Re: [PATCH] crypto: blake2b - Fix clang optimization for ARMv7-M

From: Nathan Chancellor <natechancellor@gmail.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Herbert Xu" <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	"David Sterba" <dsterba@suse.com>,
	"Horia Geantă" <horia.geanta@nxp.com>,
	"Eric Biggers" <ebiggers@google.com>,
	"open list:HARDWARE RANDOM NUMBER GENERATOR CORE"
	<linux-crypto@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] crypto: blake2b - Fix clang optimization for ARMv7-M
Date: Fri, 8 May 2020 18:43:30 -0700	[thread overview]
Message-ID: <20200509014330.GA2153306@ubuntu-s3-xlarge-x86> (raw)
In-Reply-To: <CAK8P3a2LAgEG7epWFtUZrcgk9OwpVJd+ji9Ru_rq4L-Qk_dYbg@mail.gmail.com>

On Fri, May 08, 2020 at 11:31:07PM +0200, Arnd Bergmann wrote:
> On Wed, May 6, 2020 at 7:12 AM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> > > -
> > > +#ifdef CONFIG_CC_IS_CLANG
> >
> > Given your comment in the bug:
> >
> > "The code is written to assume no loops are unrolled"
> >
> > Does it make sense to make this unconditional and take compiler
> > heuristics out of it?
> >
> > > +#pragma nounroll /* https://bugs.llvm.org/show_bug.cgi?id=45803 */
> > > +#endif
> > >       for (i = 0; i < 8; ++i)
> > >               S->h[i] = S->h[i] ^ v[i] ^ v[i + 8];
> 
> No, that would not work, as gcc does not support this pragma.
> 
>         Arnd

Ah fair enough.

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>