Comment by munch117
5 months ago
Both are wrong. It means "this standard does not constrain the behaviour of code that does this".
It's entirely legal for implementations to have predictable behaviour, documented or not, for code that is undefined by the standard. In their quest for maxxing benchmark performance they generally choose not to, but there's really nothing in any standard that stops you from making an implementation that prioritises safety.
Every implementation so far has predictable behavior in all cases. Sometimes the rules for predicting it are very obscure. But it's all fully defined within the compiler's binary code. And none of them link to nasal portals.
How do you propose to predict the behavior of a true race condition with only the binary, faithfully translated by the compiler?
Moreover, this is at best an incredibly pedantic point, not something that changes how programmers need to approach UB. You can't review the source code of a compiler that hasn't been written yet.
I didn't suggest that implementations should entirely eliminate every form of UB. There is plenty of middle ground. For example, you could easily limit the consequences of integer overflow by specifying or partially specifying overflow behaviour, with very little runtime cost.
I'm not suggesting you change how you write code, but with a better implementation the code that you do write - that lives in the real world where mistakes are made - might work better. How is that being pedantic?
An interesting case where compiler writers did something like that is casting via union members, but I'm running out of time, so we can talk about that another day.
It's fully defined by your CPU's silicon masks and your compiler's binary code that one of several things will happen.
2 replies →