linux-kernel - Re: [PATCH v5 1/2] lib/test_bitops: Add benchmark test for fns()

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Date: Mon, 6 May 2024 10:56:45 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Yury Norov <yury.norov@...il.com>, Kuan-Wei Chiu <visitorckw@...il.com>,
	kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
	oe-kbuild-all@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] lib/test_bitops: Add benchmark test for fns()

On Sun, May 05, 2024 at 12:42:53PM +0200, Miguel Ojeda wrote:
> On Fri, May 3, 2024 at 11:56 PM Yury Norov <yury.norov@...il.com> wrote:
> >
> > + * The __used attribute guarantees that the attributed variable will be
> 
> We should probably mention functions as Nathan said (unless it does
> not work for some reason).

Yeah, it should work for functions. I think clarifying it will not work
for local variables would probably be good as well, since __used__ does
not work on those like I replied in my other email, but it is not that
big of a deal.

> > + * is actually used. It's a compiler implementation details either emit
> > + * the warning in that case or not.
> 
> Is it an implementation detail or rather that they took different
> alternatives/options on purpose (even if not documented)? If we think
> it is just a consequence of their implementation, perhaps we should
> mention that and what GCC/Clang do today in their latest version, in
> case it changes (so that we know whether we need to remove the macro,
> for instance).

Yeah it is entirely possible that this is not intentional but when
-Wunused-but-set-variable was introduced in Clang, I know there was a
lot of discussion around making the warning match GCC in certain ways as
well as breaking from GCC in others. I have not tried to dig up those
discussions to confirm though.

Cheers,
Nathan