3 ms·

No. The compiler isn't the only component of the system that will determine what happens when you trigger UB, either. There is UB all the way down to hardware s

No. The compiler isn't the only component of the system that will determine what happens when you trigger UB, either. There is UB all the way down to hardware specifications. I used to be one of the folks who defined the behavior of both languages and hardware at various companies. UB does not mean "documented elsewhere". Please stop spreading misinformation.

gerdesj 2y ago
"I used to be one of the folks who defined the behavior of both languages and hardware at various companies" But not at all companies, orgs or even in Heaven and certainly (?) not at ISO/OSI/LOL. It appears that someone wants to redefine the word "undefined" - are they sure that is wise?
motorest 2y ago
> No. The compiler isn't the only component of the system that will determine what happens when you trigger UB, either. There is UB all the way down to hardware specifications. I don't think you know what undefined behavior is. That's a concept relevant to language specifications alone. It does not trickle up or down what language specifications cover. It just means that the authors of the specification intentionally left the behavior expected from a specific scenario as undefined. For those who write software targeting language specifications this means they are introducing a bug because they are presuming their software will show a behavior which is not required by the standard. For those targeting specific combinations of compiler and hardware, they need to do their homework to determine if the behavior is guaranteed.
- immibis 2y ago
  Hardware also has UB, but what happens is still dictated by the circuitry. The relevant circuitry is both complicated enough and not useful enough for the CPU designer to specify it. Often they use the word "unpredictable" instead. The behavior is perfectly predictable by an omniscient silicon demon, but you may not be able to predict it. The effect that speculative execution has on cache state turned out to be unpredictable, so we have all the different Spectre vulnerabilities. Hardware unpredictability doesn't overlap much with language UB, anyway. It's unlikely that something not defined by the language is also not defined by th hardware. It's much more likely that the compiler's source code fully defines the behaviour.

gerdesj 2y ago

"I used to be one of the folks who defined the behavior of both languages and hardware at various companies" But not at all companies, orgs or even in Heaven and certainly (?) not at ISO/OSI/LOL. It appears that someone wants to redefine the word "undefined" - are they sure that is wise?

motorest 2y ago

> No. The compiler isn't the only component of the system that will determine what happens when you trigger UB, either. There is UB all the way down to hardware specifications. I don't think you know what undefined behavior is. That's a concept relevant to language specifications alone. It does not trickle up or down what language specifications cover. It just means that the authors of the specification intentionally left the behavior expected from a specific scenario as undefined. For those who write software targeting language specifications this means they are introducing a bug because they are presuming their software will show a behavior which is not required by the standard. For those targeting specific combinations of compiler and hardware, they need to do their homework to determine if the behavior is guaranteed.

immibis 2y ago
Hardware also has UB, but what happens is still dictated by the circuitry. The relevant circuitry is both complicated enough and not useful enough for the CPU designer to specify it. Often they use the word "unpredictable" instead. The behavior is perfectly predictable by an omniscient silicon demon, but you may not be able to predict it. The effect that speculative execution has on cache state turned out to be unpredictable, so we have all the different Spectre vulnerabilities. Hardware unpredictability doesn't overlap much with language UB, anyway. It's unlikely that something not defined by the language is also not defined by th hardware. It's much more likely that the compiler's source code fully defines the behaviour.