Re: [PATCH] arc: Fix __fls() const-foldability via __builtin_clzl()

From: Kees Cook <kees@kernel.org>
To: Yury Norov <yury.norov@gmail.com>
Cc: Rasmus Villemoes <ravi@prevas.dk>,
	kernel test robot <lkp@intel.com>,
	Vineet Gupta <vgupta@kernel.org>,
	linux-snps-arc@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH] arc: Fix __fls() const-foldability via __builtin_clzl()
Date: Tue, 26 Aug 2025 19:13:03 -0700	[thread overview]
Message-ID: <202508261911.ECB628656@keescook> (raw)
In-Reply-To: <aK5eNDa5H-PNGHpj@yury>

On Tue, Aug 26, 2025 at 09:24:04PM -0400, Yury Norov wrote:
> On Tue, Aug 26, 2025 at 09:56:06AM -0700, Kees Cook wrote:
> > On Tue, Aug 26, 2025 at 03:08:59PM +0200, Rasmus Villemoes wrote:
> > > If __builtin_arc_fls() simply doesn't qualify for attr_const for
> > > $reason, I think it would be good to have that documented in the commit
> > > msg. If it does, I think a gcc ticket and link to that would be in order.
> > 
> > I already sent the patch to fix it. :)
> > 
> > https://gcc.gnu.org/pipermail/gcc-patches/2025-August/693273.html
> 
> I'm OK taking the patch if it fixes real problem for you, but it looks
> more like a GCC problem, right? Is Clang also affected?

Without it, KUnit testing of ffs/fls fails on arc. Clang is not
affected. Even with the GCC fix landed, all older GCCs with still fail,
and since it provides a improved code generation for arc, it seems worth
it (compile-time-calculable values will be emitted instead of always
running the arc instructions).

> If, say, Clang is not affected, and you expect that newer GCC versions
> will not be affected too, let's protect the new code with a proper
> ifdefery, so that it will be easier to drop the workaround later?

I think the codegen benefit is worth it as I have it.

-- 
Kees Cook