Avoid cost of cancelAll

Summary:
This is an experimental diff that may improve the performance of `HHWheelTimer::cancelAll`. If there are no timers to cancel, then it can bail early.

Since perflab is unlikely to be conclusive, and since this diff looks like a strict improvement, I'll proceed with landing it.

Reviewed By: djwatson

Differential Revision: D2735297

fb-gh-sync-id: 9f5a811ee6d9fa9434576e9abd3ef3443a7579b7

folly/io/async/HHWheelTimer.cpp

@@ -218,6 +218,9 @@ void HHWheelTimer::timeoutExpired() noexcept {
 }
 
 size_t HHWheelTimer::cancelAll() {
+  size_t count = 0;
+
+  if (count_ != 0) {
     decltype(buckets_) buckets;
 
 // Work around std::swap() bug in libc++
@@ -233,8 +236,6 @@ size_t HHWheelTimer::cancelAll() {
     std::swap(buckets, buckets_);
 #endif
 
-  size_t count = 0;
-
     for (auto& tick : buckets) {
       for (auto& bucket : tick) {
         while (!bucket.empty()) {
@@ -245,6 +246,7 @@ size_t HHWheelTimer::cancelAll() {
         }
       }
     }
+  }
 
   return count;
 }