zygoloid wrote:
> When you say that we can't detect what the front-end considers the "closest
> surrounding subobject" to be, is that mostly due to corner cases or is it a
> more general concern?
It's a more general concern: LLVM simply has no idea what the frontend
considers to be a subobject. The LLVM type doesn't carry that information.
> ```
> struct suspend_stats {
> //...
> char failed_devs[REC_FAILED_NUM][40];
> int last_failed_errno;
> int bar;
> };
> //...
> ```
>
> Without the change, the last line is:
>
> ```
> __builtin_dynamic_object_size(foo.failed_devs[argc], 1): 48
> ```
>
> Which isn't correct according to GNU's documentation. So if we can't honor
> the TYPE bit, then we should return `-1 / 0` here, right?
Perhaps according to the GCC documentation as written. But mode 0 and 1 are in
general asking for an upper bound on the accessible bytes (that is, an N so
any.access beyond N bytes is definitely out of bounds), so it seems to me that
returning -1 is strictly worse than returning 48. Do you have a use case for
which -1 is a better answer?
I suspect the only change we're missing here is a change to our documentation
to explicitly say that we give an upper/lower bound when we can't compute an
exact size.
https://github.com/llvm/llvm-project/pull/78526
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