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Comment by magicalhippo | Hacker News Reader

Subnormals or denormalized floats can lead to some unexpected results. IIRC dividing a subnormal by a subnormal returns infinity.

I experienced this where a colleague had tried to reduce the number of divisions by rewriting (a/x)*(b/y)*(c/z) to (a*b*c)/(x*y*z).

The problem was that in certain circumstances the terms were all small, but of comparable magnitude. Thus the latter lead to subnormal divided by subnormal. The fix was to undo the optimization and do the divisions first, which lead to the multiplication of three numbers of order 1.

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> IIRC dividing a subnormal by a subnormal returns infinity.

This should not be true in a conformant implementation of IEEE 754-2008. You can get infinity by dividing by a subnormal (due to overflow), but that should only be possible with sufficiently large numerators.

> The problem was that in certain circumstances the terms were all small, but of comparable magnitude. Thus the latter lead to subnormal divided by subnormal. The fix was to undo the optimization and do the divisions first, which lead to the multiplication of three numbers of order 1.

My guess is that this problem was actually the numerator being subnormal and the denominator underflowing all the way to zero and therefore the result going to infinity.

It's been over a decade, so I struggle to recall the details.

> My guess is that this problem was actually the numerator being subnormal and the denominator underflowing all the way to zero and therefore the result going to infinity.

Yes thinking about you're probably correct.

Yeah, I think a good general guideline is that if you see a bunch of divisions like that, the author did it intentionally (to avoid overflow or subnormals).

Anyway, I wonder if the optimization even helps at all. Either way the critical path is a division and two multiplications. That seems like the sort of thing that, between SIMD an OoO execution, a modern CPU ought to be able to work out, haha.

No promises but I’d want to benchmark it before caring.

Maybe depends on the platform? I tried 5e-323 / 6e-323 in javascript console and it returns 0.8333333333333334 (and e-323 should be subnormal for double precision)

Depends on control registers like e.g. MXCSR. It's an utter mess. Consider e.g. https://news.ycombinator.com/item?id=32738206

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