References: Scope and Extent
Edit history: 04-Apr-89, Version 1 by Loosemore
11-Jun-89, Version 2 by Loosemore
Proposal DYNAMIC-EXTENT:NEW-DECLARATION, passed at the March 89
meeting, provides a mechanism for declaring that the values of
variables have only dynamic (rather than indefinite) extent. It
would be useful to have similar functionality to indicate that
functional bindings may have only dynamic extent. (For example,
this would permit compilers to stack-allocate closures.)
Extend the DYNAMIC-EXTENT declaration to accept arguments that are
lists of the form (FUNCTION <name>) where <name> is a function name,
as well as symbols.
A (FUNCTION <name>) list appearing in a DYNAMIC-EXTENT declaration is
used to declare that the lexically visible functional binding of <name>
has dynamic extent. Except for the interpretation of <name> as the
name of a function instead of the name of a variable, such a declaration
otherwise has semantics that are identical to those already described
in proposal DYNAMIC-EXTENT:NEW-DECLARATION.
This permits a programmer to offer advice to an implementation about
what functions may be stack-allocated for efficiency.
It may be difficult or impossible for a compiler to infer this
same information statically.
JonL says that Lucid's compiler can stack-allocate closures, but they
have no mechanism for programmers to give the compiler permission to
HPCL-I has an UPWARD-CLOSURES declaration that pervasively affects
all closures created within the scope of the declaration.
The Symbolics Genera compiler can often infer when functions can be
implemented to have dynamic extent. Also, if a function has a
SYS:DOWNWARD-FUNCTION declaration in front of its body, then the
function is implemented with dynamic extent regardless of whether
the compiler thinks all uses are "downward". (This declaration is
rather peculiar because its scope is actually larger than the lambda
expression containing the declaration; implementationally, it's the
surrounding function definition.)
Cost to Implementors:
No cost is forced since implementations are permitted to simply
ignore the DYNAMIC-EXTENT declaration.
Cost to Users:
None. This change is upward compatible.
There may be some hidden costs to debugging using this declaration (or any
feature which permits the user to access dynamic extent objects without
the compiler proving that they are appropriate). If the user misdeclares
something and returns a pointer into the stack (or stores it in the heap),
an undefined situation may result and the integrity of the Lisp storage
mechanism may be compromised. Debugging these situations may be tricky,
but users who have asked for this feature have indicated a willingness
to deal with such costs. Nevertheless, the perils should be clearly
documented and casual users should not be encouraged to use this
Cost of Non-Adoption:
Some portable code would be forced to run more slowly (due to
GC overhead), or to use non-portable language features.
The cost of non-adoption is avoided.
This declaration allows a fairly low level optimization to work
by asking the user to provide only very high level information.
The alternatives (sharpsign conditionals, some of which may
lead to more bit-picky abstractions) are far less aesthetic.
Loosemore supports DYNAMIC-EXTENT-FUNCTION:EXTEND.
This proposal does not attempt to address the issue of specifying
dynamic extent for anonymous closures (which is really a special case
of the more general problem of specifying dynamic extent for unnamed
objects of any type). It's possible, although often awkward, to
restructure the program to give the object a name and explicitly
identify its extent.
One possible solution to the problem of dynamic extent for anonymous
lambdas would be to clarify that a reference to a closed-over variable
or function appearing lexically within a FUNCTION form is enough to
cause its value to be "saved" when the FUNCTION form is executed,
regardless of whether or not that reference is actually executed when
the resulting function is called. Then, if all of the closed-over
functions and variables referenced within a closure are declared to
have dynamic extent, the closure could be assumed to have dynamic
extent as well. (More precisely, its maximum extent would be the
intersection of the extents of the closed-over functions and