alt Hacker NewsFrom the ANSI C standard:
3.16 undefined behavior: Behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately valued objects, for which this International Standard imposes no requirements. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message).
By 'bounded', this obviously ignores the security consequences of e.g. buffer overflows, but just because UB can be exploited doesn't mean it's appropriate for e.g. the compiler to exploit it too, that clearly violates the intent of this paragraph.
Replies
Author here.
I touched on this in the "it's not about optimizations" section. It's not the compiler is out to get you. It's that you told it to do something it cannot express.
It's like if you slipped in a word in French, and not being programmed for French, it misheard the word as a false friend in English. The compiler had no way to represent the French word in it's parse tree.
So no, it's not overly legalistic. Like if the compiler knows that this hardware can do unaligned memory access, but not atomic unaligned access, should it check for alignment in std::atomic<int> ptr but not in int ptr? Probably not, right?
The behaviour is bounded by the capability of your machine. It is unlikely that your desktop computer launches a nuclear missile, unless you worked for it to be able to do that.
> Is it just me or did compiler writers apply overly legalistic interpretation to the "no requirements" part in this paragraph?
I've (fruitlessly) had this discussion on HN before - super-aggressive optimisations for diminishing rewards are the norm in modern compilers.
In old C compilers, dereferencing NULL was reliable - the code that dereferenced NULL will always be emitted. Now, dereferencing NULL is not reliable, because the compiler may remove that and the program may fail in ways not anticipated (i.e, no access is attempted to memory location 0).
The compiler authors are on the standard, and they tend to push for more cases of UB being added rather than removing what UB there is right now (for exampel, by replacing with Implementation Defined Behaviour).
> but that the consequence of this should be somewhat bounded or as expected for the target machine.
Aren't "unpredictable results" and "no requirements" contrary to the idea that the behavior would be "somewhat bounded"?