linux-kernel - Re: [PATCH] lib/raid6: use vdupq_n

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Message-ID: <CAKwvOdmzb1WF6NU5LYWGXK_O2cbqwMjbWGAa5xk5x+EMz7A1yw@mail.gmail.com>
Date:   Tue, 26 Feb 2019 12:44:44 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/raid6: use vdupq_n_u8 to avoid endianness warnings

On Mon, Feb 25, 2019 at 11:19 PM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Tue, 26 Feb 2019 at 05:03, <ndesaulniers@...gle.com> wrote:
> >
> > Clang warns: vector initializers are not compatible with NEON intrinsics
> > in big endian mode [-Wnonportable-vector-initialization]
> >
> > While this is usually the case, it's not an issue for this case since
> > we're initializing the uint8x16_t (16x uint8_t's) with the same value.
> >
> > Instead, use vdupq_n_u8 which both compilers lower into a single movi
> > instruction: https://godbolt.org/z/vBrgzt
> >
> > This avoids the static storage for a constant value.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/214
> > Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> Much better, thanks,
>
> Did you double check that the intrinsic exists on 32-bit ARM as well?
> I assume it does, but please make sure if you haven't yet.

Thanks for the review!
Looking through Clang's generated arm_neon.h, vdupq_n_u8 seems to have
2 definitions predicated on __LITTLE_ENDIAN__ (not __arch64__ or
__ARM_ARCH >= 8 like some of the other types and functions).

So NEON got some additions in v8?  Is there a doc that lists them?
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491g/BABDBBJB.html
is where I found vdupq_n_u8, but it doesn't seem to mention
compatibility (so I assume it's been around since the introduction of
NEON?).
-- 
Thanks,
~Nick Desaulniers