Re: [Qemu-devel] [PATCH v5 2/9] Add rle_encode and rle_decode functions Implement Run Length Encoding compression

[Avi Kivity <avi@redhat.com>]

On 01/04/2012 03:35 PM, Stefan Hajnoczi wrote:
> > I think we should specialize this for out case, where we expect runs of
> > zeros (so no need to encode the repeated byte) and runs of non-repeating
> > nonzeros.  I propose this encoding:
> >
> >  page = zrun
> >       | zrun nzrun
> >       | zrun nzrun page
> >
> >  zrun = length
> >
> >  nzrun = length byte...
> >
> >  length = uleb128 encoded integer
> >
> > take for example a xor-encoded page:
> >
> >  { 1000*0, 1, 2, 3, 4, 3092*0 }
> >
> > representing a page that had a single 32-bit write in the middle.  The
> > current encoding would generate
> >
> >  ff 00 ff 00 ff 00 eb 00 01 01 01 02 01 03 01 04 ff 00 ff 00 ff 00 ff
> > 00 ff 00 ff 00ff 00 ff 00 ff 00 ff 00 ff 00 ff 00 20 00
> >
> > while the zrle encoding generates
> >
> >  e8 07 04 01 02 03 04 94 18
> >
> > (e8 07 = uleb128 encoding for 1000)
>
> Had to look up the Unsigned Little-Endian Base 128 encoding, but it's
> a nice idea and simple to implement (though we probably want to
> constrain ULEB128 to max 32-bit or 64-bit in practice; we don't want
> arbitrarily long integers).
>
> http://en.wikipedia.org/wiki/LEB128
>

Sorry, should have provided the URL.  And yes, for this use we're
limited to TARGET_PAGE_BITS, so having 13-bit encoders/decoders would
suffice:

   int uleb128_encode_small(uint8_t *out, uint32_t n)
   {
       if (n < 0x80) {
           *out++ = n;
           return 1;
       } else {
           *out++ = (n & 0x7f) | 0x80;
           *out++ = n >> 7;
       }
   }

   int uleb128_decode_small(const uint128 *in, uint32_t *n)
   {
       if (!(*in & 0x80)) {
           *n = *in++;
           return 1;
       } else {
           *n = *in++ & 0x7f;
           *n |= *in++ << 7;
           return 2;
       }
   }

-- 
error compiling committee.c: too many arguments to function