netdev - RE: [PATCH] arm64: do_csum: implement accelerated scalar version

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Date:   Wed, 15 May 2019 10:15:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Will Deacon' <will.deacon@....com>,
        Robin Murphy <robin.murphy@....com>
CC:     Zhangshaokun <zhangshaokun@...ilicon.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "huanglingyan (A)" <huanglingyan2@...wei.com>,
        "steve.capper@....com" <steve.capper@....com>
Subject: RE: [PATCH] arm64: do_csum: implement accelerated scalar version

...
> > 	ptr = (u64 *)(buff - offset);
> > 	shift = offset * 8;
> >
> > 	/*
> > 	 * Head: zero out any excess leading bytes. Shifting back by the same
> > 	 * amount should be at least as fast as any other way of handling the
> > 	 * odd/even alignment, and means we can ignore it until the very end.
> > 	 */
> > 	data = *ptr++;
> > #ifdef __LITTLE_ENDIAN
> > 	data = (data >> shift) << shift;
> > #else
> > 	data = (data << shift) >> shift;
> > #endif

I suspect that
#ifdef __LITTLE_ENDIAN
	data &= ~0ull << shift;
#else
	data &= ~0ull >> shift;
#endif
is likely to be better.

	David

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