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Date: Fri, 7 Oct 2022 09:38:04 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH] powerpc/64s: POWER10 CPU Kconfig build option
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Cc: linuxppc-dev@lists.ozlabs.org, Nicholas Piggin <npiggin@gmail.com>
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On Fri, Oct 07, 2022 at 04:31:28PM +1100, Michael Ellerman wrote:
> "Nicholas Piggin" <npiggin@gmail.com> writes:
> > On Fri Oct 7, 2022 at 9:23 AM AEST, Segher Boessenkool wrote:
> >> On Fri, Oct 07, 2022 at 07:56:09AM +1000, Nicholas Piggin wrote:
> >> > On Fri Oct 7, 2022 at 5:54 AM AEST, Segher Boessenkool wrote:
> >> > > On Fri, Sep 23, 2022 at 01:30:04PM +1000, Nicholas Piggin wrote:
> ...
> >> > > > +# No AltiVec or VSX or MMA instructions when building kernel
> >> > > >  KBUILD_CFLAGS += $(call cc-option,-mno-altivec)
> >> > > >  KBUILD_CFLAGS += $(call cc-option,-mno-vsx)
> >> > > > +KBUILD_CFLAGS += $(call cc-option,-mno-mma)
> >> > >
> >> > > MMA code is never generated unless the code asks for it explicitly.
> >> > > This is fundamental, not just an implementations side effect.
> >> > 
> >> > Well, now it double won't be generated :)
> >>
> >> Yeah, but there are many other things you can unnecessarily disable as
> >> well!  :-)
> >>
> >> VMX and VSX are disabled here because the compiler *will* use those
> >> registers if it feels like it (that is, if it thinks that will be
> >> faster).  MMA is a very different beast: the compiler can never know if
> >> it will be faster, to start with.
> >
> > True, but now I don't have to find the exact clause and have my lawyer
> > confirm that it definitely probably won't change in future and break
> > things.
> 
> Right. If someone asks "does the kernel ever use MMA instructions?" we
> can just point at that line and we have a definite answer. No need to
> audit the behaviour of all GCC and Clang versions ever released.

As I said, no sane compiler can use MMA ever (unless asked for it
directly of course).  But yeah, who knows what clang does!


Segher