Eliminate unnecessary Adjust{Size|Offset}ForExtras* calls

This CL fixes a performance regression introduced by r829145.

When both partition cookies and reference counts are disabled at
compile time (which is the current default configuration),
|AdjustSizeForExtrasAdd| and similar functions become no-op.

Make the compiler aware of that.

Bug: 1150917
Change-Id: I24e91177d4575a5ea013a9c647641c6389aa01a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571281Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Sergei Glazunov <glazunov@google.com>
Cr-Commit-Position: refs/heads/master@{#833310}

base/allocator/partition_allocator/partition_alloc-inl.h

@@ -16,6 +16,9 @@
 #include <windows.h>
 #endif
 
+#define PARTITION_EXTRAS_REQUIRED \
+  (DCHECK_IS_ON() || ENABLE_REF_COUNT_FOR_BACKUP_REF_PTR)
+
 namespace base {
 
 namespace internal {

base/allocator/partition_allocator/partition_root.cc

@@ -355,6 +355,7 @@ void PartitionRoot<thread_safe>::Init(PartitionOptions opts) {
   // ref-count at the beginning of the slot.
   allow_extras = (opts.alignment != PartitionOptions::Alignment::kAlignedAlloc);
 
+#if PARTITION_EXTRAS_REQUIRED
   size_t size = 0, offset = 0;
   if (allow_extras) {
     size += internal::kPartitionCookieSizeAdjustment;
@@ -365,6 +366,7 @@ void PartitionRoot<thread_safe>::Init(PartitionOptions opts) {
   }
   extras_size = static_cast<uint32_t>(size);
   extras_offset = static_cast<uint32_t>(offset);
+#endif
 
   pcscan_mode = PartitionOptionsToPCScanMode<thread_safe>(opts.pcscan);
   if (pcscan_mode == PCScanMode::kEnabled) {

base/allocator/partition_allocator/partition_root.h

@@ -146,8 +146,16 @@ struct BASE_EXPORT PartitionRoot {
   const bool is_thread_safe = thread_safe;
 
   bool allow_extras;
+
+#if !PARTITION_EXTRAS_REQUIRED
+  // Teach the compiler that `AdjustSizeForExtrasAdd` etc. can be eliminated
+  // in builds that use no extras.
+  static constexpr uint32_t extras_size = 0;
+  static constexpr uint32_t extras_offset = 0;
+#else
   uint32_t extras_size;
   uint32_t extras_offset;
+#endif
 
   // Not used on the fastest path (thread cache allocations), but on the fast
   // path of the central allocator.