I need to implement the following calculation:
f =3D (a*b) + (c*d)
where a,b,c,d are given double values and f is a double variable for
the result
I found out that using two different implementations gives two
different results:
1) In the first implementation I compute the entire equation in memory
2) In the second implementation I store intermediate to variables, and
then load them to compute the final result.
The results differ by a tiny amount (1E-15). I suspect that the reason
for the difference is from round off errors of the compiler.
Nonetheless, I need to know the reason for the difference as I need to
have an absolute reference for optimization work.
The two ways are as follows:
1)
Calculating two double values and then summing them to a single double
value
double f1 =3D (a*b) + (c*d)
2)
Calculating two double values and storing them to two double variables
Summing the two double variables to a single double value
double e1 =3D (a*b);
double e2 =3D (c*d);
double f2 =3D e1 + e2;
The difference is calculated as follows:
double diff =3D f2-f1;
I expect the difference to be exactly 0 but instead, I get a tiny
value.
Could someone explain the reason for differences in the result?
I=92m compiling with gcc
Thanks
Avi
