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

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Date:   Thu, 4 May 2023 10:36:11 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] selftests/rseq: Implement
 rseq_unqual_scalar_typeof

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@...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.

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