m68k peephole weirdness

[Paul Brook]

In m68k.md we have:

;; Speed up pushing a single byte but leaving four bytes of space.

(define_peephole
  [(set (mem:QI (pre_dec:SI (reg:SI SP_REG)))
        (match_operand:QI 1 "general_operand" "dami"))
   (set (reg:SI SP_REG) (minus:SI (reg:SI SP_REG) (const_int 2)))]


The (const_int 2) looks very suspect. Shouldn't this be (const_int 3) ?
If not, could someone please explain to me what bit of arcane magic I'm 
missing?

I'm also intrigued to know under what circumstances this peephole would 
trigger.

Paul