linux-kernel - Re: [PATCH 2/2] word-at-a-time: provide generic big-endian zero

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Date:	Thu, 12 Dec 2013 17:02:34 +0000
From:	Will Deacon <will.deacon@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/2] word-at-a-time: provide generic big-endian
 zero_bytemask implementation

Hi Linus,

On Tue, Dec 10, 2013 at 08:08:20PM +0000, Linus Torvalds wrote:
> Hmm. Did you try to time this?

I took it for a spin on a big-endian ARMv7 platform (32-bit) and the hashing is
around 40% faster.

> Also, I really have #ifdef's in code, and I think we'd be better off
> just exposing a function that does this
> 
> +#ifdef __BIG_ENDIAN
> +       mask = ~(~0ul >> tcount*8);
> +#else
>         mask = ~(~0ul << tcount*8);
> +#endif
> 
> thing. I think it would logically go together with zero_bytemask(),
> call it something like "bytemask_from_count()" or something. Hmm? It's
> basically just the reverse of "count_masked_bytes()", which we also
> have an abstraction for.

Yup, that's a good idea, I can probably just add something to dcache.h.
I'll spin a v2.

Cheers,

Will
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