+ ;; The pseudo-atomic bit in alloc-tn is set. If the
+ ;; lowtag also has a 1 bit in the same position, we're all
+ ;; set. Otherwise, we need to subtract the pseudo-atomic
+ ;; bit.
+ (inst or result alloc-tn (if (logbitp 0 lowtag) lowtag
+ (1- lowtag)))))