gistfile1.txt

cl-user> (defun foo (a j)
           (declare (optimize (safety 0) (speed 3))
                    (type (array (unsigned-byte 8)) a))
           (let ((ptr (excl::ll :+ a #.(sys::mdparam 'comp::md-lvector-data0-norm)))
                 (off (excl::ll :fixnum-to-mi j)))
             (declare (fixnum ptr off))
             (excl::ll :aset-byte ptr off
                       (excl::ll :- (excl::ll :aref-ubyte ptr j) (excl::ll :fixnum-to-mi 1)))))
foo
cl-user> (compile 'foo)
foo
nil
nil
cl-user> (disassemble 'foo)
;; disassembly of #<Function foo>
;; formals: a j

;; code start: #x1000ca26e8:
   0: 48 83 c7 f0    add	rdi,$-16
   4: 4c 8b ee       movq	r13,rsi
   7: 49 c1 fd 03    sar	r13,$3
  11: 48 0f b6 34 37 movzbq	rsi,[rdi+rsi+0]
  16: 48 83 ee 01    sub	rsi,$1
  20: 42 88 34 2f    movb	[rdi+r13+0],sil
  24: 48 8b fe       movq	rdi,rsi
  27: f8             clc
  28: 4c 8b 74 24 10 movq	r14,[rsp+16]
  33: c3             ret
; No value