RE: [PATCH v5 07/13] minmax: Introduce {min,max}_array()

[David Laight]

From: Herve Codina
> Sent: 15 June 2023 16:26
> 
> Introduce min_array() (resp max_array()) in order to get the
> minimal (resp maximum) of values present in an array.
> 
> Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> ---
>  include/linux/minmax.h | 64 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> index 396df1121bff..1672985b02a3 100644
> --- a/include/linux/minmax.h
> +++ b/include/linux/minmax.h
> @@ -133,6 +133,70 @@
>   */
>  #define max_t(type, x, y)	__careful_cmp((type)(x), (type)(y), >)
> 
> +/*
> + * Remove a const qualifier from integer types
> + * _Generic(foo, type-name: association, ..., default: association) performs a
> + * comparison against the foo type (not the qualified type).
> + * Do not use the const keyword in the type-name as it will not match the
> + * unqualified type of foo.
> + */
> +#define __unconst_integer_type_cases(type)	\
> +	unsigned type:  (unsigned type)0,	\
> +	signed type:    (signed type)0
> +
> +#define __unconst_integer_typeof(x) typeof(			\
> +	_Generic((x),						\
> +		char: (char)0,					\
> +		__unconst_integer_type_cases(char),		\
> +		__unconst_integer_type_cases(short),		\
> +		__unconst_integer_type_cases(int),		\
> +		__unconst_integer_type_cases(long),		\
> +		__unconst_integer_type_cases(long long),	\
> +		default: (x)))

Those are probably more generally useful and belong elsewhere.

> +
> +/*
> + * Do not check the array parameter using __must_be_array().
> + * In the following legit use-case where the "array" passed is a simple pointer,
> + * __must_be_array() will return a failure.
> + * --- 8< ---
> + * int *buff
> + * ...
> + * min = min_array(buff, nb_items);
> + * --- 8< ---

Is that needed in the .h file?

> + *
> + * The first typeof(&(array)[0]) is needed in order to support arrays of both
> + * 'int *buff' and 'int buf[N]' types.
> + *
> + * The array can be an array of const items.
> + * typeof() keeps the const qualifier. Use __unconst_typeof() in order to
> + * discard the const qualifier for the __element variable.
> + */
> +#define __minmax_array(op, array, len) ({				\
> +	typeof(&(array)[0]) __array = (array);				\
> +	typeof(len) __len = (len);					\
> +	__unconst_integer_typeof(__array[0]) __element = __array[--__len]; \

s/__element/__bound/

> +	while (__len--)							\
> +		__element = op(__element, __array[__len]);		\
> +	__element; })

I'm not all sure that all the shenanigans required to use min()
is really needed here.

It would also be generally better to process the array forwards.
So something like:
	typeof (&array[0]) __ptr = array, __limit = array + len;
	typeof (array[0] + 0) __element, __bound = *__ptr++;
	while (ptr < __limit) {
		__element = *__ptr++;
		if (__element > __bound)
			__bound = __element;
	}
	(typeof (array[0]))__bound; })
seems fine to me.
The final cast is there to convert 'int' back to un/signed char|short.
Not really needed and might generate worse code.

But if you insist on using min/max ignore this bit.

	David

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