On Mon, Dec 14, 2020 at 03:57:27PM +0100, Piotr Kubaj wrote:
> > It is both, actually (-mcpu= implies -mtune=)
> Yes, but -mtune doesn't imply -mcpu. If I set up only -mtune, -mcpu is the
> generic one (ppc970 for BE)
But that is not what the patch does?
> > > Linux also does similarly in linux64.h:
> > > 74 #undef PROCESSOR_DEFAULT
> > > 75 #define PROCESSOR_DEFAULT PROCESSOR_POWER7
> > > 76 #undef PROCESSOR_DEFAULT64
> > > 77 #define PROCESSOR_DEFAULT64 PROCESSOR_POWER8
> >
> > Yes, but we still default to power4 on BE Linux (and power8 on LE). I
> > get lost in the macro maze here, not sure how that works :-)
I still am baffled how this works :-/
> > > Although there is hard to explain (for me) setting to tune for POWER7 on
> > > 32-bits. POWER7 is 64-bit and it should be more reasonable to optimize
> > > for the last 32-bit machines that still may be in use (so G4).
> >
> > 7450 has very different optimisation than 970. Power7 and 970 have
> > largely similar pipelines.
> Yes, but the point is that users on 32-bit machines don't use POWER7, but
> most likely G4.
Ah, but most users using -m32 are on a 64-bit machine!
> > > That said, on the 64-bit front, we will just match the Linux setting.
> >
> > Compile a very simple test and look at the .machine in the generated .s?
> > Does that do what you want?
> Yes, I just tested it (on BE).
> pkubaj@talos:$/usr/home/pkubaj$ cat test.c
> #include <stdio.h>
>
> int main()
> {
> printf("Hello world!");
> }
>
> I built it twice, with GCC 9.3 without my patch and with GCC 10.1 with my
> patch. The only difference is:
> pkubaj@talos:$/usr/home/pkubaj$ diff -u test.s.9 test.s.10
> --- test.s.9 2020-12-14 15:51:43.203263000 +0100
> +++ test.s.10 2020-12-14 15:52:01.147238000 +0100
> @@ -43,5 +43,5 @@
> .cfi_endproc
> .LFE1:
> .size main,.-main
> - .ident "GCC: (FreeBSD Ports Collection) 9.3.0"
> + .ident "GCC: (FreeBSD Ports Collection) 10.2.0"
> .section .note.GNU-stack,"",@progbits
>
> Both have .machine power4:
> pkubaj@talos:$/usr/home/pkubaj$ grep .machine test.s.*
> test.s.10: .machine power4
> test.s.9: .machine power4
>
> I'm not sure where GCC sets power8 for LE, but it's definitely somewhere else.
Yes... And I do not know where this power4 comes from. Mysteries :-)
> So IMO this patch is good to go.
Yes, thanks!
Segher
