;; Code
(defpackage #:bits
  (:use :cl :sb-ext :sb-c))
(in-package :bits)

(defknown popcnt (fixnum) fixnum)

(in-package :sb-vm)
(define-vop (bits::popcnt)
  (:translate bits::popcnt)
  (:policy :fast-safe)
  (:args (x :target r))
  (:arg-types fixnum)
  (:results (r))
  (:result-types fixnum)
  (:generator 1
	      (inst popcnt r x)))
(in-package :bits)

(defun popcnt (x)
  (popcnt x))
;; Usage
(disassemble #'bits::popcnt) =>
; disassembly for BITS::POPCNT
; Size: 21 bytes. Origin: #x52B0166A
; 6A:       498B4560         MOV RAX, [R13+96]                ; no-arg-parsing entry point
                                                              ; thread.binding-stack-pointer
; 6E:       488945F8         MOV [RBP-8], RAX
; 72:       F3480FB8DA       POPCNT RBX, RDX
; 77:       488BE5           MOV RSP, RBP
; 7A:       F8               CLC
; 7B:       5D               POP RBP
; 7C:       C3               RET
; 7D:       CC0F             BREAK 15                         ; Invalid argument count trap

(popcnt 5) => 5
(popcnt -1) => -1
;; POPCNT only acts as #'identity, but why?