No, most compilers are not themselves reproducible, even within very restrictive sandboxes (e.g. they may do some work concurrently and collect the results base

No, most compilers are not themselves reproducible, even within very restrictive sandboxes (e.g. they may do some work concurrently and collect the results based on when it completes, then build on top of that. If they don't add a timing-insensitive sorting step, the resulting binary will (assuming no bugs) be functionally equivalent, but may not be bit-by-by equal), and a build tool can only do so much.