And that will be slower than calling memset() unless you have a really smart compiler.

I don't trust any memset at all. Literally every single memset I looked at was either broken, too slow or insecure. Mostly the wellknown glibc, freebsd, msvcrt and compiler implementations.

That's quite a bold claim. Care to explain further?

I already explained it. Noone implements memory barriers in its safe variants, only compiler barriers. And the trivial variants work byte-wise, not word-wise, which is usually 8x slower.