That would depend on the implementation. But it doesn't really matter, no one should be using atoi.

Yeah, iirc it was exploitable because it doesn’t do any bounds checking or something like that, right?

It bounds checks just fine, as long as you pass it a null-terminated string. It just returns 0 on error, so there's no way to know if the string was for example, larger than would fit in an int, or even just totally invalid, or if the user actually entered 0.

In general almost nothing uses SIMD for processing strings :(

Not literally atoi() because most people don't use that anymore, but the C++ std::from_chars() uses SIMD in GCC 12+, thanks to some of the same people behind the OP: https://lemire.me/blog/2022/07/27/comparing-strtod-with-from...