dlmalloc: avoid unnecessary atomics in realloc()

* Atomics are much cheaper than mutexes for such rarely contented
  fields, but they are still much slower than plain arithmetics.
* For realloc(), we'd expect relatively small changes between calls
  which means that the chunk's usable size probably won't change.
  Therefore it makes sense to check whether we'd get a zero addition
  to teco_memory_usage.
* We will now have at most one atomic add in realloc() instead of
  always 2 atomic operations.

0 files changed, 0 insertions, 0 deletions