Of course, implementation details are different between microarchitectures - that's the definition, after all. However, "agnostic" usually means that behavior

Of course, implementation details are different between microarchitectures - that's the definition, after all. However, "agnostic" usually means that behavior doesn't vary. Performance is orthogonal to behavior in this case. I definitely don't expect code optimized for Haswell to be as fast on a Broadwell as code optimized for Broadwell, but I do at least expect it to run.