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A comment on A Usable C++ Dialect That Is Safe Against Memory Corruption. Parent comment

Re:A Usable C++ Dialect That Is Safe Against Memory Corruption by staticassertion on Thursday March 15, 2018 @04:09PM (#16596145)
It's more complicated than this. C++ compilers can't just "reject" UB code - they don't have enough information at all times to prove that code invokes UB, the languages is not powerful enough to represent this in all cases.

When people say "just fix this in compilers, don't compile when they run into UB" it's a fundamental misunderstanding of the problem. Yes, in some cases the compiler can prove UB, and uses it to write optimiations, but it doesn't prove that all code is not invoking UB.

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Re:A Usable C++ Dialect That Is Safe Against Memory Corruption by gpderetta on Thursday March 15, 2018 @06:03PM (#16596879)
> a fundamental misunderstanding of the problem

No kidding! There is one right here:

> Yes, in some cases the compiler can prove UB, and uses it to write optimiation

Compilers do not prove UB , on the contrary they postulate no-UB and then use this axiom to prove other properties of the code.

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Re:A Usable C++ Dialect That Is Safe Against Memory Corruption by staticassertion on Thursday March 15, 2018 @07:02PM (#16597244)
I suppose that's true, yes. My point was more that your compiler is not hitting a line of code and saying "I know this is or is not UB" because it can not do that in the general case - it is definitely incorrect to say it is 'proving' this. Would you consider that an accurate representation?
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Re:A Usable C++ Dialect That Is Safe Against Memory Corruption by masklinn on Friday March 16, 2018 @08:14AM (#16600079)
> My point was more that your compiler is not hitting a line of code and saying "I know this is or is not UB" because it can not do that in the general case

I think it's still a (cause of) misunderstanding. For the compiler, UBs are situations which axiomatically can not occur. "This is UB" is not a concept, because the compiler assumes at all point that UBs can not occur.

The compiler doesn't go "oh you're dereferencing a pointer which may be null, fuck you", it goes "you're dereferencing a pointer so it can't be null, and thus I can remove anything assuming possible nullability".

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Re:A Usable C++ Dialect That Is Safe Against Memory Corruption by staticassertion on Friday March 16, 2018 @12:25PM (#16601699)
> The compiler doesn't go "oh you're dereferencing a pointer which may be null, fuck you", it goes "you're dereferencing a pointer so it can't be null, and thus I can remove anything assuming possible nullability".

:) This is what I'm trying to say.

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