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

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Date:   Fri, 8 May 2020 23:31:07 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        David Sterba <dsterba@...e.com>,
        Horia Geantă <horia.geanta@....com>,
        Eric Biggers <ebiggers@...gle.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] crypto: blake2b - Fix clang optimization for ARMv7-M

On Wed, May 6, 2020 at 7:12 AM Nathan Chancellor
<natechancellor@...il.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