stop compiler optimizations with -ffast-math from break sum accumulator

include/boost/histogram/accumulators/sum.hpp

@@ -48,12 +48,21 @@ public:
 
   /// Increment sum by value
   sum& operator+=(const RealType& value) {
-    auto temp = large_ + value; // prevent optimization
-    if (std::abs(large_) >= std::abs(value))
-      small_ += (large_ - temp) + value;
-    else
-      small_ += (value - temp) + large_;
-    large_ = temp;
+    // prevent compiler optimization from destroying the algorithm
+    // when -ffast-math is enabled
+    volatile RealType l;
+    RealType s;
+    if (std::abs(large_) >= std::abs(value)) {
+      l = large_;
+      s = value;
+    } else {
+      l = value;
+      s = large_;
+    }
+    large_ += value;
+    l -= large_;
+    l += s;
+    small_ += l;
     return *this;
   }
 

include/boost/histogram/accumulators/weighted_sum.hpp

@@ -16,9 +16,12 @@ namespace histogram {
 namespace accumulators {
 
 /// Holds sum of weights and its variance estimate
-template <typename RealType>
+template <class RealType>
 class weighted_sum {
 public:
+  using value_type = RealType;
+  using const_reference = const RealType&;
+
   weighted_sum() = default;
   explicit weighted_sum(const RealType& value) noexcept
       : sum_of_weights_(value), sum_of_weights_squared_(value) {}
@@ -67,16 +70,13 @@ public:
   }
 
   /// Return value of the sum.
-  const RealType& value() const noexcept { return sum_of_weights_; }
+  const_reference value() const noexcept { return sum_of_weights_; }
 
   /// Return estimated variance of the sum.
-  const RealType& variance() const noexcept { return sum_of_weights_squared_; }
+  const_reference variance() const noexcept { return sum_of_weights_squared_; }
 
   // lossy conversion must be explicit
-  template <class T>
-  explicit operator T() const {
-    return static_cast<T>(sum_of_weights_);
-  }
+  explicit operator const_reference() const { return sum_of_weights_; }
 
   template <class Archive>
   void serialize(Archive& ar, unsigned /* version */) {
@@ -85,8 +85,8 @@ public:
   }
 
 private:
-  RealType sum_of_weights_ = RealType();
-  RealType sum_of_weights_squared_ = RealType();
+  value_type sum_of_weights_ = value_type();
+  value_type sum_of_weights_squared_ = value_type();
 };
 
 } // namespace accumulators

test/Jamfile

@@ -38,7 +38,10 @@ alias odr :
     ;
 
 alias cxx14 :
-    [ run accumulators_test.cpp ]
+    [ run accumulators_test.cpp : : :
+      # make sure sum accumulator works even with -ffast-math and optimizations 
+      <toolset>gcc:<cxxflags>"-O3 -ffast-math"
+      <toolset>clang:<cxxflags>"-O3 -ffast-math" ]
     [ run algorithm_project_test.cpp ]
     [ run algorithm_reduce_test.cpp ]
     [ run algorithm_sum_test.cpp ]