[musl] Options for mallocng+ldso reclaim_gaps

[Rich Felker <dalias@libc.org>]

I have an out-of-tree mockup of the ldso reclaim_gaps equivalent for
mallocng. Right now the way it works is by repeatedly placing a
single-slot group of each power-of-two-sized class (in which nested
groups can be allocated) that fits, in order of decreasing powers. So
a 2600 byte gap would get filled with

- 2048 (16 header + 1x2032)
- 512 (16 header + 1x496)
- (remainder 40 bytes too small to be useful)

This yields a total of 5x496 for groups of small allocations, and even
some larger things like 2x240 that might be useful for struct dso. But
it's only one option.

The other option is treating each donation as a blank slate for bump
allocation of permanent groups of any size that fit. For example the
above could fit:

- 1x1632, 1x676, 1x240
- 1x1344, 1x672, 1x496[, 1x64]
- 1x1008, 1x1344, 1x240
- etc.

Desptie this seeming attractive, the permanence of the division (it
really doesn't make sense to make any complex dlmalloc reinvention for
merging these back for reuse in different divisions) means it's very
possible that small programs that benefit the most from reclaim_gaps
might never get to use the reclaimed memory again, if they don't need
the same size class again. On the other hand, the largest-fit
power-of-two mockup I have now gives slots that can hold almost any
nested group of smaller size.

Thoughts on any of this? Anything I might be missing in pros or cons
or other reasonable choices that might be better?