[libc] Cache the most recently used slot for a chunk size (#149751)

Summary:
This patch changes the `find_slab` logic to simply cache the most
successful slot. This means the happy fast path is now a single atomic
load on this index. I removed the SIMT shuffling logic that did slab
lookups wave-parallel. Here I am considering the actual traversal to be
comparatively unlikely, so it's not overly bad that it takes longer.
ideally one thread finds a slot and shared it with the rest so we only
pay that cost once.

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Co-authored-by: Shilei Tian <i@tianshilei.me>

0 files changed, 0 insertions, 0 deletions