Refactor sleeper

Summary:
Propose refactoring changes, that were pending from landed diff D70250664

Two main changes are preferring C++ over pre-processor and structuring the code in a way spinCount<kMaxActiveSpin invariant is guaranteed to hold

Reviewed By: yfeldblum

Differential Revision: D70581321

fbshipit-source-id: 8db35d9e7cb314e6d21026011a4891a3bc7e5c05

Author: Nicoshev

folly/synchronization/detail/Sleeper.h

@@ -34,34 +34,36 @@ namespace detail {
  */
 class Sleeper {
   const std::chrono::nanoseconds delta;
-  uint32_t spinCount;
-
-#if FOLLY_AARCH64 && !FOLLY_MOBILE
-  uint32_t spinCountLog = 1;
-#endif
 
   static constexpr uint32_t kMaxActiveSpin = 4096;
+  static constexpr bool useBackOff = kIsArchAArch64 && !kIsMobile;
+
+  uint32_t spinCount = 0;
+
+  uint32_t spinCountTarget = 1;
 
  public:
   static constexpr std::chrono::nanoseconds kMinYieldingSleep =
       std::chrono::microseconds(500);
 
-  constexpr Sleeper() noexcept : delta(kMinYieldingSleep), spinCount(0) {}
+  constexpr Sleeper() noexcept : delta(kMinYieldingSleep) {}
 
-  explicit Sleeper(std::chrono::nanoseconds d) noexcept
-      : delta(d), spinCount(0) {}
+  explicit Sleeper(std::chrono::nanoseconds d) noexcept : delta(d) {}
 
   void wait() noexcept {
-    if (spinCount < kMaxActiveSpin) {
-#if FOLLY_AARCH64 && !FOLLY_MOBILE
-      do {
+    bool doSpin = useBackOff
+        ? spinCountTarget <= kMaxActiveSpin
+        : spinCount < kMaxActiveSpin;
+    if (doSpin) {
+      if constexpr (useBackOff) {
+        do {
+          asm_volatile_pause();
+        } while (++spinCount < spinCountTarget);
+        spinCountTarget <<= 1;
+      } else {
+        ++spinCount;
         asm_volatile_pause();
-      } while (++spinCount < spinCountLog);
-      spinCountLog <<= 1;
-#else
-      ++spinCount;
-      asm_volatile_pause();
-#endif
+      }
     } else {
       /* sleep override */
       std::this_thread::sleep_for(delta);