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issue DYNAMIC-EXTENT-FUNCTION, version 1
- To: firstname.lastname@example.org
- Subject: issue DYNAMIC-EXTENT-FUNCTION, version 1
- From: email@example.com (Sandra J Loosemore)
- Date: Tue, 4 Apr 89 11:27:21 MDT
As promised, here's a first cut at this issue. I'm not particularly
attached to the name DYNAMIC-EXTENT-FUNCTION, but I'm still feeling
too burned out from arguing over what to rename DEFPROCLAIM to want to
waste a lot of time on thinking up alternate names for this one too. :-(
References: Scope and Extent
Edit history: 04-Apr-89, Version 1 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.)
Introduce a new declaration called DYNAMIC-EXTENT-FUNCTION. This is
identical to the DYNAMIC-EXTENT declaration, except that the
arguments name functions instead of variables.
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.
Cost to Implementors:
No cost is forced since implementations are permitted to simply
ignore the DYNAMIC-EXTENT-FUNCTION 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:NEW-DECLARATION.