It's quite lucky a spare bit is free in the tag encoding, this means we
don't need a reserved length value as originally planned. We end up using
all of the bits that overlap the alt pointer encoding, which is nice and
unexpected.
In theory spreading rbyd trees across multiple commits shouldn't change
anything, but the whole point of testing is to catch things that are not
in the theory.
This was a inter-abstraction-layer assumption that won't hold true with
the rbyd blocks.
The whole caching layer probably deserves a rewrite at this point.
It turns out statefulness works quite well with this algorithm (The
prototype was in Haskell, which created some artificial problems. I
think it may have just been too high-level a language for this
near-instruction-level algorithm).
This bias makes it so that tag lookups always find a tag strictly >= the
requested tag, unless we are at the end of the tree.
This makes tree traversal trivial, which is quite nice.
Need to remove ntag now, it's no longer needed.
- Moved alt encoding 0x1 => 0x4, which can lead to slightly better
lookup tables, the perturb bit takes the same place as the color bit,
which means both can be ignored in readonly operations.
- Dropped lfs_rbyd_fetchmatch, asking each lfs_rbyd_fetch to include NULL
isn't that bad.
New encoding:
tags:
iiii iiiiiii iiiiiTT TTTTTTt ttt0tpv
^--------^------^^^- 16-bit id
'------|||- 8-bit type2
'||- 5-bit type1
'|- perturb bit
'- valid bit
llll lllllll lllllll lllllll lllllll
^- n-bit length
alts:
wwww wwwwwww wwwwwww wwwwwww www1dcv
^^^-^- 28-bit weight
'|-|- color bit
'-|- direction bit
'- valid bit
jjjj jjjjjjj jjjjjjj jjjjjjj jjjjjjj
^- n-bit jump
- primitive lfs_rbyd_fetch
- primitive lfs_rbyd_commit
- tag reading/progging and encoding machinery
The tag encoding scheme here uses pairs of leb128s, encoding either
a normal tag:
iiii iiiiiii iiiiiTT TTTTTTt ttttt0v
^--------^------^-^- 16-bit id
'------|-|- 8-bit type2
'-|- 6-bit type1
'- valid bit
llll lllllll lllllll lllllll lllllll
^- n-bit length
Or an alt pointer:
wwww wwwwwww wwwwwww wwwwwww wwwcd1v
^^^-^- 28-bit weight
'|-|- color bit
'-|- direction bit
'- valid bit
jjjj jjjjjjj jjjjjjj jjjjjjj jjjjjjj
^- n-bit jump
Note that two bits overlap the alt pointer dir/color encoding, this
is actually not a problem at all since some tags (crcs/fcrcs) don't
participate in the rbyd tree and can use these bits.
There's a number of benefits to using leb128s, which should probably
be written about, most notably is the abstraction of the device's
word-size. The "n-bits" above can be whatever word size works on the
device, trading off code-size for storage capabilities without breaking
compatibility with other devices. This will eventually be negotiated via
the superblock.
Christopher Haster