[glowcoil]

> If you write code that tries to get a pointer to the first variable in the stack, and guess the stack size and read everything in it, Odin does not prevent that, it also (AFAIK) does not prevent the compiler from promoting local variables to registers.

This is exactly what I described above. Odin does not define the behavior of a program which indirectly pokes at stack memory, and it is thus able to perform optimizations which exploit the fact that that behavior is left undefined.

> The compiler does not get to look at that and say "well this looks like undefined behavior, let me get rid of this line!".

This is a misleading caricature of the relationship between optimizations and undefined behavior. C compilers do not hunt for possible occurrences of undefined behavior so they can gleefully get rid of lines of code. They perform optimizing transformations which are guaranteed to preserve the behavior of valid programs. Some programs are considered invalid (those which execute invalid operations like out-of-bounds array accesses at runtime), and those same optimizing transformations are simply not required to preserve the behavior of such programs. Odin does not work fundamentally differently in this regard.

If you want to get rid of a particular source of undefined behavior entirely, you either have to catch and reject all programs which contain that behavior at compile time, or you have to actually define the behavior (possibly at some runtime cost) so that compiler optimizations can preserve it. The way Odin defines the results of integer overflow and bit shifts larger than the width of the operand is a good example of the latter.

C does have a particularly broad and programmer-hostile set of UB-producing operations, and I applaud Odin both for entirely removing particular sources of UB (integer overflow, bit shifts) and for making it easier to avoid it in general (bounds-checked slices, an optional type). These are absolutely good things. However, I consider it misleading and false to claim that Odin has no UB whatsoever; you can insist on calling it something else, but that doesn't change the practical implications.

[hsn915]

> They perform optimizing transformations which are guaranteed to preserve the behavior of valid programs. Some programs are considered invalid (those which execute invalid operations like out-of-bounds array accesses at runtime), and those same optimizing transformations are simply not required to preserve the behavior of such programs.

I think this is the core of the problem and it's why people don't like these optimizations and turn them off.

Again I'm not the odin designer nor a core maintainer, so I can't speak on behalf of the language, but from what I understand, Odin's stance is that the compiler may not make assumptions about what kind of code is invalid and whose behavior therefore need not be preserved by the transformations it makes.

[Gibbon1]

> C compilers do not hunt for possible occurrences of undefined behavior so they can gleefully get rid of lines of code.

Yes they do, if they detect UB they consider the result poisoned and delete any code that depends on it.