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

[Nick Desaulniers]

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