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

[Konstantin Belousov]

On Sat, Mar 04, 2017 at 02:54:56PM -0800, John Baldwin wrote:
> On Saturday, March 04, 2017 11:16:11 PM Konstantin Belousov wrote:
> > On Sat, Mar 04, 2017 at 03:49:52PM -0500, Pedro Giffuni wrote:
> > > 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.
> 
> Hmm, I originally wanted to use -mtune=i686 (spelled as -mcpu=pentiumpro) on
> i386 builds for this reason, but I removed it at bde@'s request in r125252.
> I would be happy to go back to adding -mtune for i386 when CPUTYPE isn't
> specified.

I just rechecked.
gcc, at least 4.9 and 6.3, generate 'right' prologue, i.e.
	call	__x86.get_pc_thunk.cx (ecx or whatever register
					which is used to address GOT)
__x86.get_pc_thunk.cx:
	movl	(%esp), %ecx
	ret
even when compiling for -march=i486.

OTOH, clang 3.9.1 uses
	calll	.L0
.L0:	popl	%eax
to get the base even for native nehalem and newer CPUs.

So indeed there is no reason to bother. gcc become too good to require any
tuning, and clang generates unoptimal code even when hinted.  I did not
checked 4.0.