Improve cpu id caching in DigestBuilder

Summary:
`DigestBuilder` caches the cpu id and invalidates it when it detects contention, but the critical section is small enough that it is possible for two thread to end up on a conflicting slot but not detect it for some time, causing cache-line pingponging in the meantime.

Switch to `cachedCurrent()` instead, which uses the number of accesses to invalidate the cache, and is successfully adopted by other high-concurrency primitives.

Reviewed By: yfeldblum

Differential Revision: D27726518

fbshipit-source-id: 05d70ad9f101df4a8797e44cc3cbe94cbd1cb7b8

folly/stats/detail/DigestBuilder-inl.h

@@ -23,8 +23,6 @@
 namespace folly {
 namespace detail {
 
-static thread_local uint32_t tls_lastCpuBufferSlot = 0;
-
 template <typename DigestT>
 DigestBuilder<DigestT>::DigestBuilder(size_t bufferSize, size_t digestSize)
     : bufferSize_(bufferSize), digestSize_(digestSize) {
@@ -71,14 +69,9 @@ DigestT DigestBuilder<DigestT>::build() {
 
 template <typename DigestT>
 void DigestBuilder<DigestT>::append(double value) {
-  auto& which = tls_lastCpuBufferSlot;
-  auto cpuLocalBuf = &cpuLocalBuffers_[which];
-  std::unique_lock<SpinLock> g(cpuLocalBuf->mutex, std::try_to_lock_t());
-  if (!g.owns_lock()) {
-    which = AccessSpreader<>::current(cpuLocalBuffers_.size());
-    cpuLocalBuf = &cpuLocalBuffers_[which];
-    g = std::unique_lock<SpinLock>(cpuLocalBuf->mutex);
-  }
+  auto cpuLocalBuf = &cpuLocalBuffers_[AccessSpreader<>::cachedCurrent(
+      cpuLocalBuffers_.size())];
+  std::unique_lock<SpinLock> g(cpuLocalBuf->mutex);
   cpuLocalBuf->buffer.push_back(value);
   if (cpuLocalBuf->buffer.size() == bufferSize_) {
     if (!cpuLocalBuf->digest) {