https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63644
Bug ID: 63644
Summary: Kahan Summation with fast-math, pattern not always
recognized
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vincenzo.innocente at cern dot ch
in the following example (compiled with -Ofast -std=c++11) the kahan summation
pattern is recognized in "sum", not in "counter"
see
http://goo.gl/aJn61B
#include<cstdio>
template<typename T>
struct KahanSum {
KahanSum(T is=0) : sum(is){}
KahanSum<T> operator+=(T a) { add(a); return *this;}
void add(T a) {
float x = a - eps;
float s = sum + x;
eps = (s-sum) - x;
sum = s;
}
T result() const { return sum;}
T sum;
T eps=0;
};
float a[1204];
float sum() {
KahanSum<float> res;
for (int i=0; i<1024; ++i) res+= a[i];
return res.result();
}
float counter(int maxl) {
float tenth=0.1f;
KahanSum<float> sum = tenth;
int n=0;
while(n<maxl) {
sum += tenth;
++n;
// if (n<21 || n%36000==0) printf("%d %f
%a\n",n,sum.result(),sum.result());
}
// use eps to avoid optimization out
float count = float(60*60*100*10);
printf("\n\n%f %f %a\n\n",count,float(count*tenth),float(count*tenth));
return sum.result();
}
