[x264-devel] commit: add AltiVec implementation of x264_pixel_var_16x16 and x264_pixel_var_8x8 (Guillaume Poirier )

[Guillaume POIRIER]

Hello,

On Sun, Jan 25, 2009 at 12:33 AM, Loren Merritt <lorenm at u.washington.edu> wrote:
> On Sat, 24 Jan 2009, Guillaume POIRIER wrote:

>> I have no experience in using LUT: what's the right way to compute the
>> index fast?
>
> perm_tab needs to be static const, otherwise it gets written to the stack
> at every function call.

Damn, I thought that modern compilers were smart enough to catch that.
Fixed. That brings the overall number of cycles down to 16 cycles as
previously.


> gcc might also be failing to optimize the ! (it's not a simple arithmetic
> op). Fix that by
> perm_tab[(((uintptr_t)pix & 8) >> 3) + ((i_stride & 8) >> 2)]

I tried that, and interestingly it doesn't change cycle count. I used
your expression anyway since it's simpler.


> And then promote chroma planes to 16 byte alignment, and change it to
> perm_tab[((uintptr_t)pix & 8) >> 3]

I implemented this, and it's brilliant! I shaves one more cycle overall.

Attached patch aligns chroma planes on mod16, and has the rest of the
PPC optimizations you suggested.

Please check I didn't screw up something.

Thanks a lot for all your suggestions!

Guillaume
-- 
Only a very small fraction of our DNA does anything; the rest is all
comments and ifdefs.

Marilyn Monroe  - "It's not true that I had nothing on. I had the radio on."
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