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

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Message-ID: <CAHk-=wjk_thfjRnAYRoGX7LjJ8AyiPTmBqjJEPu6JiKDLG2isg@mail.gmail.com>
Date: Fri, 6 Dec 2024 11:02:46 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Vincent Mailhol <vincent.mailhol@...il.com>
Cc: David Laight <David.Laight@...lab.com>, 
	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>, uecker@...raz.at
Subject: Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

On Fri, 6 Dec 2024 at 10:52, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> This may be a case of "we just need to disable that incorrect compiler
> warning". Or does anybody see a workaround?

Hmm. The "== 0" thing does work, but as mentioned, that causes (more
valid, imho) warnings with pointers.

And it's not necessarily require that a pointer expression actually be
marked as a constant, as for the fact that these macros often get used
in various arbitrary contexts where things *might* be pointers, even
if "not constant" is a perfectly fine answer.

We do actually consciously use __builtin_constant_p() on pointers.
It's very convenient for format strings in particular, where
__builtin_constant_p() is a good test for a constant string, which
sometimes gets treated differently.

And in fact, dealing with NULL pointers statically might be worth it
too, so I do think it's worth keeping in mind.

               Linus