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

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Message-ID: <CAHk-=win8afdcergvJ6f2=rRrff8giGUW62qmYs9Ae6aw=wcnA@mail.gmail.com>
Date: Fri, 6 Dec 2024 11:15:20 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: Vincent Mailhol <vincent.mailhol@...il.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" <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 11:07, David Laight <David.Laight@...lab.com> wrote:
>
> I'm missing the compiler version and options to generate the error.

Just -Wall with most recent gcc versions seems to do it. At least I
can repro it with gcc-14.2.1 and something silly like this:

  $ cat t.c
  int fn(int a) { return (a<<2)?1:2; }
  $ gcc -Wall -S t.c
  t.c: In function ‘fn’:
  t.c:1:26: warning: ‘<<’ in boolean context, did you mean ‘<’?
[-Wint-in-bool-context]

> Does a '+ 0' help?  "(var << 2) + 0 ? 0 : 0"

Yeah, that actually works.

And "+0" is nice in that it should work in any context.

> #define const_NULL(x) _Generic(0 ? (x) : (char *)0, char *: 1, void *: 0)
> #define const_true(x) const_NULL((x) ? NULL : (void *)1L))
> #define const_expr(x) const_NULL((x) ? NULL : NULL))
> I send this morning.
> Needs 's/char/struct kjkjkjkjui/' applied.

Oh Christ. You really are taking this whole ugly to another level.

           Linus