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Comment by cogman10 | Hacker News Reader

Historically, -O3 has been a bit less stable (producing incorrect code) and more experimental (doesn't always make things faster).

Flags from -O3 often flow down into -O2 as they are proven generally beneficial.

That said, I don't think -O3 has the problems it once did.

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-O3 gained a reputation of being more likely to "break" code, but in reality it was almost always "breaking" code that was invalid to start with (invoked undefined behavior). The problem is C and C++ have so many UB edge cases that a large volume of existing code may invoke UB in certain situations. So -O2 thus had a reputation of being more reliable. If you're sure your code doesn't invoke undefined behavior, though, then -O3 should be fine on a modern compiler.

Exactly. A lot of people didn’t understand the contract between the programmer and the compiler that is required to use -O3.

That's a little vague, I'd put that more pointedly: they don't understand how the C and C++ languages are defined, have a poor grasp of undefined behaviour in particular, and mistakenly believe their defective code to be correct.

Of course, even with a solid grasp of the language(s), it's still by no means easy to write correct C or C++ code, but if your plan it to go with this seems to work, you're setting yourself up for trouble.

Oh, there are also plenty of bugs. And Clang still does not implement the aliasing model of C. For C, I would definitely recommend -O2 -fno-strict-aliasing

Indeed, e.g. Rust by default (release builds) use -O3.

Don't forget about -Oz!

Thanks

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