Issue DYNAMIC-EXTENT-FUNCTION Writeup

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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