[dmoy]

Because sometimes it's very wrong

Way back when during my EE course days, we had like a whole semester devoted to weird edge cases like this, and spent month on ieee754 (precision loss, Nan, divide by zero, etc)

When you took an ieee754 divide by zero value as gospel and put it in the context of a voltage divisor that is always negative or zero, getting a positive infinity value out of divide by zero was very wrong, in the sense of "flip the switch and oh shit there's the magic smoke". The solution was a custom divide function that would know the context, and yield negative infinity (or some placeholder value). It was a contrived example for EE lab, but the lesson was - sometimes the standard is wrong and you will cause problems if it's blindly followed.

Sometimes it's fine, but it depends on the domain

[Dylan16807]

But IEEE 754 works as you described in your last comment. It doesn't take the numerator's sign. So what's wrong?

Can you give more context on your voltage math? Was the numerator sometimes negative? If the problem is that your divisor calculation sometimes resulted in positive zero, that doesn't sound like the standard being wrong without more info.

[dmoy]

> But IEEE 754 works as you described in your last comment. It doesn't take the numerator's sign. So what's wrong?

The numerator was always positive. The denominator was always negative (negative voltage is a pretty common thing), except when it became zero. That led to surprising behavior.

Right the whole point of the exercise was that sometimes the standard is wrong for your specific problem at hand. We spent lecture after lecture going over exactly how ieee754 precision loss worked, and other edge cases, so we could know how to exactly follow the standard.

Then we had an example where the sudden sign flip from a/-0.00000000001 = <huge_negative_number> to a/0 = <positive_infinity> would cause big problems with a calculation. If you didn't explicitly handle the divide by zero case and do the "correct for domain, but not following ieee754 standard" way, then you'd fry a component.

It's been a long time so I don't remember the exact setup, just the higher level lesson of "don't blindly follow standards and assume you don't need to check edge cases (exception or otherwise) because the standard does things a certain way".

[Dylan16807]

It's a good lesson in defensiveness! But if your value was always either less than zero or negative zero it would have done the right thing, both domain correct and standard correct. It's hard to say exactly why you got positive zero, but my bet is that it's more subtle than the standard doing something you can actually call "wrong".

[nivertech]

With IEEE 754 you can always explicitly check for edge cases.

But with exceptions you can’t use SIMD / vectorization.

[dmoy]

Yea that's totally fair, you'd need to build it in as a first class behavior of your code, doesn't necessarily mean that exceptions is the right way to do it.