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Seriously wtf someone comes up with "X is UB" and even worse, "Since it's UB this gives a license to do whatever the f we want, including something that's clea

Seriously wtf someone comes up with "X is UB" and even worse, "Since it's UB this gives a license to do whatever the f we want, including something that's clearly not at all what the dev intended" No wonder the languages being developed to solve real problems by people with real jobs are moving forward

masklinn 2y ago
> Since it's UB this gives a license to do whatever the f we want, including something that's clearly not at all what the dev intended That’s really not how it works. Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is). Of note, Rust very much has UBs, and hitting them is as bad as in C++, but the “safe” subset of the langage is defined such that you should not be able to hit UBs from there at all (such feasibility is what “soundness” is about, and why “unsoundness” is one of the few things justifying breaking BC: it undermines the entire point of the langage).
- deleted 2y ago
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- Measter 2y ago
  > Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is). I think a good way to view this would be that optimization passes have invariants. The passes transform code from one shape to another while ensuring that the output from running the code remains the same. But in order for the transformation to be valid certain invariants must be upheld, and if they are not then the result of the pass will have different output (UB).
  - masklinn 2y ago
    That’s part of it, but compilers also use the information more directly especially in languages with inexpressive type systems e.g. dereferencing a null pointer is UB so the compiler will tag a dereferenced pointer as “non-null”, then will propagate this constraint and remove unnecessary checks (e.g. any check downstream from the dereference, or unconditionally leading to it).

masklinn 2y ago

> Since it's UB this gives a license to do whatever the f we want, including something that's clearly not at all what the dev intended That’s really not how it works. Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is). Of note, Rust very much has UBs, and hitting them is as bad as in C++, but the “safe” subset of the langage is defined such that you should not be able to hit UBs from there at all (such feasibility is what “soundness” is about, and why “unsoundness” is one of the few things justifying breaking BC: it undermines the entire point of the langage).

deleted 2y ago
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Measter 2y ago
> Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is). I think a good way to view this would be that optimization passes have invariants. The passes transform code from one shape to another while ensuring that the output from running the code remains the same. But in order for the transformation to be valid certain invariants must be upheld, and if they are not then the result of the pass will have different output (UB).
- masklinn 2y ago
  That’s part of it, but compilers also use the information more directly especially in languages with inexpressive type systems e.g. dereferencing a null pointer is UB so the compiler will tag a dereferenced pointer as “non-null”, then will propagate this constraint and remove unnecessary checks (e.g. any check downstream from the dereference, or unconditionally leading to it).