[svat]

> manually make some allocations downward from the top and some allocations upward from the bottom

Incidentally, this is what Knuth does in TeX, if I understand correctly: http://mirrors.ctan.org/info/knuth-pdf/tex/tex.pdf#page=43 (section 116):

> The mem array is divided into two regions that are allocated separately, but the dividing line between these two regions is not fixed; they grow together until finding their “natural” size in a particular job. Locations less than or equal to lo_mem_max are used for storing variable-length records consisting of two or more words each. […] Locations greater than or equal to hi_mem_min are used for storing one-word records…

(Different allocators are used for the two regions and neither seems to be a bump allocator, so it's probably not very relevant to this thread, but I was reminded of it so just sharing…)