linux-kernel - Re: [PATCH next v2 2/5] minmax: Allow min()/max()/clamp() if the arguments have the same signedness.

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Message-ID: <20230730170546.GA1666163@dev-arch.thelio-3990X>
Date:   Sun, 30 Jul 2023 10:05:46 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'kernel test robot' <lkp@...el.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        'Andy Shevchenko' <andriy.shevchenko@...ux.intel.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        "'Matthew Wilcox (Oracle)'" <willy@...radead.org>,
        'Christoph Hellwig' <hch@...radead.org>,
        "'Jason A. Donenfeld'" <Jason@...c4.com>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "oe-kbuild-all@...ts.linux.dev" <oe-kbuild-all@...ts.linux.dev>
Subject: Re: [PATCH next v2 2/5] minmax: Allow min()/max()/clamp() if the
 arguments have the same signedness.

On Sun, Jul 30, 2023 at 02:55:50PM +0000, David Laight wrote:
> From: kernel test robot
> > Sent: 29 July 2023 03:01
> > 
> > kernel test robot noticed the following build errors:
> > 
> ...)
> > compiler: clang version 15.0.7 (https://github.com/llvm/llvm-project.git
> > 8dfdcc7b7bf66834a761bd8de445840ef68e4d1a)
> > reproduce: (https://download.01.org/0day-ci/archive/20230729/202307290943.ODVeyeK6-
> > lkp@...el.com/reproduce)
> > 
> ...
> > >> mm/percpu.c:3102:10: error: static assertion expression is not an integral constant expression
> >                    base = min(ptr, base);
> >                           ^~~~~~~~~~~~~~
> ...                    ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    include/linux/minmax.h:23:2: note: expanded from macro '__types_ok'
> >            (is_signed_type(typeof(x)) == is_signed_type(typeof(y)))
> >            ^
> ...
> >    mm/percpu.c:3102:10: note: cast that performs the conversions of a reinterpret_cast is not allowed
> > in a constant expression
> 
> That is a C++ error that seems to have crept into C.
> The relevant definition is:
> 
> #define is_signed_type(type)  (((type)(-1)) < (type)1)
> 
> This seems to have been fixed in clang 16.0.0.

Indeed, it looks like

https://github.com/llvm/llvm-project/commit/a181de452df311d7647329120d05f4eb9c158b6c

fixed this as a result of the discussion at

https://github.com/llvm/llvm-project/issues/57687, which certainly makes
sense.

Cheers,
Nathan