

As far as I have understood, mfence is a hardware memory barrier while asm volatile (\"\" : : : \"memory\") is a compiler barrier. But,can asm vo

Answer 1

• asm volatile ("" ::: "memory") is just a compiler barrier.
• asm volatile ("mfence" ::: "memory") is both a compiler barrier and MFENCE
• __sync_synchronize() is also a compiler barrier and a full memory barrier.

so asm volatile ("" ::: "memory") will not prevent CPU reordering independent data instructions per se. As pointed out x86-64 has a strong memory model, but StoreLoad reordering is still possible. If a full memory barrier is needed for your algorithm to work then you neeed __sync_synchronize