rcu: narrow some variable to avoid warnings

Summary:
On Windows, MSVC rightfully complains about conversion from uint64_t to uint8_t
and uint32_t. In both cases, the narrowing is acceptable, so let's do that.

Reviewed By: yfeldblum

Differential Revision: D25352231

fbshipit-source-id: aa900fc14a666f1a99bfaa45b1afac02e7209fa3

folly/synchronization/Rcu-inl.h

@@ -52,10 +52,10 @@ rcu_domain<Tag>::~rcu_domain() {
 template <typename Tag>
 rcu_token<Tag> rcu_domain<Tag>::lock_shared() {
   auto idx = version_.load(std::memory_order_acquire);
-  idx &= 1;
-  counters_.increment(idx);
+  uint8_t epoch = to_narrow(idx & 1);
+  counters_.increment(epoch);
 
-  return rcu_token<Tag>(idx);
+  return rcu_token<Tag>(epoch);
 }
 
 template <typename Tag>
@@ -129,7 +129,7 @@ void rcu_domain<Tag>::synchronize() noexcept {
       }
       std::atomic<uint32_t> cutoff{100};
       // Wait for someone to finish the work.
-      turn_.tryWaitForTurn(work, cutoff, false);
+      turn_.tryWaitForTurn(to_narrow(work), cutoff, false);
     }
   }
 }
@@ -171,7 +171,7 @@ void rcu_domain<Tag>::half_sync(bool blocking, list_head& finished) {
 
   version_.store(next, std::memory_order_release);
   // Notify synchronous waiters in synchronize().
-  turn_.completeTurn(curr);
+  turn_.completeTurn(to_narrow(curr));
 }
 
 } // namespace folly

folly/synchronization/Rcu.h

@@ -308,10 +308,10 @@ class rcu_token {
   rcu_token& operator=(rcu_token&& other) = default;
 
  private:
-  explicit rcu_token(uint64_t epoch) : epoch_(epoch) {}
+  explicit rcu_token(uint8_t epoch) : epoch_(epoch) {}
 
   friend class rcu_domain<Tag>;
-  uint64_t epoch_;
+  uint8_t epoch_;
 };
 
 // Defines an RCU domain.  RCU readers within a given domain block updaters