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

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Message-ID: <b3bf85a9-85de-3800-787e-2b00d3edd939@arm.com>
Date:   Thu, 28 Feb 2019 18:00:15 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] lib/raid6: use vdupq_n_u8 to avoid endianness warnings

On 26/02/2019 20:44, Nick Desaulniers wrote:
> 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?).

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.