On Wed, 4 Jan 2012, Linus Torvalds wrote: > On Wed, Jan 4, 2012 at 9:00 AM, Christoph Lameter <cl@xxxxxxxxx> wrote: > > > > xadd is 3 cycles. add is one cycle. > > On some uarchs. On new uarchs it can be a single cycle, I think, and > on some uarchs it will even be microcoded and/or only go in one pipe > because it has that odd behavior that it writes both to memory and a > register, and thus doesn't fit the normal fastpaths. > The point is, xadd isn't actually any faster than just doing the > regular "add and read". It's *slower*. Ok that assumes that both add and read are this_cpu operations that use the segment override to relocate the address. I thought you wanted to open code everything. So then we would have to use the following? this_cpu_add(var, count); result = this_cpu_read(var); XADD and ADD have the same cycle count if the ADD is used to add to a memory location. Both use 4 microops. The MOVE from memory costs another 2 so we are at 6. Two segment overrides for each instruction add 2 more (nothing in the optimization manual but as far as I can recall another uop is needed for the address calculation). So this ends up at 8. On the other hand result = this_cpu_add_return(var, count) is 4 + 1. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
