It can take quite a bit of engineering just to get the same source to produce the same results in many C or C++ toolchains. "Reproducible builds" require work; all sorts of trivial things like the length of pathnames can perturb the results. Not to mention having to have the same optimizer flags.

"Do these two binaries always behave the same for the same inputs" is practically an unsolvable problem in general. You can get fairly close with something like AFL (American fuzzy lop, a fuzzer and also a type of rabbit).

(Someone should really make an LLM bot that scans HN for instances of "just" and explain why you can't just do that, it's such a red flag word)