linux-kernel - Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

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Date:   Tue, 29 Oct 2019 21:12:18 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
 reference to bcmp

On Tue, Oct 22, 2019 at 03:57:09AM -0500, Segher Boessenkool wrote:
> On Mon, Oct 21, 2019 at 10:15:29PM -0700, Nathan Chancellor wrote:
> > On Fri, Oct 18, 2019 at 03:02:10PM -0500, Segher Boessenkool wrote:
> > > I think the proper solution is for the kernel to *do* use -ffreestanding,
> > > and then somehow tell the kernel that memcpy etc. are the standard
> > > functions.  A freestanding GCC already requires memcpy, memmove, memset,
> > > memcmp, and sometimes abort to exist and do the standard thing; why cannot
> > > programs then also rely on it to be the standard functions.
> > > 
> > > What exact functions are the reason the kernel does not use -ffreestanding?
> > > Is it just memcpy?  Is more wanted?
> > 
> > I think Linus summarized it pretty well here:
> > 
> > https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/
> 
> GCC recognises __builtin_memcpy (or any other __builtin) just fine even
> with -ffreestanding.
> 
> So the kernel wants a warning (or error) whenever a call to one of these
> library functions is generated by the compiler without the user asking
> for it directly (via a __builtin)?  And that is all that is needed for
> the kernel to use -ffreestanding?
> 
> That shouldn't be hard.  Anything missing here?
> 
> 
> Segher

Yes, I suppose that would be good enough.

I don't know if there are any other optimizations that are missed out on
by using -ffreestanding. It would probably be worth asking other kernel
developers on a separate thread (or the one I linked above).

Would be nice to get this shored up soon since our PowerPC builds have
been broken since the beginning of August :/

Cheers,
Nathan