[Patch] mt_allocator: spare mem & fix alignment problems

[Paolo Carlini]

Hi,

the below does two, in principle unrelated, things:

  1- Changes struct block_record to a union: in fact next and
     thread_id are never used at the same time. The former only
     when the block is unused and linked to the others of the
     same thread, the latter only when used. This allows to spare
     a good amount of memory (see attachments)

  2- Adds a configurable _M_min_bin, which sets the smallest bin
     size. The current 1 byte, frankly, doesn't make much sense
     and, anyway, raising it to 8 bytes allows to fix the alignment
     problems we were experiencing on ia64 (now the alignment is
     the same used by default by pool_allocator).

Overall, the performance seem slightly improved (see attachments), but
perhaps the heuristics in deallocate should be tweaked again.

Tested x86/x86_64/ia64 linux: now same results on all three, with the
only unsolved problem (wrt new/pool-allocator) of pthread7-rope.

Will wait a while for comments, then commit to mainline.

Paolo.

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