Re: [PATCH] lib/raid6: use vdupq_n_u8 to avoid endianness warnings

[Robin Murphy]

On 26/02/2019 20:44, Nick Desaulniers wrote:
> On Mon, Feb 25, 2019 at 11:19 PM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> > On Tue, 26 Feb 2019 at 05:03, <ndesaulniers@xxxxxxxxxx> 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@xxxxxxxxx>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> >
> > 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?).

FWIW the most recent 'proper' spec document I know of is this one:

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073b/index.html


Apparently we have a more interactive playground on the new site, too:

https://developer.arm.com/technologies/neon/intrinsics


Robin.