Statistic: Initialize in the constructor

[MatzeB]

Summary

Statistics should be fast enough that we can enable them in release builds without compiletime impact. This does not work today because the lazy initialization in use today puts a memory fence on every increment operation!

The obvious solution for this is initializing the statistic variable in the constructor. Which is easy to do and works better in every way than todays code IMO except that introduce additional static constructors into llvm which in this specific instance look fine to me; the alternative of forbidding global variables for statistic variables appears arguably worse.

[MatzeB]

People don't seem to accept static constructors for this.

include/llvm/ADT/Statistic.h

 class raw_fd_ostream;
 
 class Statistic {
 public:
   const char *DebugType;
   const char *Name;
   const char *Desc;
   std::atomic<unsigned> Value;
-  bool Initialized;
+  bool Initialized = false;
 
   unsigned getValue() const { return Value.load(std::memory_order_relaxed); }
   const char *getDebugType() const { return DebugType; }
   const char *getName() const { return Name; }
   const char *getDesc() const { return Desc; }
 
   /// construct - This should only be called for non-global statistics.
   void construct(const char *debugtype, const char *name, const char *desc) {
     DebugType = debugtype;
     Name = name;
     Desc = desc;
     Value = 0;
     Initialized = false;
   }
 
   // Allow use of this class as the value itself.
   operator unsigned() const { return getValue(); }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_STATS)
-   const Statistic &operator=(unsigned Val) {
+  Statistic(const char *DebugType, const char *Name, const char *Desc)
+      : DebugType(DebugType), Name(Name), Desc(Desc) {
+    init();
+  }
+
     Value.store(Val, std::memory_order_relaxed);
-    return init();
+    return *this;
   }
 
   const Statistic &operator++() {
     Value.fetch_add(1, std::memory_order_relaxed);
-    return init();
+    return *this;
   }
 
   unsigned operator++(int) {
-    init();
     return Value.fetch_add(1, std::memory_order_relaxed);
   }
 
   const Statistic &operator--() {
     Value.fetch_sub(1, std::memory_order_relaxed);
-    return init();
+    return *this;
   }
 
   unsigned operator--(int) {
-    init();
     return Value.fetch_sub(1, std::memory_order_relaxed);
   }
 
   const Statistic &operator+=(unsigned V) {
     if (V == 0)
       return *this;
     Value.fetch_add(V, std::memory_order_relaxed);
-    return init();
+    return *this;
   }
 
   const Statistic &operator-=(unsigned V) {
     if (V == 0)
       return *this;
     Value.fetch_sub(V, std::memory_order_relaxed);
-    return init();
+    return *this;
   }
 
 #else  // Statistics are disabled in release builds.
+  Statistic(const char *, const char *, const char *)
+      : DebugType(nullptr), Name(nullptr), Desc(nullptr) {}
 
   const Statistic &operator=(unsigned Val) {
     return *this;
   }
 
   const Statistic &operator++() {
     return *this;
   }
protected:
   }
 
   void RegisterStatistic();
 };
 
 // STATISTIC - A macro to make definition of statistics really simple.  This
 // automatically passes the DEBUG_TYPE of the file into the statistic.
 #define STATISTIC(VARNAME, DESC)                                               \
-  static llvm::Statistic VARNAME = {DEBUG_TYPE, #VARNAME, DESC, {0}, false}
+  static llvm::Statistic VARNAME = {DEBUG_TYPE, #VARNAME, DESC}
 
 /// \brief Enable the collection and printing of statistics.
 void EnableStatistics(bool PrintOnExit = true);
 
 /// \brief Check if statistics are enabled.
 bool AreStatisticsEnabled();
 
 /// \brief Return a file stream to print our output on.