[PATCH] kbuild: add -Werror=implicit-function-declaration

[Simon Glass]

Hi Masahiro,

On Thu, 7 May 2020 at 19:54, Masahiro Yamada <masahiroy at kernel.org> wrote:
>
> On Fri, May 8, 2020 at 10:39 AM Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Masahiro,
> >
> > On Thu, 7 May 2020 at 06:21, Masahiro Yamada
> > <yamada.masahiro at socionext.com> wrote:
> > >
> > > Add -Werror=implicit-function-declaration as Linux does.
> > >
> > > If you do not check the prototype, it may go wrong run-time.
> > > It is better to break the build, and require to include correct
> > > headers.
> > >
> > > Signed-off-by: Masahiro Yamada <yamada.masahiro at socionext.com>
> > > ---
> > >
> > >  Makefile | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > NAK
> >
> > We already get a warning in this situation. This makes it painful for
> > development since things that should be warnings end up being errors.
> > So your build fails when really it should work well enough to do a
> > test run with your new code. I don't think it has any benefit on code
> > quality since we already detect warnings in gitlab, etc.
> >
> > U-Boot is set up so that warnings are reported and are easy to spot if
> > you use 'make -s' (i.e. not buried in the output).
> >
> > Regards,
> > Simon
>
>
>
> Linux added this flag in 2007.
>
> The intention seems to break the build earlier
> when a non-existing function is used.
>
> I have not seen compliant about this flag in Linux.
> What does it make different for U-Boot ?

Well that commit message is quite misleading. The author is presumably
ignoring the warnings that come out in the compile phase. For me they
come up loud and clear. I don't know why it takes half an hour to get
to the link stage. My average incremental build time is under 4
seconds including the link.

Finally, the warning does not tell you anything about whether the
function doesn't exist. It just tells you you have left out a header
file.

I know how much of a pain this is, because coreboot does this. It does
it partly because there is so much build output that the warnings are
invisible unless they actually halt the build. Even then you have to
search for what went wrong.

Regards,
SImon

>
>
>
>
> commit 94bed2a9c4ae980838003f5d32681eef794ecc28
> Author: Dave Jones <davej at redhat.com>
> Date:   Sun Jul 15 23:41:38 2007 -0700
>
>     Add -Werror-implicit-function-declaration
>
>     Add -Werror-implicit-function-declaration
>     This makes builds fail sooner if something is implicitly defined instead
>     of having to wait half an hour for it to fail at the linking stage.
>
>     Signed-off-by: Dave Jones <davej at redhat.com>
>     Cc: Sam Ravnborg <sam at ravnborg.org>
>     Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
>     Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
>
>
>
> --
> Best Regards
> Masahiro Yamada