On Thu, Dec 13, 2012 at 7:21 AM, Jakub Jelinek <ja...@redhat.com> wrote: > On Wed, Dec 12, 2012 at 10:09:14PM -0800, Xinliang David Li wrote: >> On Wed, Dec 12, 2012 at 5:19 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> >> > libcall is not faster up to 8KB to rep sequence that is better for >> >> > regalloc/code >> >> > cache than fully blowin function call. >> >> >> >> Be careful with this. My recollection is that REP sequence is good for >> >> any size -- for smaller size, the REP initial set up cost is too high >> >> (10s of cycles), while for large size copy, it is less efficient >> >> compared with library version. >> > >> > Well this is based on the data from the memtest script. >> > Core has good REP implementation - it is a win from rather small blocks (16 >> > bytes if I recall) and it does not need alignment. >> > Library version starts to be interesting with caching hints, but I think >> > till 80KB >> > it is still not a win for my setup (glibc-2.15) >> >> A simple test shows that -mstringop-strategy=libcall always beats >> -mstringop-strategy=rep_8byte (on core2 and corei7) except for size >> smaller than 8 where the rep_8byte strategy simply bypasses REP movs. >> Can you share your memtest ? > > I can't believe that say 16 byte or 32 byte memcpy can be ever faster using a > libcall. The PLT call overhead is simply too high.
I believe the PLT call overhead may be effectively zero if the benchmarking is just a loop around a memcpy. Thus for measuring the PLT overhead I call the benchmark broken ;) Richard. > Jakub
