Optimization question

[Izaak_Beekman]

Hi,

Under what conditions, if any, will (current versions of) ifort assume associativity of arithmatic when working with floating point variables, and optimize away corrections such as those found in the Kahan summation algorithm: ( http://en.wikipedia.org/wiki/Compensated_summation )

i.e. Does the compiler assume associativity of arithmatic by default? When -O3 is passed? Is there a way to turn it off, if it's on?

Thanks,

Zaak

[TimP]

The default will break Kahan summation except at -O0.  I would suggest setting "-assume protect_parens" so you will not require O0.  -fp-model source|precise also ought to work.

If the compiler ignored parentheses consistently (but in violation of the standard) the Kahan summation would collapse into an ordinary sum with no Kahan corrections, but with correct results at lower accuracy.

[Izaak_Beekman]

Thanks Tim,

Good to know.