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Comment by erlehmann_ | Hacker News Reader

To me, undefined behaviour analysis and typing analysis seem very similar: Both allow a compiler to infer things that can help improve performance … So why do you see the claims that both can help performance as “opposite”?

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The type system proves things about my program that must be true and disallows invalid programs. The undefined behavior system decides things about my program that may or may not be true and just breaks my program when one of us is wrong (and I don't even care who: whether it is the compiler that came up with something insane or me trying to do the impossible <- I honestly don't care).

I think when undefined behaviour is used for optimizations the programmer does not want it is more of a misunderstanding than one party (programmer or compiler) necessarily being “wrong”, as a compiler assumes that a program should make sense (i.e. is not undefined) and infers things from that. To clarify, would you say that assumption is problematic?

One claims to be undefined. One claims to use typed things to produce other typed things. I could see them doing similar or dissimilar things. Undefined vs specific types just seemed opposite to me.

The thing about undefined behavior is that it means the compiler can do whatever it wants. Most of the time, since the compiler wants to create efficient code, it can just assume that invalid values won't occur the same way a type system can allow a compiler to know that invalid values can't occur.

EDIT: More information here http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

Appreciate the link.

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