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Comment by charleslmunger | Hacker News Reader

This is sort of true, except that behavior can be undefined in the standard but defined in your implementation. One of the best examples is -fwrapv, which makes signed integer overflow not UB. And Fil-C has no undefined behavior that I'm aware of. But most implementations target the standard and "define" what the standard doesn't to be "whatever I feel like, in service of implementation simplicity and performance". What matters is what your implementation guarantees, which should be a superset of what the standard does.

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> This is sort of true, except that behavior can be undefined in the standard but defined in your implementation.

But then it's not UB, and not the topic at hand. So I wouldn't call it "sort of true" for a statement about UB, because it doesn't apply to not-UB.

It's also different in a consteval context. But that doesn't make it less true outside consteval context.

Fil-C does still have UB. Signed-overflow, for instance, is still UB in Fil-C, and can still lead to things like the optimizer removing checks in the presence of UB.

https://news.ycombinator.com/item?id=46388460

Even if that's the case, it's not "undefined" in the sense that it doesn't permit "now your program is the attacker's program".

It does not permit taking over pointers and doing a plain old RCE, sure, but it still does permit a variety of optimizations that could break the logic of your program in surprising ways.

Here are two examples of where it's clear that the UB from the underlying C is still present and the optimization passes are still able to do things that wouldn't be possible without UB, like falling through functions and deleting safety checks inserted by the Fil-C runtime:

https://news.ycombinator.com/item?id=49677463 https://news.ycombinator.com/item?id=49681273

I'm not really convinced you can take LLVM and just flip a few poison bits off and remove a couple optimization passes and know that UB isn't going to explode in your face.

I stand corrected! I had assumed that Fil-C actually removed poison and unreachable states in the IR but thinking about it more that seems very challenging to do comprehensively.

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