A cache for InlineExecutor singleton

Summary: [Folly] A cache for `InlineExecutor` singleton, which can be inlined into callers with little extra code compared to calling the backing function directly.

Reviewed By: marshallcline

Differential Revision: D8339793

fbshipit-source-id: d2be0128e617cfefe66809eb44ce40e1a8ae2dfb

folly/executors/InlineExecutor.cpp

@@ -20,9 +20,12 @@
 
 namespace folly {
 
-InlineExecutor& InlineExecutor::instance() {
+InlineExecutor& InlineExecutor::instance_slow() noexcept {
   static auto instance = Indestructible<InlineExecutor>{};
+  cache.store(&*instance, std::memory_order_release);
   return *instance;
 }
 
+std::atomic<InlineExecutor*> InlineExecutor::cache;
+
 } // namespace folly

folly/executors/InlineExecutor.h

@@ -15,6 +15,11 @@
  */
 
 #pragma once
+
+#include <atomic>
+
+#include <folly/CPortability.h>
+#include <folly/CppAttributes.h>
 #include <folly/Executor.h>
 
 namespace folly {
@@ -24,11 +29,20 @@ namespace folly {
 /// QueuedImmediateExecutor.
 class InlineExecutor : public Executor {
  public:
-  static InlineExecutor& instance();
+  FOLLY_ATTR_VISIBILITY_HIDDEN FOLLY_ALWAYS_INLINE static InlineExecutor&
+  instance() noexcept {
+    auto const value = cache.load(std::memory_order_acquire);
+    return value ? *value : instance_slow();
+  }
 
   void add(Func f) override {
     f();
   }
+
+ private:
+  FOLLY_COLD static InlineExecutor& instance_slow() noexcept;
+
+  static std::atomic<InlineExecutor*> cache;
 };
 
 } // namespace folly