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

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Message-ID: <c3d21364d9ad43279352dc4d7348c97d@AcuMS.aculab.com>
Date: Sat, 7 Dec 2024 21:45:47 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Martin Uecker' <muecker@...g.de>, 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>, 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"
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	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH 02/10] compiler.h: add is_const() as a replacement of
 __is_constexpr()

From: Martin Uecker
> Sent: 07 December 2024 21:06
> 
> 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

(The preprocess output is about 500 bytes for each line.)

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

Some of those hacks look excessive.
Isn't IF_CONST(x, y, z) just
	_Generic(0 ? (void *)((x) ? 0L : 0L) : (char *)0, char *: y, void *: z)
and that gets rid some of the grossness.
Just having that as a builtin would simplify some things.
Although you could use:
	__builtin_choose_expr(IS_CONST(x), y, z)
if you need y and z to have different types, and just:
	IS_CONST(x) ? y : z
otherwise.
Since AFAICT they are otherwise equivalent.

	David

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