Re: Lisp_Marker size on 32bit systems

[Paul Eggert <eggert@cs.ucla.edu>]

Stefan Monnier wrote:
> I think 32 byte objects aren't supposed to be much more likely to be
> aligned on 32 byte boundaries than on 32B + 16B.

True, I miscalculated. If the cache line size is 64 bytes and objects are 
allocated on 16-byte boundaries (the case now), the probability that a 
randomly-placed 24-byte marker (allocated as 32 bytes) will straddle into two 
cache lines is (2-1)/4, or 25%. Whereas if objects are allocated in 8-byte 
boundaries as you're suggesting, the probability that the same marker will 
straddle is (3-1)/8, which is still 25%. So for this particular case the 
straddling issue should be a wash.

> Those perf-stats also show improved I$ performance, which isn't
> explained by your suggested explanation.  Similarly, they show a reduced
> number of instructions.

Yes, it could well be that the 32-byte allocation is faster than the 24 partly 
due to some reason other than d-cache effects. Although there is a smaller 
percentage of cache misses in the 32-byte version, it could be that this is 
because the 32-byte version uses simpler code that would be faster even if the 
cache miss rate were the same.