Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

[Masahiro Yamada]

On Tue, Aug 25, 2020 at 11:02 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Tue, Aug 25, 2020 at 5:30 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> >
> > On Tue, Aug 25, 2020 at 4:10 PM Nick Desaulniers
> > <ndesaulniers@xxxxxxxxxx> wrote:
> > >
> > > Masahiro, are you implying that we shouldn't take the
> > > -fno-builtin-stpcpy patch, because Clang is inconsistent? (That can be
> > > fixed.) Even though -fno-builtin-stpcpy works here as intended?
> > > https://lore.kernel.org/lkml/20200817220212.338670-2-ndesaulniers@xxxxxxxxxx/
> > >
> > > Otherwise we need to provide an implementation of this symbol in the kernel.
> > > https://lore.kernel.org/lkml/20200815020946.1538085-1-ndesaulniers@xxxxxxxxxx/
> > >
> > > Please, pick your poison.
> >
> >
> >
> > I am not a compiler expert.
> >
> > Nor am I sure if I am the right person who makes this decision.
> > But, if so, I would choose the latter.
> > (implement stpcpy() in the kernel)
> >
> > I was addressed last night, so
> > I should write up my thoughts.
> >
> > I do not think -fno-builtin-stpcpy is a
> > general solution.
> >
> > -fno-builtin-stpcpy will work for now
> > because only Clang implements the transformation
> > from 'sprintf(dest, "%s", str)' into
> > 'stpcpy(dest, str) - dest'.
> >
> > If GCC implements it some day,
> > we would run into a problem because
> > in GCC, it is not -fno-builtin-stpcpy, but
> > -fno-builtin-sprintf that disables that optimization.
> >
> > In this regard, 'KBUILD_CFLAGS += -fno-builtin-sprintf'
> > would be more future-proof, but it is potentially
> > an overkill.
> > We want to disable optimization from sprintf() to stpcpy(),
> > but we may still benefit from the optimization from
> > sprintf() into something else.
> >
> >
> > Linus is uncomfortable with this kind of compiler magic.
> > If we take compiler's freedom away,
> > -ffreestanding is a big hammer to solve this problem.
> >
> > If we welcome the compiler's optimization,
> > we should implement stpcpy(), bcmp(), and whatever
> > until we solve all link errors.
>
> Speculating that -ffreestanding is untenable, submitted v3:
> https://lore.kernel.org/lkml/20200825135838.2938771-1-ndesaulniers@xxxxxxxxxx/T/#u
> --
> Thanks,
> ~Nick Desaulniers

FYI:
Andrew Morton picked up this patch.




-- 
Best Regards
Masahiro Yamada