Ask HN: How is it possible to get -0.0 in a sum?
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)
Bonus question: What happens in subtractions? I only know
(-0.0) - (+0.0)
i would guess that because of how *** * floats are in binary computers, you have something like -0.0000000000000000000000000000000000001 and when you round it you end up with -0.0. Same goes for positive value, you're just not used to write the + sign before every number, so seeing the minus feels strange.
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
What happens if we take the smallest (as in closest to zero) negative subnormal and add it to itself?
Copying the example by sparkie, something like this? https://godbolt.org/z/xhdnb9ax3 I get +0.0 if I comment the round to negative option.