On Sun, Jul 31, 2016 at 11:11:25PM +1000, Bruce Evans wrote: > On Haswell, "rep stos" takes about 25 cycles to start up, and the function > call overhead is in the noise. 25 cycles is a lot. Haswell can move > 32 bytes/cycle from L2 to L2, so it misses moving 800 bytes or 1/5 of a > page in its startup overhead. Oops, that is for "rep movs". "rep stos" > is similar. > The commit message contained a probable explanation of the reason why the change demonstrated measurable improvement in non-microbenchmark load.
That said, the only thing I am answering and asking there is the above claim about 25 cycles overhead of rep;stosq on hsw. I am curious how the overhead was measured. Note: Agner Fog' tables state that fast mode takes <2n uops and has reciprocal throughput of 0.5n worst case and do not demostrate any setup overhead for hsw. _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
