Re: [PATCH 04/31] stddef: Avoid warning with clang with offsetof()

[Tom Rini <trini@konsulko.com>]

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On Thu, Jan 13, 2022 at 09:08:13AM +0100, Rasmus Villemoes wrote:
> On 01/11/2021 02.17, Simon Glass wrote:
> > Some bright sparks have decided that a cast on a constant cannot be a
> > constant, so offsetof() produces this warning on clang-10:
> > 
> > include/intel_gnvs.h:113:1: error: static_assert expression is not an
> > 	integral constant expression
> > check_member(acpi_global_nvs, unused2, GNVS_CHROMEOS_ACPI_OFFSET);
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/kernel.h:284:2: note: expanded from macro 'check_member'
> >         offsetof(struct structure, member) == (offset), \
> >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/stddef.h:20:32: note: expanded from macro 'offsetof'
> >                                 ^
> > include/intel_gnvs.h:113:1: note: cast that performs the conversions of
> > 	a reinterpret_cast is ot allowed in a constant expression
> > include/linux/stddef.h:20:33: note: expanded from macro 'offsetof'
> > 
> > Fix it by using the compiler built-in version, if available.
> > 
> > Signed-off-by: Simon Glass <sjg@chromium.org>
> > ---
> > 
> >  include/linux/stddef.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> > index c540f6100d4..a7f546fdfe5 100644
> > --- a/include/linux/stddef.h
> > +++ b/include/linux/stddef.h
> > @@ -1,6 +1,8 @@
> >  #ifndef _LINUX_STDDEF_H
> >  #define _LINUX_STDDEF_H
> >  
> > +#include <linux/compiler_types.h>
> > +
> >  #undef NULL
> >  #if defined(__cplusplus)
> >  #define NULL 0
> > @@ -14,7 +16,11 @@
> >  
> >  #ifndef __CHECKER__
> >  #undef offsetof
> > -#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> > +#ifdef __compiler_offsetof
> > +#define offsetof(TYPE, MEMBER)	__compiler_offsetof(TYPE, MEMBER)
> > +#else
> > +#define offsetof(TYPE, MEMBER)	((size_t)&((TYPE *)0)->MEMBER)
> > +#endif
> >  #endif
> 
> 
> Can we please just drop the useless indirections? Any compiler we care
> about provides __builtin_offsetof(), so just make the whole thing
> 
>   #undef offsetof
>   #define offsetof(TYPE, MEMBER)  __builtin_offsetof(TYPE, MEMBER)
> 
> And by "compilers we care about", that includes sparse - it has had
> __builtin_offsetof ever since 2007.
> 
> And we can nuke the __compiler_offsetof definition in compiler_types.h,
> and if any users exist, just mechanically convert them to offsetof().

We should re-sync this with upstream, but that will keep the indirection
you're objecting to here, I think.

-- 
Tom

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