The default will break Kahan

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.