From 7ae684a8b48aee6bb67f1acd3221b458fb7cea05 Mon Sep 17 00:00:00 2001
From: Chandler Carruth <chandlerc@gmail.com>
Date: Thu, 2 Jun 2016 08:44:05 +0000
Subject: [PATCH] Switch statistics to use relaxed updates to a std::atomic.

This removes usage of the hacky, incorrect, and TSan-unfriendly
home-grown atomics. It should actually be more efficient in some cases.

Based on our existing usage of <atomic>, all of this is portably
available AFAICT. One small challenge is initializing the stastic, but
I've tried a comparable sample out on MSVC (the most likely to complain
here) and it seems to work. Will have to watch the build bots of course.

llvm-svn: 271504
---
 llvm/include/llvm/ADT/Statistic.h | 43 ++++++++++++++-----------------
 1 file changed, 19 insertions(+), 24 deletions(-)

diff --git a/llvm/include/llvm/ADT/Statistic.h b/llvm/include/llvm/ADT/Statistic.h
index 320e5c12259b..661fea72cf0e 100644
--- a/llvm/include/llvm/ADT/Statistic.h
+++ b/llvm/include/llvm/ADT/Statistic.h
@@ -28,6 +28,7 @@
 
 #include "llvm/Support/Atomic.h"
 #include "llvm/Support/Compiler.h"
+#include <atomic>
 #include <memory>
 
 namespace llvm {
@@ -38,10 +39,10 @@ class Statistic {
 public:
   const char *Name;
   const char *Desc;
-  volatile llvm::sys::cas_flag Value;
+  std::atomic<unsigned> Value;
   bool Initialized;
 
-  llvm::sys::cas_flag getValue() const { return Value; }
+  unsigned getValue() const { return Value.load(std::memory_order_relaxed); }
   const char *getName() const { return Name; }
   const char *getDesc() const { return Desc; }
 
@@ -52,51 +53,45 @@ public:
   }
 
   // Allow use of this class as the value itself.
-  operator unsigned() const { return Value; }
+  operator unsigned() const { return getValue(); }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_STATS)
    const Statistic &operator=(unsigned Val) {
-    Value = Val;
+    Value.store(Val, std::memory_order_relaxed);
     return init();
   }
 
   const Statistic &operator++() {
-    // FIXME: This function and all those that follow carefully use an
-    // atomic operation to update the value safely in the presence of
-    // concurrent accesses, but not to read the return value, so the
-    // return value is not thread safe.
-    sys::AtomicIncrement(&Value);
+    Value.fetch_add(1, std::memory_order_relaxed);
     return init();
   }
 
   unsigned operator++(int) {
     init();
-    unsigned OldValue = Value;
-    sys::AtomicIncrement(&Value);
-    return OldValue;
+    return Value.fetch_add(1, std::memory_order_relaxed);
   }
 
   const Statistic &operator--() {
-    sys::AtomicDecrement(&Value);
+    Value.fetch_sub(1, std::memory_order_relaxed);
     return init();
   }
 
   unsigned operator--(int) {
     init();
-    unsigned OldValue = Value;
-    sys::AtomicDecrement(&Value);
-    return OldValue;
+    return Value.fetch_sub(1, std::memory_order_relaxed);
   }
 
-  const Statistic &operator+=(const unsigned &V) {
-    if (!V) return *this;
-    sys::AtomicAdd(&Value, V);
+  const Statistic &operator+=(unsigned V) {
+    if (V == 0)
+      return *this;
+    Value.fetch_add(V, std::memory_order_relaxed);
     return init();
   }
 
-  const Statistic &operator-=(const unsigned &V) {
-    if (!V) return *this;
-    sys::AtomicAdd(&Value, -V);
+  const Statistic &operator-=(unsigned V) {
+    if (V == 0)
+      return *this;
+    Value.fetch_sub(V, std::memory_order_relaxed);
     return init();
   }
 
@@ -145,8 +140,8 @@ protected:
 
 // 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, DESC, 0, 0 }
+#define STATISTIC(VARNAME, DESC)                                               \
+  static llvm::Statistic VARNAME = {DEBUG_TYPE, DESC, {0}, 0}
 
 /// \brief Enable the collection and printing of statistics.
 void EnableStatistics();