On Sat, Mar 04, 2017 at 03:49:52PM -0500, Pedro Giffuni wrote: > > > Il giorno 04 mar 2017, alle ore 14:43, John Baldwin <j...@freebsd.org> ha > > scritto: > > > > On Saturday, March 04, 2017 10:52:46 AM Pedro Giffuni wrote: > >> > >> On 03/04/17 10:32, Slawa Olhovchenkov wrote: > >>> On Sat, Mar 04, 2017 at 03:04:17PM +0000, Pedro F. Giffuni wrote: > >>> > >>>> Author: pfg > >>>> Date: Sat Mar 4 15:04:17 2017 > >>>> New Revision: 314669 > >>>> URL: https://svnweb.freebsd.org/changeset/base/314669 > >>>> > >>>> Log: > >>>> Drop i486 from the default i386 GENERIC kernel configuration. > >>>> > >>>> 80486 production was stopped by Intel on September 2007. Dropping the > >>>> 486 > >>>> configuration option from the GENERIC kernel improves performance > >>>> slightly. > >>>> > >>>> Removing I486_CPU is consistent at this time: we don't support any > >>>> processor without a FPU and the PC-98 arch, which frequently involved > >>>> i486 > >>>> CPUs, is also gone so we don't test such platforms anymore. > >>> > >>> What is realy mean? > >> > >> This means we don't do work-arounds that would be required for raw 486. > >> Instead we will use the 586 instructions by default. > > > > This doesn't change that. The kernel already has runtime tests in place > > for new things on 486 and later via cpuid. > > > > Hmm ..then I am wondering if I effectively changed anything? > > >>> Some Via CPU is like i486 (by instruction set). > >>> > >>> CPU: VIA Ezra (800.04-MHz 686-class CPU) > >>> Origin="CentaurHauls" Id=0x678 Family=0x6 Model=0x7 Stepping=8 > >>> Features=0x803035<FPU,DE,TSC,MSR,MTRR,PGE,MMX> > >>> AMD Features=0x80000000<3DNow!> > >>> > >> > >> 486 never had MMX extensions. > >> This is a 686, performance should improve ~4%. > > > > How did you measure the improvement? Keeping I486_CPU doesn't really > > do anything except remove a some #ifdef'd conditionals in identcpu.c > > and initcpu.c. It doesn't affect whether we use the TSC, MMX, etc. Those > > are all runtime checks based the CPU feature flags from cpuid. > > > > The number came out from an old posting involving buildworld times, which I > can???t find now :(. > Things seem to have changed a lot: it was surely using GCC back then, I > don???t believe clang does much distinction about 486 at all. > > BTW, does it make sense to keep i586 in the configuration still? Both i486 > and i586 were once removed but later re-instated in r205336. > What did make significant impact on 32bit shared libraries some time ago was to compile them with -mtune=i686. Default PIC prologue effectively neutered return stack predictor, adding uneccessary overhead to already expensive PIC code. I think that this is even measureable, i.e. it might give >= 5% of difference.
I did not rechecked modern compilers WRT the generated PIC code, but I doubt that the thing changed recently. Several notes: -mtune is not -march, i.e. the code would be still targeted for 486 instruction set, but scheduling is optimized for more modern CPUs. Also, recent gcc puts specific meaning into -mtune=i686, interpreting it as request for scheduling for generic modern CPUs. We already compile 32bit compat libs on amd64 with -march=i686. Working on this stuff would be much more useful than tweaking kernel config for CPU detection. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
