Re: x86: faster strncpy_from_user()

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2012-04-10 at 17:57 -0700, Linus Torvalds wrote:
> On Tue, Apr 10, 2012 at 5:50 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > .. and you don't have a double shift, right? So you'd need to do two
> > shifts and the or for each word.
> 
> Actually, if sparc has a "rotate" instruction, you can do with a
> single shift (rotate) per word loop.
> 
> You need to set up a mask register based on the alignment, and
> pre-load the first word, but if you do have a rotate you can rotate
> and then use "and mask" first to generate the "high bits" of the
> current word, and then use the "andn mask" to generate the low bits of
> the next word. So then you just need a single rotate per loop, and
> some (very minor) loop prep.
> 
> Of course, RISC people tended to throw out rotate too, so maybe you
> don't have even that.

Well, we do have a very nice & flexible rotate & mask on ppc at
least :-)

Cheers,
Ben.