[all-commits] [llvm/llvm-project] ce52f9: [libc] Search empty bits after failed allocation (...

[Joseph Huber via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ce52f9cdc4c6873d1d5b3a970393d4b6aff12e70
      https://github.com/llvm/llvm-project/commit/ce52f9cdc4c6873d1d5b3a970393d4b6aff12e70
  Author: Joseph Huber <huberjn at outlook.com>
  Date:   2025-07-23 (Wed, 23 Jul 2025)

  Changed paths:
    M libc/src/__support/GPU/allocator.cpp

  Log Message:
  -----------
  [libc] Search empty bits after failed allocation (#149910)

Summary:
The scheme we use to find a free bit is to just do a random walk. This
works very well up until you start to completely saturate the bitfield.
Because the result of the fetch_or yields the previous value, we can
search this to go to any known empty bits as our next guess. This
effectively increases our liklihood of finding a match after two tries
by 32x since the distribution is random.

This *massively* improves performance when a lot of memory is allocated
without freeing, as it now doesn't takea one in a million shot to fill
that last bit. A further change could improve this further by only
*mostly* filling the slab, allowing 1% to be free at all times.



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