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> The standard offers no interpretation of any undefined behavior "The standard" is not compiling my program, the real compiler is doing that, and the authors

by acqq 7y ago

> The standard offers no interpretation of any undefined behavior "The standard" is not compiling my program, the real compiler is doing that, and the authors of that specific implementation should do the right thing, in spite of the standard leaving something "undefined." > The compiler has no choice, it has to define its own interpretation. Correct. And it can define it as to not to be an adversary to the writer of the code.

ryandrake 7y ago
What is the “right” thing to do with UB, and who defines what is right, if not the standard? You?
- acqq 7y ago
  > What is the “right” thing to do with UB It is actually quite clear in most of specific cases, as Linus pointed even 18 years ago: https://gcc.gnu.org/legacy-ml/gcc/2002-01/msg00418.html https://gcc.gnu.org/legacy-ml/gcc/2002-01/msg00418.html "When it comes to optimization, there are things that are "obvious", and that will never generate any discussion at all, because they are faster ways of getting the exact same result, with no question about it. Nobody will ever fault a compiler for evaluating constant expressions at compile-time rather than doing the math at run-time and getting the same result. The two are indistinguishable from a result standpoint. But optimizations that can change the value of an expression are slightly different. They should make sense. You should be able to _explain_ them." "If you cannot explain the results, you end up having to argue about word-weaseling in the standard, and people _will_ disagree on what the damn thing means. Because standards aren't mathematically exact things."
MaxBarraclough 7y ago
> it can define it as to not to be an adversary to the writer of the code. We see peculiar things happen when there's undefined behaviour, because of the way compilers' optimisers work. It would be possible to tame the way undefined behaviour manifests, but this would trade off against the compiler's ability to optimise. If that weren't the case, everyone would be doing it.

by acqq 7y ago

ryandrake 7y ago

What is the “right” thing to do with UB, and who defines what is right, if not the standard? You?

acqq 7y ago
> What is the “right” thing to do with UB It is actually quite clear in most of specific cases, as Linus pointed even 18 years ago: https://gcc.gnu.org/legacy-ml/gcc/2002-01/msg00418.html https://gcc.gnu.org/legacy-ml/gcc/2002-01/msg00418.html "When it comes to optimization, there are things that are "obvious", and that will never generate any discussion at all, because they are faster ways of getting the exact same result, with no question about it. Nobody will ever fault a compiler for evaluating constant expressions at compile-time rather than doing the math at run-time and getting the same result. The two are indistinguishable from a result standpoint. But optimizations that can change the value of an expression are slightly different. They should make sense. You should be able to _explain_ them." "If you cannot explain the results, you end up having to argue about word-weaseling in the standard, and people _will_ disagree on what the damn thing means. Because standards aren't mathematically exact things."

MaxBarraclough 7y ago

> it can define it as to not to be an adversary to the writer of the code. We see peculiar things happen when there's undefined behaviour, because of the way compilers' optimisers work. It would be possible to tame the way undefined behaviour manifests, but this would trade off against the compiler's ability to optimise. If that weren't the case, everyone would be doing it.