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SubjectRe: [PATCH 2/4] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use
On Sat, Oct 05, 2019 at 07:17:36PM +0200, Miguel Ojeda wrote:
> Hi Joe,
>
> On Sat, Oct 5, 2019 at 6:46 PM Joe Perches <joe@perches.com> wrote:
> >
> > Reserve the pseudo keyword 'fallthrough' for the ability to convert the
> > various case block /* fallthrough */ style comments to appear to be an
> > actual reserved word with the same gcc case block missing fallthrough
> > warning capability.
> >
> > All switch/case blocks now must end in one of:
> >
> > break;
> > fallthrough;
> > goto <label>;
> > return [expression];
> > continue;
> >
> > fallthough is gcc's __attribute__((__fallthrough__)) which was introduced
> > in gcc version 7..
>
> Nits: double period, missing "r" in fallthough.
>
> > fallthrough devolves to an empty "do {} while (0)" if the compiler
> > version (any version less than gcc 7) does not support the attribute.
>
> Perhaps add a short note why (empty statement warnings maybe? I don't
> remember them but it was months ago so maybe it changed).
>
> > Signed-off-by: Joe Perches <joe@perches.com>
> > Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> Please add Dan's Suggested-by and copy the things I wrote in the
> commit message when I proposed this:
>
> https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4

Perhaps just take this patch directly with Miguel's authorship into the
series and tweak the __fallthrough to fallthrough in subject and body.

-Kees

>
> > ---
> > include/linux/compiler_attributes.h | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> > diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> > index 6b318efd8a74..cdf016596659 100644
> > --- a/include/linux/compiler_attributes.h
> > +++ b/include/linux/compiler_attributes.h
> > @@ -40,6 +40,7 @@
> > # define __GCC4_has_attribute___noclone__ 1
> > # define __GCC4_has_attribute___nonstring__ 0
> > # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
> > +# define __GCC4_has_attribute___fallthrough__ 0
>
> This goes after __externally_visible__.
>
> > #endif
> >
> > /*
> > @@ -185,6 +186,22 @@
> > # define __noclone
> > #endif
> >
> > +/*
> > + * Add the pseudo keyword 'fallthrough' so case statement blocks
> > + * must end with any of these keywords:
> > + * break;
> > + * fallthrough;
> > + * goto <label>;
> > + * return [expression];
> > + *
> > + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
>
> This also goes after __externally_visible__.
>
> Please add:
>
> * Optional: only supported since gcc >= 7.1
> * Optional: only supported since clang >= 10
> * Optional: not supported by icc
>
> As well as:
>
> clang: https://clang.llvm.org/docs/AttributeReference.html#fallthrough
>
> See how I did it in the link above:
>
> https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4
>
> > + */
> > +#if __has_attribute(__fallthrough__)
> > +# define fallthrough __attribute__((__fallthrough__))
> > +#else
> > +# define fallthrough do {} while (0) /* fallthrough */
> > +#endif
> > +
> > /*
> > * Note the missing underscores.
> > *
> > --
> > 2.15.0
> >
>
> Cheers,
> Miguel

--
Kees Cook

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