3-bit Waymarking
(a.k.a. Son of Use-Diet)
Gabor Greif
Weekend LLVM-hobbyist
<picture : array of Uses embedded/pointered in User>
Use has 4 pointers
- drop pointer to
User - allocate
Uses beforeUserin memory - make
Prevpointer tagged (2-bits, since always 4-byte aligned) - seen 12% space savings on big C++ programs
- landed in the LLVM codebase: May 2008
Employ a framed serial code in consecutive Uses
S→ full stops→ stop0,1→ binary digits
Read off binary digits to obtain distance to User
:-)
2.5% runtime increase
(but it was worth it!)
When two feet permit just so much speed, then you have to upgrade to three feet!
I really did not mean to do something cruel as this!
But no earthly life-form provides this feature, so...
Clearly I was in need of some alien technology!
...then I took a page from the book of space exploration and found this gem:
Alien tricks from Mars! :-)
On today's predominantly 64-bit platforms, pointers are 8-byte aligned
We have 8 distinct tags for disposal
- double digits:
00,01,10,11 - 3 stop tags:
q,r,s(always in this order) - full stop:
S
Originally modelled in Haskell (+QuickCheck)
Now in LLVM repo (on a branch), with automatic algorithm selection
- stop tags allow longer hops while hunting down the framed digits
- any stop tag encodes the distance to the framed payload
- harvesting 2-bits at a time
| tag-bits | frames |
|---|---|
| 2 | …1s100000s11010s10100s1111s1010s110s11s1S |
| accesses | …87CBA9876BA9876A987659876587654654343221 |
| 3 | …rs203qrs131qrs113qrs101qrs30qrs13qrs3rsS |
| accesses | …5566655566655566655555544455444443332221 |
| Δ | …3265443205443204332104332132210211011000 |
- unroll tag initialisation loops
- distance relative to stopped frame (microoptimization)
rol(rotate) instructions with condition flags- examining resultant assembly (on all archs!)
Credits:
- NASA (image)
- Wikipedia (image)
- W3C Slidy