Re: [RFC PATCH 1/3] selftests/rseq: Implement rseq_unqual_scalar_typeof

[Mathieu Desnoyers]

On 2023-05-04 02:09, Peter Zijlstra wrote:
> On Wed, May 03, 2023 at 04:13:22PM -0400, Mathieu Desnoyers wrote:
> > Allow defining variables and perform cast with a typeof which removes
> > the volatile and const qualifiers.
> >
> > This prevents declaring a stack variable with a volatile qualifier
> > within a macro, which would generate sub-optimal assembler.
> >
> > This is imported from the "librseq" project.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > ---
> >   tools/testing/selftests/rseq/compiler.h | 27 +++++++++++++++++++++++++
> >   1 file changed, 27 insertions(+)
> >
> > diff --git a/tools/testing/selftests/rseq/compiler.h b/tools/testing/selftests/rseq/compiler.h
> > index f47092bddeba..8dc7f881e253 100644
> > --- a/tools/testing/selftests/rseq/compiler.h
> > +++ b/tools/testing/selftests/rseq/compiler.h
> > @@ -33,4 +33,31 @@
> >   #define RSEQ_COMBINE_TOKENS(_tokena, _tokenb)	\
> >   	RSEQ__COMBINE_TOKENS(_tokena, _tokenb)
> >   
> > +#ifdef __cplusplus
> > +#define rseq_unqual_scalar_typeof(x)					\
> > +	std::remove_cv<std::remove_reference<decltype(x)>::type>::type
> > +#else
> > +/*
> > + * Use C11 _Generic to express unqualified type from expression. This removes
> > + * volatile qualifier from expression type.
> > + */
> > +#define rseq_unqual_scalar_typeof(x)					\
> > +	__typeof__(							\
> > +		_Generic((x),						\
> > +			char: (char)0,					\
> > +			unsigned char: (unsigned char)0,		\
> > +			signed char: (signed char)0,			\
> > +			unsigned short: (unsigned short)0,		\
> > +			signed short: (signed short)0,			\
> > +			unsigned int: (unsigned int)0,			\
> > +			signed int: (signed int)0,			\
> > +			unsigned long: (unsigned long)0,		\
> > +			signed long: (signed long)0,			\
> > +			unsigned long long: (unsigned long long)0,	\
> > +			signed long long: (signed long long)0,		\
> > +			default: (x)					\
> > +		)							\
> > +	)
>
> FWIW, I like how the kernel version uses a little helper for the
> signed/unsigned pairs. Makes it a little more readable.

OK, will update for next round:

#define rseq_scalar_type_to_expr(type)                                  \
        unsigned type: (unsigned type)0,                                \
        signed type: (signed type)0

/*
 * Use C11 _Generic to express unqualified type from expression. This removes
 * volatile qualifier from expression type.
 */
#define rseq_unqual_scalar_typeof(x)                                    \
        __typeof__(                                                     \
                _Generic((x),                                           \
                        char: (char)0,                                  \
                        rseq_scalar_type_to_expr(char),                 \
                        rseq_scalar_type_to_expr(short),                \
                        rseq_scalar_type_to_expr(int),                  \
                        rseq_scalar_type_to_expr(long),                 \
                        rseq_scalar_type_to_expr(long long),            \
                        default: (x)                                    \
                )                                                       \
        )

Thanks,

Mathieu

> > +#endif
> > +
> >   #endif  /* RSEQ_COMPILER_H_ */
> > --
> > 2.25.1

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com