SAPs are not stack-allocated

[Andrew Kravchuk]

I'm trying to optimize performance of some code heavily relying on CFFI and I want to stack-allocate C pointers. But the following minimal code

((lambda (x)
   (declare (type sb-sys:system-area-pointer x))
   (let ((p (sb-sys:sap+ x 42)))
     (declare (dynamic-extent p))
     (print (sb-introspect:allocation-information p))
     (print p)))
 (sb-sys:int-sap 0))

being tested on latest SBCL 2.4.1 (both Linux and Windows build), prints :HEAP despite dynamic-extent declaration on P; the results of DISASSEMBLE confirm that P pointer is indeed a 16 bytes allocated on heap via ALLOC-TRAMP.

Is it perhaps because SAPs have some complex internal structure that can't be laid out on stack?