Explicitly stack-allocated data

Question

I think in most implementations of Common Lisp cons cells are generally/always heap allocated (see Why is consing in Lisp slow?)

Common Lisp does provide a facility for returning multiple values from a function (using values when returning and multiple-value-bind at the call-site). I'm speculating a bit here, but I think the motivation for this construction is two-fold: 1) make functions like truncate easier to use in the typical case where you don't care about the discarded value and 2) make it possible to return multiple values without using a heap-allocated data structure at all and (depending on the implementation (?)) avoiding the heap entirely (and GC overhead later down the road).

Does Common Lisp (or a specific implementation like SBCL maybe) give you the ability to use stack-allocated data (maybe in conjunction with something like weak references) or create composite/large-ish value types (something like structs in C)?

You can do stack allocation with the `DYNAMIC-EXTENT` declaration. See SBCL manual [6.2 Dynamic-extent allocation](http://www.sbcl.org/manual/#Dynamic_002dextent-allocation) for example. — jkiiski, Oct 16 '17 at 00:38
Please see [values function in Common Lisp](https://stackoverflow.com/a/22796346/850781). — sds, Oct 16 '17 at 13:14

Rainer Joswig · Accepted Answer · 2017-10-16T09:02:31.343

Common Lisp has a DYNAMIC-EXTENT declaration. Implementations can use this information to stack allocate some data structures - they can also ignore this declaration.

See the respective documentation how some implementations support it:

Other implementations support it also, but they may lack explicit documentation about it.

The main motivation for explicit support of returning multiple values was to get rid of consing / destructuring lists of return values or even putting some results in global variables. Thus one may now be able to return multiple values in registers or via a stack.

Explicitly stack-allocated data

1 Answers1