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

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Message-ID: <20260207102551.74a42680@pumpkin>
Date: Sat, 7 Feb 2026 10:25:51 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: 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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH next] minmax.h: Use auto for variables in
 __minmax_array()

On Fri, 6 Feb 2026 14:41:35 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

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

If you have:
int f(const int x)
{
	auto y = x;
	y++;
	return y;
}
gcc prior to 11.0 error that y is const.
So in this case the loop can't change __element.

The constness is also kept by 'auto y = +x' which does integer promotion
(useful for converting enums) and both -x and ~x.

> > 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  
> 
> s/Type type/Type/ ?

Actually s/Type/the/

> > 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.  
> 
> I'm not understanding the motivation for this change.  Is there some
> compilation issue to be addressed?

Mainly unqual_scalar_typeof() being horrid.
There is an ongoing long thread about its use in the arm64 LTO READ_ONCE().
Newer compilers do have a builtin, and there are some shorter alternatives
that work in some places.
But here is just isn't needed.
So one less place to check.

I did mean to copy the main contributers to that thread, but forgot.

	David