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

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Message-ID: <63f538fceadf4e2ba93cdcaae9fab266e5a000ac.camel@gwdg.de>
Date: Sat, 7 Dec 2024 22:06:04 +0100
From: Martin Uecker <muecker@...g.de>
To: David Laight <David.Laight@...LAB.COM>, Linus Torvalds
	<torvalds@...ux-foundation.org>, Vincent Mailhol <vincent.mailhol@...il.com>
CC: "w@....eu" <w@....eu>, 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>,
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	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Rikard Falkeborn
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Subject: Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of
 __is_constexpr()

Am Samstag, dem 07.12.2024 um 21:00 +0000 schrieb David Laight:
> From: Martin Uecker
> > Sent: 07 December 2024 19:52
...

> 
> > There exist proposals along those lines for C2Y.
> > 
> > From a more near-term solution, I wonder if making it possible (or
> > easier) to return integer constant expressions from statement
> > expressions and allowing a restricted form of statement expressions
> > at file scope would help?
> 
> It would help a lot if a #define that just used local variables
> to avoid arguments being re-expanded and for CSE could still
> generate a constant value.
> Does need to be a #define - to get token pasting and 'stringify'.
> Although you would need something for reporting detected errors,
> and builtin compiler support for const_true() for the detection
> itself.

We are super close:

https://godbolt.org/z/Tarq89bha

(if we ignore the grotesque hacks to get there, but this would go
away if the compiler does it internally)

Martin