square_norm question

[kostrzewa]

I don't really understand what's going on in the linalg/square_norm function (at least the unoptimized one). It seems to me like there are four empty operations taking place.

At the third iteration of the loop we have:

tr = ds_3
ts = ds_1 + ds_2 + ds_3
tt = ds_3
ks = ds_1 + ds_2 + ds_3
kc = 0

Since all the variables are overwritten from one call to the next I presume they were declared static for performance purposes rather than data persistence, so I don't really know why the additions and subtractions are carried out and then discarded.

[deuzeman]

Are you referring to the Kahan summation starting at line 75 of linalg/square_norm.c? Because I don't see anything getting directly discarded there...

[kostrzewa]

I didn't know this had such a profound effect on numerical errors. Thanks.

[deuzeman]

Neither did I, until I had the exact same reaction some time ago. :)