If a lexical variable is declared to be of fixnum , character , short-float , long-float, or their subtypes, then it is allocated on the C stack rather than on the value stack. In addition, the variable always has a raw datum as its value: 32 bit signed integer for fixnums, 8 bit character code with 24 bit padding for characters (remember that the font and bit fields of KCL characters are always 0 ), 32 bit floating point representation for short-floats, and 64 bit floating point representation for long-floats. Similarly, if a lexical variable is named in an object declaration (see Section 7.1), then it is allocated on the C stack but, in this case, the variable always has a cell pointer as its value. The user is strongly recommended to make sure that objects stored in such an object variable may never be garbage collected unexpectedly. For example,
(do ((x (foo) (cdr x)))
((endp x))
(let ((y (car x)))
(declare (object y))
(bar y)))
this object declaration is completely safe because the value of the variable y is always a substructure of the value of x, which in turn is protected against garbage collection. Incidentally, loop variables of dolist may always be declared as object variables, since the dolist form has essentially the same control structure as the do form above. On the other hand, the result of evaluation of the following form is unpredictable, because the cons cell pointed to from the object variable z may be garbage collected before bar is called.
(let ((z (cons x y)))
(declare (object z))
(foo (cons x y))
(bar z))
Lexical variables that are not declared to be of fixnum, character, short-float, long-float, or their subtypes, and that are not named in object declarations are usually allocated on the value stack, but may possibly be allocated on the C stack automatically by the compiler.