linux-kernel - Re: [PATCH next] minmax.h: Use auto for variables in __minmax

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Message-ID: <20260207105049.09c67177@pumpkin>
Date: Sat, 7 Feb 2026 10:50:49 +0000
From: David Laight <david.laight.linux@...il.com>
To: Peter Anvin <hpa@...or.com>, Andy Shevchenko
 <andriy.shevchenko@...ux.intel.com>, Arnd Bergmann <arnd@...nel.org>,
 Christoph Hellwig <hch@...radead.org>, "Jason A . Donenfeld"
 <Jason@...c4.com>, Herve Codina <herve.codina@...tlin.com>, Linus Torvalds
 <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>,
 linux-kernel@...r.kernel.org
Cc: Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH next] minmax.h: Use auto for variables in
 __minmax_array()

On Fri,  6 Feb 2026 22:25:54 +0000
david.laight.linux@...il.com wrote:

Cc the people discussing unqual_scalar_typeof() for arm64 LTO READ_ONCE().


> From: David Laight <david.laight.linux@...il.com>
> 
> While 'auto __element = _array[--__len]' should remove 'const',
> gcc prior to version 11 are buggy and retain it.
> However forcing an integer promotion by adding zero does work.
> 
> Promoting signed/unsigned char and short to int doesn't matter here,
> that happens as soon as the value is used.
> 
> Type type of the result (for char/short arrays) changes, but the value
> will always be promoted to int before it is used (for any purpose) so
> it isn't even worth casting the type back - all that is likely to do
> is make the compiler explicitly mask it to 8/16 bits before it is
> immediately promoted back to int.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
>  include/linux/minmax.h | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> index a0158db54a04..7d437f73a6d6 100644
> --- a/include/linux/minmax.h
> +++ b/include/linux/minmax.h
> @@ -239,20 +239,13 @@
>   * ...
>   * min = min_array(buff, nb_items);
>   * --- 8< ---
> - *
> - * The first typeof(&(array)[0]) is needed in order to support arrays of both
> - * 'int *buff' and 'int buff[N]' types.
> - *
> - * The array can be an array of const items.
> - * typeof() keeps the const qualifier. Use __unqual_scalar_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);					\
> -	__unqual_scalar_typeof(__array[0]) __element = __array[--__len];\
> -	while (__len--)							\
> -		__element = op(__element, __array[__len]);		\
> +#define __minmax_array(op, array, len) ({			\
> +	auto __array = &(array)[0];				\
> +	auto __len = len;					\
> +	auto __element = __array[--__len] + 0;			\
> +	while (__len--)						\
> +		__element = op(__element, __array[__len]);	\
>  	__element; })
>  
>  /**