Since Common Lisp forbids the redefinition of standard functions, the compiler can have special knowledge of these standard functions embedded in it. This special knowledge is used in various ways (open coding, inline expansion, source transformation), but the implications to the user are basically the same:
Attempts to redefine standard functions may be frustrated, since the function may never be called. Although it is technically illegal to redefine standard functions, users sometimes want to implicitly redefine these functions when they are debugging using the trace macro. Special-casing of standard functions can be inhibited using the notinline declaration.
The compiler can have multiple alternate implementations of standard functions that implement different trade-offs of speed, space and safety. This selection is based on the compiler policy.
When a function call is open coded, inline code whose effect is equivalent to the function call is substituted for that function call. When a function call is closed coded, it is usually left as is, although it might be turned into a call to a different function with different arguments. As an example, if nthcdr were to be open coded, then
(nthcdr 4 foobar)might turn into
(cdr (cdr (cdr (cdr foobar))))or even
(do ((i 0 (1+ i))
(list foobar (cdr foobar)))
((= i 4) list))
If nth is closed coded, then
(nth x l)might stay the same, or turn into something like
(car (nthcdr x l))
In general, open coding sacrifices space for speed, but some functions (such as car) are so simple that they are always open-coded. Even when not open-coded, a call to a standard function may be transformed into a different function call (as in the last example) or compiled as static call. Static function call uses a more efficient calling convention that forbids redefinition.