[PATCH 03/10] AXFS: axfs.h

[Geert Uytterhoeven]

On Fri, 22 Aug 2008, Arnd Bergmann wrote:
> On Friday 22 August 2008, Jared Hulbert wrote:
> > If I have an array of u64 numbers tracking small numbers (a[0] = 1;
> > a[1] = 2;) just throwing that onmedia is a big waste.
> > (0x0000000000000001; 0x0000000000000002)  Having different array types
> > for different images such as arrays of u8,u16,u32,u64 becomes less
> > efficient for 3,5,6 and 7 byte numbers, 3 bytes was a particularly
> > interesting size for me.
> > 
> > All I'm doing is removing the totally unnecessary zeros and aligning by bytes.
> > Take an array of u64 like this :
> > 0x0000000000000005
> > 0x0000000000001001
> > 0x00000000000a0000
> > 
> > I strip off the unneeded leading zeros:
> > 0x000005
> > 0x001001
> > 0x0a0000
> > 
> > Then pack them to byte alignment:
> > 0x0000050010010a0000
> > 
> > Sure it could be encoded more but that would make it harder to extract
> > the data.  This way I can read the data in one, maybe two, cache
> > misses.  A couple of shifts to deal with the alignment and endianness
> > and we are done.
> 
> So do I understand right that 3 bytes is your minimum size, and going
> smaller than that would not be helpful? Otherwise I would assume that
> storing a '5' should only take one byte instead of three.

>From the paper, the minimum size (called `depth') is 1.

> I don't unsterstand yet why you store the length of each word separate
> from the word. Most variable-length codes store that implicitly in
> the data itself, e.g. in the upper three bits, so that for storing
> 0x5, 0x1001, 0xa0000, this could e.g. end up as 0x054010014a0000,
> which is shorter than what you have, but not harder to decode.

AFAIU, the length (`depth') of each word is not stored separate, as the
depth is the same for all values in the same table.

With kind regards,

Geert Uytterhoeven
Software Architect

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