Replace spinlock with SharedMutex in DigestBuilder

Summary:
Spinlocks in user space are generally a bad idea due to tendency to
burn CPU in outlier events. Replace it with SharedMutex instead. The common
case cost could go up a bit should be better overall.

Reviewed By: yfeldblum

Differential Revision: D31654923

fbshipit-source-id: 77d249293458cd740904f777e00be1bd6834b62f

folly/stats/DigestBuilder-inl.h

@@ -37,7 +37,7 @@ DigestT DigestBuilder<DigestT>::build() {
   digestPtrs.reserve(cpuLocalBuffers_.size());
 
   for (auto& cpuLocalBuffer : cpuLocalBuffers_) {
-    std::unique_lock<SpinLock> g(cpuLocalBuffer.mutex);
+    std::unique_lock<SharedMutex> g(cpuLocalBuffer.mutex);
     valuesVec.push_back(std::move(cpuLocalBuffer.buffer));
     if (cpuLocalBuffer.digest) {
       digestPtrs.push_back(std::move(cpuLocalBuffer.digest));
@@ -70,7 +70,7 @@ template <typename DigestT>
 void DigestBuilder<DigestT>::append(double value) {
   auto cpuLocalBuf = &cpuLocalBuffers_[AccessSpreader<>::cachedCurrent(
       cpuLocalBuffers_.size())];
-  std::unique_lock<SpinLock> g(cpuLocalBuf->mutex);
+  std::unique_lock<SharedMutex> g(cpuLocalBuf->mutex);
   cpuLocalBuf->buffer.push_back(value);
   if (cpuLocalBuf->buffer.size() == bufferSize_) {
     if (!cpuLocalBuf->digest) {

folly/stats/DigestBuilder.h

@@ -19,7 +19,7 @@
 #include <memory>
 
 #include <folly/Memory.h>
-#include <folly/SpinLock.h>
+#include <folly/SharedMutex.h>
 
 namespace folly {
 
@@ -55,9 +55,22 @@ class DigestBuilder {
  private:
   struct alignas(hardware_destructive_interference_size) CpuLocalBuffer {
    public:
-    mutable SpinLock mutex;
+    mutable SharedMutex mutex;
     std::vector<double> buffer;
     std::unique_ptr<DigestT> digest;
+
+    CpuLocalBuffer() noexcept = default;
+
+    CpuLocalBuffer(CpuLocalBuffer&& other) noexcept
+        : buffer{std::move(other.buffer)}, digest{std::move(other.digest)} {}
+
+    CpuLocalBuffer& operator=(CpuLocalBuffer&& other) noexcept {
+      if (this != &other) {
+        buffer = std::move(other.buffer);
+        digest = std::move(other.digest);
+      }
+      return *this;
+    }
   };
 
   //  cpulocalbuffer_alloc custom allocator is necessary until C++17