75529ed

[libpas] atomic-load gets CSE-ed while it can be changed concurrently
https://bugs.webkit.org/show_bug.cgi?id=311776
rdar://174363026

Reviewed by Marcus Plutowski.

This patch fixes two issues.

1. pas_compact_atomic_ptr's null check gets eliminated.

Since it is just a normal load and store, compiler can do CSE if no
current-thread's modification possibility is found. Given that this
atomic load / store is almost always for concurrent access, we should
just use atomic relaxed load and store.

2. Debug assertion failure via `pas_bitfit_directory_get_view`'s bound
   check in `pas_bitfit_directory_get_first_free_view` call.

When we call `pas_bitfit_directory_find_first_empty`, which reloads the
new max_size. so `found_empty_index.index` can be larger than the
previously loaded max_size. This leads to hitting a debug assertion
failure in pas_bitfit_directory_get_view since we get a size from a
old `directory`, and this obtained index and loading directory does not
have any dependencies. We put a dependency between then to get the fresh
up-to-date max_size for assertion.

* Source/bmalloc/libpas/src/libpas/pas_bitfit_directory.c:
(pas_bitfit_directory_get_first_free_view):
* Source/bmalloc/libpas/src/libpas/pas_compact_atomic_ptr.h:
* Source/bmalloc/libpas/src/libpas/pas_compact_tagged_atomic_ptr.h:
* Source/bmalloc/libpas/src/libpas/pas_utils.h:

Canonical link: https://commits.webkit.org/310860@main

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