linux-kernel - Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization

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Message-ID: <CAKwvOdmCgWFgHof8fqep1hymeNc368Fn8rLDwzzCU58tV5wyXg@mail.gmail.com>
Date:   Thu, 12 Nov 2020 13:55:36 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Adrian Ratiu <adrian.ratiu@...labora.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        Collabora Kernel ML <kernel@...labora.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization

On Thu, Nov 12, 2020 at 1:50 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Wed, Nov 11, 2020 at 04:15:59PM +0200, Adrian Ratiu wrote:
> > On Tue, 10 Nov 2020, Nick Desaulniers <ndesaulniers@...gle.com>
> > wrote:
> > >
> > > Yes, though additionally Arvind points out that this code is
> > > kind of curious if there was overlap; maybe the parameters
> > > should just be restrict-qualified.
> > >
> >
> > For now I think I'll just re-send the GCC changes and leave the
> > Clang optimization as is, until we better understand what's
> > happening and what's the best way to enable it.
> >
>
> Note that the __restrict__ keywords also help GCC -- it saves it from
> having to emit the non-vectorized version and switch between the two at
> runtime. If we can verify it's safe, it's a good thing to add all
> around.

100% agree.  Even a BUILD_BUG_ON or WARN_ON in callers to validate
such an invariant might be nice.
-- 
Thanks,
~Nick Desaulniers