PartitionAlloc: Set the thread cache threshold by size, not bucket index

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}

base/allocator/partition_allocator/thread_cache.h

@@ -9,7 +9,6 @@
 #include <cstdint>
 #include <memory>
 
-#include "base/allocator/partition_allocator/checked_ptr_support.h"
 #include "base/allocator/partition_allocator/partition_alloc_forward.h"
 #include "base/allocator/partition_allocator/partition_cookie.h"
 #include "base/allocator/partition_allocator/partition_freelist_entry.h"
@@ -19,7 +18,6 @@
 #include "base/no_destructor.h"
 #include "base/partition_alloc_buildflags.h"
 #include "base/synchronization/lock.h"
-#include "build/build_config.h"
 
 namespace base {
 
@@ -95,6 +93,10 @@ class BASE_EXPORT ThreadCacheRegistry {
 #define GET_COUNTER(counter) 0
 #endif  // defined(PA_ENABLE_THREAD_CACHE_STATISTICS)
 
+ALWAYS_INLINE static constexpr int ConstexprLog2(size_t n) {
+  return n < 1 ? -1 : (n < 2 ? 0 : (1 + ConstexprLog2(n >> 1)));
+}
+
 // Per-thread cache. *Not* threadsafe, must only be accessed from a single
 // thread.
 //
@@ -170,14 +172,11 @@ class BASE_EXPORT ThreadCache {
   };
 
   // TODO(lizeb): Optimize the threshold.
-#if defined(ARCH_CPU_64_BITS) || ENABLE_TAG_FOR_MTE_CHECKED_PTR
-  static constexpr size_t kBucketCount = 41;
-#else
-  static constexpr size_t kBucketCount = 49;
-#endif
-  // Checked in ThreadCache::Init(), not with static_assert() as the size is not
-  // set at compile-time.
   static constexpr size_t kSizeThreshold = 512;
+  static constexpr size_t kBucketCount =
+      ((ConstexprLog2(kSizeThreshold) - kMinBucketedOrder + 1)
+       << kNumBucketsPerOrderBits) +
+      1;
   static_assert(
       kBucketCount < kNumBuckets,
       "Cannot have more cached buckets than what the allocator supports");