A minimal function which produces EB's bug is as follows: (DEFUN LOSS (X) (LIST (CXR 2 X) (CXR 3 X) (CXR 4 X) (CXR 5 X) (CXR 6 X) (CXR 7 X))) Evidently the compiler's particular strategy for compiling CXRs commits it to eating up registers.