[racket] [PATCH] Speeding up set-argb-pixels

These changes look good, and I'll push them.

I also tried a larger revision to lift the mode tests out of the loop;
it's only worth about 10%, but it might set up further improvements.

When I run your example, though, I get much better absolute performance
than you're reporting: 34.3 ± 0.2 msec in version 5.3.1, and 8.45 ± 0.1
msec after the changes. That's running `racket' from a command line and
on 64-bit Mac OS X, but I get similar results from other machines (and
other OSes under VirtualBox on Mac OS X). Any idea why your numbers are
so different?

At Sun, 16 Dec 2012 11:29:58 -0700, Michael Wilber wrote:
> TL;DR: About ~2.8x speedup from using local variables and unsafe
> functions. Copying each bitmap row could bring speedup to ~20x, but it
> doesn't quite work and I need your help. Pull request at
> https://github.com/plt/racket/pull/199
> 
> Hey there!
> 
> I'm writing some FFmpeg bindings for Racket. It's fast enough to decode
> video in real time, but on my machine, set-argb-pixels takes 189.35±1.3
> msec to run for a 500x500 image, which means I'm limited to displaying
> frames at ~5fps.
> 
> Here's a toy benchmark to test set-argb-pixels:
> https://gist.github.com/4a5661dfad984cfdab19
> 
> There are some very simple bottlenecks that I've started to address:
> 
> 1. It turns out that the references to b&w? and alpha-channel-local? for
>    each pixel are slow slow slow. Making them local variables drops the
>    time down to 124.8±1.0msec. This three-line change gives a speedup
>    factor of about ~1.5
> 
> 2. Using unsafe functions everywhere (unsafe-bytes-ref and friends,
>    unsafe-fx+ and friends) drops it further to 67.05±0.6msec, which is a
>    speedup factor of ~2.82 over the original on my machine
> 
> A pull request for the above is at
> https://github.com/plt/racket/pull/199
> 
> Now, if we can assume that the input bytes already contain pre-clipped,
> premultiplied data, we don't really have to loop through each pixel. If
> we copy each row using copy-bytes!, that drops the function to 9.55±6.1
> msec (!) which is a speedup factor of ~20x over the original.
> 
> The problem with that is on my little-endian machine, Cairo expects the
> input data in BGRA format, not RGBA, so the colors look wrong. Alas,
> this is why Racket's doing all the byte swizzling manually.
> 
> Is there a fast native way of switching the endianness of a byte vector
> assumed to contain 32-bit ints? Or some way to do what we want?
> 
> If there's a way to do this, this could make playing simple
> low-resolution videos from Racket pretty feasible.
> 
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