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

9 points by gus_massa 15 hours ago

I'm looking for corner cases where he result is -0.0. As far as I know, the only way to get -0.0 in a sum is

  (-0.0) + (-0.0)
Does someone know any other case in IEEE 754?

Bonus question: What happens in subtractions? I only know

  (-0.0) - (+0.0)
Is there any other case?
sparkie 12 hours ago

It depends on the FP rounding mode. If rounding mode is FE_TOWARDZERO/FE_UPWARD/FE_TONEAREST 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/5qvqsdh9P

kazinator 9 hours ago

What happens if we take the smallest (as in closest to zero) negative subnormal and add it to itself?