CL-USER> (setf *debug-mode-p* t)
T

CL-USER> (defun foo ()
           (with-debug 
             (print "debug mode!")))
WARNING: redefining COMMON-LISP-USER::FOO in DEFUN
FOO

CL-USER> (disassemble #'foo)
; disassembly for FOO
; Size: 55 bytes. Origin: #x530F019C                          ; FOO
; 9C:       498B4510         MOV RAX, [R13+16]                ; thread.binding-stack-pointer
; A0:       488945F8         MOV [RBP-8], RAX
; A4:       4883EC10         SUB RSP, 16
; A8:       488B15B9FFFFFF   MOV RDX, [RIP-71]                ; "debug mode!"
; AF:       B902000000       MOV ECX, 2
; B4:       48892C24         MOV [RSP], RBP
; B8:       488BEC           MOV RBP, RSP
; BB:       41807D3800       CMP BYTE PTR [R13+56], 0         ; thread.stepping
; C0:       7402             JEQ L0
; C2:       CC0D             INT3 13                          ; single-step trap (around)
; C4: L0:   B8A2B53850       MOV EAX, #x5038B5A2              ; #<FDEFN PRINT>
; C9:       FFD0             CALL RAX
; CB:       488BE5           MOV RSP, RBP
; CE:       F8               CLC
; CF:       5D               POP RBP
; D0:       C3               RET
; D1:       CC10             INT3 16                          ; Invalid argument count trap
NIL

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

CL-USER> (setf *debug-mode-p* nil)
NIL

CL-USER> (defun foo ()
           (with-debug 
             (print "debug mode!")))
WARNING: redefining COMMON-LISP-USER::FOO in DEFUN
FOO

CL-USER> (disassemble #'foo)
; disassembly for FOO
; Size: 21 bytes. Origin: #x530F024C                          ; FOO
; 4C:       498B4510         MOV RAX, [R13+16]                ; thread.binding-stack-pointer
; 50:       488945F8         MOV [RBP-8], RAX
; 54:       BA17001050       MOV EDX, #x50100017              ; NIL
; 59:       488BE5           MOV RSP, RBP
; 5C:       F8               CLC
; 5D:       5D               POP RBP
; 5E:       C3               RET
; 5F:       CC10             INT3 16                          ; Invalid argument count trap
NIL