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The bucket index is needed internally, but it can be computed from the
threshold size rather than being hard-coded.

This was partially fixed by 94069728
(https://chromium-review.googlesource.com/c/2428896) which added a
constant for the threshold size which can be checked by PA_CHECK.
Unfortunately, that new CHECK was violated on mac-arm64, where the
environment reports that the system allocator guarantees a minimum
8-byte alignment, thus the bucket corresponding to allocation size 512
occurs at a higher index than other 64-bit platforms, where the
environment reports a minimum 16-byte alignment. It’s possible that the
alignment reported on mac-arm64 is a bug, because the system allocator
does in fact impose a 16-byte minimum alignment, but perhaps it believes
that while this is true, it’s only guaranteeing 8-byte alignment.

Regardless, the new PA_CHECK did catch that the thread cache threshold
was not tuned as expected on mac-arm64. It seems to be intended to have
used 512 everywhere, but 256 was being used on mac-arm64 instead.

Bug: 1135726
Change-Id: Ic820842d2a6eb669a0a65afb68c34638adf598d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463445
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816239}