issue DYNAMIC-EXTENT-FUNCTION, version 2

[sandra%defun@cs.utah.edu (Sandra J Loosemore)]

I apologize for taking so long to finish this up -- I keep getting
distracted with "real work" lately....

Forum:		CLEANUP
Issue:          DYNAMIC-EXTENT-FUNCTION
References:     Scope and Extent
		Issue DYNAMIC-EXTENT
Category:       ADDITION
Edit history:   04-Apr-89, Version 1 by Loosemore
		11-Jun-89, Version 2 by Loosemore


Problem Description:

  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.)

Proposal (DYNAMIC-EXTENT-FUNCTION:EXTEND):

  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.

Rationale:

  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.

Current Practice:

  JonL says that Lucid's compiler can stack-allocate closures, but they
  have no mechanism for programmers to give the compiler permission to
  do so.

  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
  declaration.

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.

Benefits:

  The cost of non-adoption is avoided.

Aesthetics:

  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.

Discussion:

  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
  variables.)
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