linux-kernel - Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is

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Message-ID: <CAMZ6RqL+iS6GVsY20=O6GdQakRpp7XdewZJsUbmE5OCsKaHR6Q@mail.gmail.com>
Date: Sat, 7 Dec 2024 22:50:41 +0900
From: Vincent Mailhol <vincent.mailhol@...il.com>
To: Martin Uecker <muecker@...g.de>
Cc: David Laight <David.Laight@...lab.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Yury Norov <yury.norov@...il.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Kees Cook <kees@...nel.org>, 
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, Jani Nikula <jani.nikula@...ux.intel.com>, 
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, 
	Tvrtko Ursulin <tursulin@...ulin.net>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Mike Leach <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Rikard Falkeborn <rikard.falkeborn@...il.com>, 
	"linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>, 
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>, 
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"coresight@...ts.linaro.org" <coresight@...ts.linaro.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

On Sat. 7 Dec. 2024 à 22:19, Martin Uecker <muecker@...g.de> wrote:
> Am Samstag, dem 07.12.2024 um 21:45 +0900 schrieb Vincent Mailhol:
> > On Sat. 7 Dec. 2024 at 17:39, Martin Uecker <muecker@...g.de> wrote:
> > > Am Freitag, dem 06.12.2024 um 16:26 +0900 schrieb Vincent Mailhol:
>
> ...
>
> > > I find it amazing how much time the Linux kernel community spends
> > > revising code to make it work perfectly.
> > >
> > > Still, I am wondering whether some of this time and effort should not
> > > be targeted at C compilers and language work to make these macro
> > > hacks unnecessary?
> >
> > It seems to me that the long term solution to this problem are the
> > constexpr functions.
>
> How would constexpr functions help here?  (I am a bit sceptical about
> constexpr functions.)

I was thinking of some of the "side features" of constexpr functions. Namely:

  - std::is_constant_evaluated
  Link: https://en.cppreference.com/w/cpp/types/is_constant_evaluated

  - if consteval
  Link: https://en.cppreference.com/w/cpp/language/if#Consteval_if

I did not try it, but looking at these, I believe that this would
allow us to rewrite most of our macros into some constexpr functions.

> > But the core issue is that before getting this support in Linux, we
> > have to wait for this to be added to the C2Y draft, then implemented
> > in the compilers (probably just reusing the C++ constexpr functions)
> > and finally wait maybe one more decade for the C2Y support to reach
> > the kernel. For reference the kernel supports C11 only from 2022… So
> > maybe we will see those in the kernel around 2037? Meanwhile, we have
> > to deal with those hacks.
>
> If we do not collaborate on proper solutions, then you might have
> to wait much longer.

I was invited to WG14 this September. For now, I am only lurking. The
thing I have in mind right now is to write a paper to allow the use of
static_assert() in expressions (i.e. make it return 0 on success).
That should be a relatively small change, but would bring a nice
quality of life improvement.

For context, look at this:

  https://lore.kernel.org/all/CAHk-=wjLSEcZ5LdW+3C+9rtjvNPHZT6zdk0POj67T5k2ZpDbgA@mail.gmail.com/T/#m1ba33a804b4041154b72a1d0333f90ec7204c461

And I will definitely follow the progress of constexpr functions in C2Y.


Yours sincerely,
Vincent Mailhol