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Yeah. It's safe in that you won't get UB, but it's bad in that you can get arbitrarily wrong answers.

To be fair, if you're using floating point at all you can get arbitrarily wrong answers. The nice thing about ieee754 conformance is that you can, with a lot of expertise, somewhat reason about the kinds of error you're getting. But for code that wasn't written by someone skilled in numerical techniques, and that's the vast majority of fp code, is fast-math worse than the status quo?

From personal experience, yes: I've seen multiple cases of scientists finding the ultimate cause of their bugs was some fast-math-related optimization.

The problem isn't necessarily the code they wrote themselves: it is often that they've compiled someone else's code or an open source library with fast-math, which broke some internal piece.

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