Opportunity for speedup

Wade Brainerd wadetb at gmail.com
Thu Feb 19 13:56:59 EST 2009

RLE (run length encoding) compresses sequences of identical pixels ("runs")
as value/count pairs.
So abbbbbbbbbbccc would be stored as 1a 10b 3c.

The decompressor looks like:

while (cur < end)
   unsigned short count = *cur++;
   unsigned short value = *cur++;
   while (count--)
      *dest++ = value;

This can be faster than memcpy because you are reading significantly less
memory than you would with memcpy, thus fewer cache misses are incurred.

Because the startup images are mostly spans solid colors, this kind of
compression works very well.  If that were not the case, say if there were a
left-to-right gradient in the background, RLE would probably make things
worse, thus you have to be careful when choosing it.

But the smaller size on disk and in memory would probably improve
performance in other ways as well.


On Thu, Feb 19, 2009 at 1:49 PM, Bobby Powers <bobbypowers at gmail.com> wrote:

> 2009/2/19 Wade Brainerd <wadetb at gmail.com>:
> > On Thu, Feb 19, 2009 at 1:22 PM, C. Scott Ananian <cscott at laptop.org>
> wrote:
> >>
> >> I'd suggest just uncompressing the various image files and re-timing
> >> as a start.  The initial implementation was uncompressed, but people
> >> complained about space usage on the emulator images (which are
> >> uncompressed).  The current code supports both uncompressed and
> >> compressed image formats.  For uncompressed images, putting the bits
> >> on the screen is an mmap and memcpy, so I can't imagine any
> >> implementation being faster than that (it's possible, of course, that
> >> what's stealing CPU is the shell's invocation of the client program;
> >> recoding just that little part in C should be trivial, since it does
> >> nothing but write to a socket IIRC.)
> >
> > I implemented a RLE compressor specifically for these 16bit image files
> the
> > last time this question came up.  This can certainly be faster than
> memcpy
> > since we are talking memory performance.
> Can you explain this?  I don't think I have enough knowledge to
> evaluate your claim.
> bobby
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