Change Abseil's SpinLock adaptive_spin_count to a class static variable that can be set by tcmalloc friend classes.

PiperOrigin-RevId: 826261810
Change-Id: I48eedaf091aff3d9c6b97b7eb8c71958711348cf

Author: Abseil Team

absl/base/internal/spinlock.cc

@@ -71,23 +71,17 @@ void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
 }
 
 // Monitor the lock to see if its value changes within some time period
-// (adaptive_spin_count_ loop iterations). The last value read from the lock
+// (adaptive_spin_count loop iterations). The last value read from the lock
 // is returned from the method.
-ABSL_CONST_INIT std::atomic<int> SpinLock::adaptive_spin_count_{0};
 uint32_t SpinLock::SpinLoop() {
   // We are already in the slow path of SpinLock, initialize the
   // adaptive_spin_count here.
-  if (adaptive_spin_count_.load(std::memory_order_relaxed) == 0) {
-    int current_spin_count = 0;
-    int new_spin_count = NumCPUs() > 1 ? 1000 : 1;
-    // If this fails, the value will remain unchanged. We may not spin for the
-    // intended duration, but that is still safe. We will try again on the next
-    // call to SpinLoop.
-    adaptive_spin_count_.compare_exchange_weak(
-        current_spin_count, new_spin_count, std::memory_order_relaxed,
-        std::memory_order_relaxed);
-  }
-  int c = adaptive_spin_count_.load(std::memory_order_relaxed);
+  ABSL_CONST_INIT static absl::once_flag init_adaptive_spin_count;
+  ABSL_CONST_INIT static int adaptive_spin_count = 0;
+  LowLevelCallOnce(&init_adaptive_spin_count,
+                   []() { adaptive_spin_count = NumCPUs() > 1 ? 1000 : 1; });
+
+  int c = adaptive_spin_count;
   uint32_t lock_value;
   do {
     lock_value = lockword_.load(std::memory_order_relaxed);

absl/base/internal/spinlock.h

@@ -47,7 +47,6 @@ namespace tcmalloc {
 namespace tcmalloc_internal {
 
 class AllocationGuardSpinLockHolder;
-class Static;
 
 }  // namespace tcmalloc_internal
 }  // namespace tcmalloc
@@ -174,16 +173,6 @@ class ABSL_LOCKABLE ABSL_ATTRIBUTE_WARN_UNUSED SpinLock {
   // Provide access to protected method above.  Use for testing only.
   friend struct SpinLockTest;
   friend class tcmalloc::tcmalloc_internal::AllocationGuardSpinLockHolder;
-  friend class tcmalloc::tcmalloc_internal::Static;
-
-  static int GetAdaptiveSpinCount() {
-    return adaptive_spin_count_.load(std::memory_order_relaxed);
-  }
-  static void SetAdaptiveSpinCount(int count) {
-    adaptive_spin_count_.store(count, std::memory_order_relaxed);
-  }
-
-  static std::atomic<int> adaptive_spin_count_;
 
  private:
   // lockword_ is used to store the following: