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by asgeir 1295 days ago
The compiler may make optimization decisions based on the presence of UB so the code only needs to be present.

The compiler can't know that an arbitrary part of a function is unreachable if, for instance, that code path is controlled by a parameter or global state since that equates to solving the halting problem.

https://blog.llvm.org/2011/05/what-every-c-programmer-should...
1 comments
haberman 1295 days ago
I don't think this is correct. If UB is never invoked, the program is conforming and must be translated/executed correctly.

DR #109 specifically states that a compiler may not fail to translate a strictly conforming program just because some possible executions could trigger UB: https://www.open-std.org/jtc1/sc22/wg14/docs/rr/dr_109.html

Edit: downvoters, please explain your disagreement. I cited a primary document in support of my position.

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asgeir 1295 days ago
The undefined behavior can still affect code generation even though the program gets translated.

It's just that the generated code may not do exactly what you expect it to do because the presence of undefined behavior allowed the compiler to make assumptions which may surprise you.

edit: It seems that I am incorrect

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haberman 1295 days ago
DR 109 allows that a program may be strictly conforming even if some possible executions of the program invoke UB:

> A conforming implementation must not fail to translate a strictly conforming program simply because some possible execution of that program would result in undefined behavior.

This text specifically allows for the case that a program is strictly conforming even if there is a possible execution that invokes UB. If a program is strictly conforming, it must produce the correct behavior.

I would challenge you to show an example where GCC or Clang will break the correctness of a program on account of UB that is not reached during program execution.

Here is an example where GCC and Clang specifically respects the correctness of a program as long as it does not reach the UB: https://godbolt.org/z/befWah77W

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asgeir 1295 days ago
It seems that you may be correct.

At the very least I am unable to construct a counter-example so I concede this point

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