That just is quite ironic, because it is even version dependent across releases of the same compiler, which means anything fine tuned for version X can break on version X + 1, due to changes on the optimizer.
And I doubt anyone is regularly reviewing the produced Assembly every single time they change compilers.
Assembly doesn't have more than 200 documented cases that hardly any human is capable of remembering.
You just need to know your compiler flags/configuration to produce exact assembly output (if you want that). And about assembly not having UB, BSF and BSR are pretty good examples for that.