Ask HN: How is it possible to get -0.0 in a sum?

It depends on the FP rounding mode. If rounding mode is FE_TOWARDZERO then the case you gave is the only one I'm aware of. If rounding mode is FE_DOWNWARD (towards negative infinity) then other calculations that result in a zero will give a -0.0.

Here's an example of -1.0f + 1.0f resulting in -0.0: https://godbolt.org/z/z853nWMqa

NB: If using C23 you can use:

    #include<fenv.h>

    #pragma STDC FENV_ROUND FE_DOWNWARD
  
    // or

    fesetround(FE_DOWNWARD);

Which would be more portable. I just edited the CSR directly because godbolt was complaining about undefined reference to `fesetround` even with -std=c23.