svn commit: r314669 - head/sys/i386/conf

[Ian Lepore]

On Sat, 2017-03-04 at 15:49 -0500, Pedro Giffuni wrote:
> > 
> > Il giorno 04 mar 2017, alle ore 14:43, John Baldwin <jhb at freebsd.or
> > g> 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  Steppi
> > > > ng=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.
> 
> Pedro.

i586 hardware which is not also i686 is still alive and well, but I
think at this point it's mostly used in specialized applications.  I
have no problem with removing i586 from GENERIC, but please don't
remove underlying support for setting it in custom kernel configs.

-- Ian