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> If it optimizes and there are invalid assumptions then there is likely UB. It's the distinguishing from bugs that concerns me.
by NoTeslaThrow 1y ago
> If it optimizes and there are invalid assumptions then there is likely UB.
It's the distinguishing from bugs that concerns me.
- lmm 1y agoAnything a compiler does with code which is UB is not a bug in the compiler. That's pretty much the definition of UB.
- fc417fc802 1y agoI don't follow. Isn't UB a subset of bugs or alternatively a follow on consequence that causes observable behavior to further deviate?
- NoTeslaThrow 1y ago> Isn't UB a subset of bugs No, not at all. UB can still produce correct and expected results for the entire input domain.
- fc417fc802 1y agoIf I have a bug that only triggers between 9 and 10 am EST on Mondays that is still a bug, no? Now extend that to "rand(1.0) < 0.01". Now extend that to a check using __TIME__ that goes off at compile time instead of runtime (some binaries are buggy, some aren't). Now extend that to UB.
- deleted 1y ago[deleted]
- pharrington 1y ago"can" is extremely different than "will"!
- vlovich123 1y agoIt is a bug - you’ve violated the contract between the language and the compiler. Just like segfault or logic bug, it’s a class of bugs. Why is special though is that in most bugs you just hit an invalid state. In UB you can end up executing code that never existed or not executing code that does exist. Or any number of other things can happen because the compiler applies an optimization assuming a runtime state you promised it would never occur but did. It’s slightly different from being a strict subset because UB is actually exploited to perform optimizations - UB is not allowed so the compiler is able to emit more efficient code is taught to exploit that and the language allows for it (eg the niche optimization the blog describes)