use relaxed_atomic in CacheLocality

Summary: The stripe table, which is a table of atomics, is its own data and does not guard any associated data. It must technically be atomic to avoid technical data race, but all accesses are relaxed.

Differential Revision: D31524230

fbshipit-source-id: 782b515f3845b4567ca258fd8d096f22248d60ff

folly/concurrency/CacheLocality.cpp

@@ -374,17 +374,14 @@ bool AccessSpreaderBase::initialize(
       assert(index < n);
       // as index goes from 0..n, post-transform value goes from
       // 0..numStripes
-      row[cpu].store(
-          static_cast<CompactStripe>((index * numStripes) / n),
-          std::memory_order_relaxed);
+      row[cpu] = static_cast<CompactStripe>((index * numStripes) / n);
       assert(row[cpu] < numStripes);
     }
     size_t filled = n;
     while (filled < kMaxCpus) {
       size_t len = std::min(filled, kMaxCpus - filled);
       for (size_t i = 0; i < len; ++i) {
-        row[filled + i].store(
-            row[i].load(std::memory_order_relaxed), std::memory_order_relaxed);
+        row[filled + i] = row[i].load();
       }
       filled += len;
     }

folly/concurrency/CacheLocality.h

@@ -35,6 +35,7 @@
 #include <folly/detail/StaticSingletonManager.h>
 #include <folly/lang/Align.h>
 #include <folly/lang/Exception.h>
+#include <folly/synchronization/RelaxedAtomic.h>
 
 namespace folly {
 
@@ -191,7 +192,8 @@ class AccessSpreaderBase {
       kMaxCpus - 1 <= std::numeric_limits<CompactStripe>::max(),
       "stripeByCpu element type isn't wide enough");
 
-  using CompactStripeTable = std::atomic<CompactStripe>[kMaxCpus + 1][kMaxCpus];
+  using CompactStripeTable =
+      relaxed_atomic<CompactStripe>[kMaxCpus + 1][kMaxCpus];
 
   struct GlobalState {
     /// For each level of splitting up to kMaxCpus, maps the cpu (mod
@@ -281,8 +283,7 @@ struct AccessSpreader : private detail::AccessSpreaderBase {
 
     unsigned cpu;
     s.getcpu.load(std::memory_order_relaxed)(&cpu, nullptr, nullptr);
-    return s.table[std::min(size_t(kMaxCpus), numStripes)][cpu % kMaxCpus].load(
-        std::memory_order_relaxed);
+    return s.table[std::min(size_t(kMaxCpus), numStripes)][cpu % kMaxCpus];
   }
 
   /// Returns the stripe associated with the current CPU.  The returned
@@ -296,8 +297,7 @@ struct AccessSpreader : private detail::AccessSpreaderBase {
     if (kIsMobile) {
       return current(numStripes);
     }
-    return s.table[std::min(size_t(kMaxCpus), numStripes)][cpuCache().cpu(s)]
-        .load(std::memory_order_relaxed);
+    return s.table[std::min(size_t(kMaxCpus), numStripes)][cpuCache().cpu(s)];
   }
 
   /// Returns the maximum stripe value that can be returned under any