Update src/machine.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

src/tools/miri/src/machine.rs

@@ -1192,6 +1192,8 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
         if frame.extra.is_user_relevant {
             // All that we store is whether or not the frame we just removed is local, so now we
             // have no idea where the next topmost local frame is. So we recompute it.
+            // (If this ever becomes a bottleneck, we could have `push` store the previous
+            // user-relevant frame and restore that here.)
             ecx.active_thread_mut().recompute_top_user_relevant_frame();
         }
         let timing = frame.extra.timing.take();