linux-kernel - Re: [RFC PATCH 1/3] selftests/rseq: Implement rseq_unqual_scalar

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Date:   Thu, 4 May 2023 08:09:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] selftests/rseq: Implement
 rseq_unqual_scalar_typeof

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@...icios.com>
> ---
>  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.

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