Which is a better write barrier on x86: lock+addl or xchgl?

Question

The Linux kernel uses lock; addl $0,0(%%esp) as write barrier, while the RE2 library uses xchgl (%0),%0 as write barrier. What\'s the difference an

Answer 1

The important part of lock; addl and xchgl is the lock prefix. It's implicit for xchgl. There is really no difference between the two. I'd look at how they assemble and choose the one that's shorter (in bytes) since that's usually faster for equivalent operations on x86 (hence tricks like xorl eax,eax)

The presence of SSE2 is probably just a proxy for the real condition which is ultimately a function of cpuid. It probably turns out that SSE2 implies the existence of lfence and the availability of SSE2 was checked/cached at boot. lfence is required when it's available.