From 821b0732f23af1c73155e71d4c46b8023664aacf Mon Sep 17 00:00:00 2001
From: Martijn Vels <mvels@google.com>
Date: Fri, 9 Dec 2022 13:44:40 -0800
Subject: [PATCH] Cleanup ThreadCache and LifeCycledId

PiperOrigin-RevId: 494254080
---
 src/google/protobuf/arena.cc     | 17 +++++++----------
 src/google/protobuf/arena_impl.h | 29 +++++++----------------------
 2 files changed, 14 insertions(+), 32 deletions(-)

diff --git a/src/google/protobuf/arena.cc b/src/google/protobuf/arena.cc
index 2a76aa13f5..c47cfbf85a 100644
--- a/src/google/protobuf/arena.cc
+++ b/src/google/protobuf/arena.cc
@@ -441,8 +441,8 @@ ThreadSafeArena::SerialArenaChunk* ThreadSafeArena::SentrySerialArenaChunk() {
 }
 
 
-ThreadSafeArena::CacheAlignedLifecycleIdGenerator
-    ThreadSafeArena::lifecycle_id_generator_;
+ABSL_CONST_INIT alignas(kCacheAlignment)
+    std::atomic<ThreadSafeArena::LifecycleId> ThreadSafeArena::lifecycle_id_{0};
 #if defined(PROTOBUF_NO_THREADLOCAL)
 ThreadSafeArena::ThreadCache& ThreadSafeArena::thread_cache() {
   static internal::ThreadLocalStorage<ThreadCache>* thread_cache_ =
@@ -451,14 +451,12 @@ ThreadSafeArena::ThreadCache& ThreadSafeArena::thread_cache() {
 }
 #elif defined(PROTOBUF_USE_DLLS)
 ThreadSafeArena::ThreadCache& ThreadSafeArena::thread_cache() {
-  static PROTOBUF_THREAD_LOCAL ThreadCache thread_cache_ = {
-      0, static_cast<LifecycleIdAtomic>(-1), nullptr};
-  return thread_cache_;
+  static PROTOBUF_THREAD_LOCAL ThreadCache thread_cache;
+  return thread_cache;
 }
 #else
-PROTOBUF_THREAD_LOCAL ThreadSafeArena::ThreadCache
-    ThreadSafeArena::thread_cache_ = {0, static_cast<LifecycleIdAtomic>(-1),
-                                      nullptr};
+ABSL_CONST_INIT PROTOBUF_THREAD_LOCAL
+    ThreadSafeArena::ThreadCache ThreadSafeArena::thread_cache_;
 #endif
 
 ThreadSafeArena::ThreadSafeArena() : first_arena_(*this) { Init(); }
@@ -549,8 +547,7 @@ uint64_t ThreadSafeArena::GetNextLifeCycleId() {
     // On platforms that don't support uint64_t atomics we can certainly not
     // afford to increment by large intervals and expect uniqueness due to
     // wrapping, hence we only add by 1.
-    id = lifecycle_id_generator_.id.fetch_add(1, std::memory_order_relaxed) *
-         kInc;
+    id = lifecycle_id_.fetch_add(1, std::memory_order_relaxed) * kInc;
   }
   tc.next_lifecycle_id = id + kDelta;
   return id;
diff --git a/src/google/protobuf/arena_impl.h b/src/google/protobuf/arena_impl.h
index c4810d5de9..a9ca93ba99 100644
--- a/src/google/protobuf/arena_impl.h
+++ b/src/google/protobuf/arena_impl.h
@@ -109,8 +109,6 @@ struct ArenaBlock {
   // data follows
 };
 
-using LifecycleIdAtomic = uint64_t;
-
 enum class AllocationClient { kDefault, kArray };
 
 class ThreadSafeArena;
@@ -582,16 +580,6 @@ class PROTOBUF_EXPORT ThreadSafeArena {
 #pragma warning(disable : 4324)
 #endif
   struct alignas(kCacheAlignment) ThreadCache {
-#if defined(PROTOBUF_NO_THREADLOCAL)
-    // If we are using the ThreadLocalStorage class to store the ThreadCache,
-    // then the ThreadCache's default constructor has to be responsible for
-    // initializing it.
-    ThreadCache()
-        : next_lifecycle_id(0),
-          last_lifecycle_id_seen(-1),
-          last_serial_arena(nullptr) {}
-#endif
-
     // Number of per-thread lifecycle IDs to reserve. Must be power of two.
     // To reduce contention on a global atomic, each thread reserves a batch of
     // IDs.  The following number is calculated based on a stress test with
@@ -599,11 +587,11 @@ class PROTOBUF_EXPORT ThreadSafeArena {
     static constexpr size_t kPerThreadIds = 256;
     // Next lifecycle ID available to this thread. We need to reserve a new
     // batch, if `next_lifecycle_id & (kPerThreadIds - 1) == 0`.
-    uint64_t next_lifecycle_id;
+    uint64_t next_lifecycle_id{0};
     // The ThreadCache is considered valid as long as this matches the
     // lifecycle_id of the arena being used.
-    uint64_t last_lifecycle_id_seen;
-    SerialArena* last_serial_arena;
+    uint64_t last_lifecycle_id_seen{static_cast<uint64_t>(-1)};
+    SerialArena* last_serial_arena{nullptr};
   };
 
   // Lifecycle_id can be highly contended variable in a situation of lots of
@@ -612,12 +600,9 @@ class PROTOBUF_EXPORT ThreadSafeArena {
 #ifdef _MSC_VER
 #pragma warning(disable : 4324)
 #endif
-  struct alignas(kCacheAlignment) CacheAlignedLifecycleIdGenerator {
-    constexpr CacheAlignedLifecycleIdGenerator() : id{0} {}
-
-    std::atomic<LifecycleIdAtomic> id;
-  };
-  static CacheAlignedLifecycleIdGenerator lifecycle_id_generator_;
+  using LifecycleId = uint64_t;
+  ABSL_CONST_INIT alignas(
+      kCacheAlignment) static std::atomic<LifecycleId> lifecycle_id_;
 #if defined(PROTOBUF_NO_THREADLOCAL)
   // iOS does not support __thread keyword so we use a custom thread local
   // storage class we implemented.
@@ -627,7 +612,7 @@ class PROTOBUF_EXPORT ThreadSafeArena {
   // wrap them in static functions.
   static ThreadCache& thread_cache();
 #else
-  static PROTOBUF_THREAD_LOCAL ThreadCache thread_cache_;
+  ABSL_CONST_INIT static PROTOBUF_THREAD_LOCAL ThreadCache thread_cache_;
   static ThreadCache& thread_cache() { return thread_cache_; }
 #endif