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by torstenvl 1034 days ago
Yes. But there's middle ground between fully-defined behavior (lots of slow checks) and what current compiler-writers think UB is (do whatever I want).

Specifically, implement UB the way it is described in the standard: pretend it isn't UB, do it anyway, consequences be damned. That's what "ignore the situation with unpredictable results" actually means.
1 comments
muldvarp 1034 days ago
> compiler-writers think UB is

The current standard is _very_ explicit that undefined behavior is indeed undefined, i.e. "do whatever you want".

> pretend it isn't UB, do it anyway, consequences be damned.

This explicitly isn't a requirement, but even if it were, "ignoring the situation completely with unpredictable results" can be interpreted in numerous ways. One of these ways is "ignoring any cases in which UB is encountered" which is exactly what compilers are doing. Then again, saying "the compiler didn't ignore the situation and as a consequence I got results I didn't predict" isn't a strong argument when the standard specifically told you that you will get unpredictable results.

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torstenvl 1034 days ago
"The Party told you to reject the evidence of your eyes and ears. It was their final, most essential command."

There's a certain poetic consistency in ignoring arbitrary portions of the standard to justify ignoring arbitrary portions of the input code.

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muldvarp 1033 days ago
Which part of the standard is ignored? Again, the standard is _very_ explicit about what undefined behavior means. If you don't like that you can either try to change the standard or use the numerous command line options provided by most compilers to tell your compiler that you would like certain undefined behaviors have a defined meaning.

Saying that compilers shouldn't ignore code with undefined behavior is like saying compilers shouldn't ignore the body of an if-statement just because the condition evaluated to false.

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torstenvl 1033 days ago
You're right on one point: the standard is very explicit.

And because it is explicit—a fact you yourself just admitted—the fact that silent erasure of non-dead code is not a listed option in response to UB means that it is not allowed.

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muldvarp 1033 days ago
The standard is explicit that the behavior of code with undefined behavior is well... undefined and that implementations can do whatever they want.
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torstenvl 1033 days ago
Reasonable people can disagree as to whether that interpretation is valid.

No reasonable person can say that it is explicit. It simply, factually, is not. At no point in any version of the C Standard does the text "implementations can do whatever they want" appear.

I have no time for blatant and insulting dishonesty. We're done here.

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