All C and C++ compilers in common use are single-threaded, and parallelism is achieved by compiling multiple files at once. An optimizer pass producing nondeterministic results due to a bug is not impossible, but would be a bug to fix. Multithreaded linkers which are expected to have nondeterministic results do exist, and you have to not use that for reproduce able builds.
There's a long tail of tricky problems to solve for reproducible builds, but a specific build of a compiler producing different executable code from run to run hasn't really been one of them.
Now I'm wondering how differently my multithreaded tcc fork produces code. I don't think it should but it always wouldn't surprise me if there was some shared state remaining.
There's a long tail of tricky problems to solve for reproducible builds, but a specific build of a compiler producing different executable code from run to run hasn't really been one of them.