Issue DYNAMIC-EXTENT Writeup

Status:	      passed, as amended, Mar 89 X3J13.

Forum:	      Cleanup

Issue:        DYNAMIC-EXTENT

References:   Scope and Extent

Category:     ADDITION

Edit history: 27-Jun-88, Version 1 by Pitman (as issue STACK-LET)

  	      15-Nov-88, Version 2 by Pitman (issue renamed, major revision)

	      11-Jan-89, Version 3 by Masinter (Moon's proposal)

	      05-Apr-89, Version 4 by Pitman and Steele (changes per X3J13)

Related-Issues: REST-ARGUMENT-EXTENT, WITH-DYNAMIC-EXTENT


Problem Description:


  Sometimes a programmer knows that a particular data structure

  will have only dynamic extent. In some implementations, it is

  possible to allocate such structures in a way that will make them

  easier to reclaim than by general purpose garbage collection

  (eg, on the stack or in some temporary area). Currently, however,

  there is no way to request the use of such an allocation mechanism.


Proposal (DYNAMIC-EXTENT:NEW-DECLARATION):


  Introduce a new declaration called DYNAMIC-EXTENT. The arguments to

  this declaration are names of variables.


  It is permissible for an implementation to simply ignore this declaration.

  In implementations which do not ignore it, the compiler (or interpreter)

  is free to make whatever optimizations are appropriate given this

  information; the most common optimization is to stack-allocate the

  initial value of the object. What data types (if any) can have dynamic

  extent will can vary from implementation to implementation.


  Definition: Object <x> is an ``otherwise inaccessible part'' (OIP)

    of <y> iff making <y> inaccessible would make <x> inaccessible.

    (Note that every object is an OIP of itself.)


  Suppose that construct <c> contains a DYNAMIC-EXTENT declaration for

  variable <v> (which need not be bound by <c>).  Consider the values

  <w1>, ..., <wN> taken on by <v> during the course of some execution of

  <c>.  The declaration asserts that if object <x> is an OIP of <wI>

  when <wI> ever becomes the value of <v>, then just after execution of

  <c> terminates <x> will be either inaccessible or still an OIP of <v>.


  If the assertion is ever violated, the conseqeuences are undefined.


Examples:


  Since stack allocation of the initial value entails knowing at the

  object's creation time that the object can be stack-allocated, it is

  not generally useful to declare DYNAMIC-EXTENT for variables for

  which have no lexically apparent initial value. For example,


	(DEFUN F ()

	  (LET ((X (LIST 1 2 3)))

	    (DECLARE (DYNAMIC-EXTENT X))

	    ...))


  would permit those compilers which wish to do so to stack-allocate the

  list in X. However,


	(DEFUN G (X) (DECLARE (DYNAMIC-EXTENT X)) ...)

	(DEFUN F () (G (LIST 1 2 3)))


  could not typically permit a similar optimization in G because it would

  be a modularity violation for the compiler to assume facts about G from

  within F. Only an implementation which was willing to be responsible for

  recompiling F if G's definition changed incompatibly could stack-allocate

  the list argument to G in F.


  Other interesting cases are:


	(PROCLAIM '(INLINE G))

	(DEFUN G (X) (DECLARE (DYNAMIC-EXTENT X)) ...)

	(DEFUN F () (G (LIST 1 2 3)))


    and	(DEFUN F ()

	  (FLET ((G (X) (DECLARE (DYNAMIC-EXTENT X)) ...))

	    (G (LIST 1 2 3))))


  where some compilers might realize the optimization was possible and others

  might not.


  An interesting variant of this is the so-called `stack allocated rest list'

  which can be achieved (in implementations supporting the optimization) by:


	(DEFUN F (&REST X)

	  (DECLARE (DYNAMIC-EXTENT X))

	  ...)


  Note here that although the initial value of X is not explicit, the F

  function is responsible for assembling the list X from the passed arguments,

  so the F function can be optimized by the compiler to construct a 

  stack-allocated list instead of a heap-allocated list in implementations

  which support such.



In

            (LET ((X (LIST 'A1 'B1 'C1))

                  (Y (CONS 'A2 (CONS 'B2 (CONS 'C2 NIL)))))

              (DECLARE (DYNAMIC-EXTENT X Y))

              ...)

The OIP's of X are three conses, and the OIP's of Y are three other

conses.  None of the symbols A1, B1, C1, A2, B2, C2, or NIL is an

OIP of X or Y.  However, if a freshly allocated uninterned symbol had

been used, it would have been an OIP.


- - - - - - - -

          (DOTIMES (I N) 

            (DECLARE (DYNAMIC-EXTENT I))


This is particularly instructive.  Since I is an integer by the

definition of DOTIMES, but EQ and EQL are not necessarily equivalent for

integers, what are the OIP's of I, which this declaration

requires the body of the DOTIMES not to "save"?  If the value of I is 3,

and the body does (SETQ FOO 3), is that an error?  The answer is no, but

the interesting thing is that it depends on the implementation-dependent

behavior of EQ on numbers.  In an implementation where EQ and EQL are

equivalent for 3, then 3 is not an OIP because (EQ I (+ 2 1)) is true,

and therefore there is another way to access the object besides

going through I.  On the other hand, in an implementation where EQ and

EQL are not equivalent for 3, then the particular 3 that is the value of

I is an OIP, but any other 3 is not.  Thus (SETQ FOO 3) is valid

but (SETQ FOO I) is erroneous.  Since (SETQ FOO I) is erroneous in some

implementations, it is erroneous in all portable programs, but some other

implementations may not be able to detect the error.


- - - - - - - -


  (LET ((X (LIST 1 2 3)))

    (DECLARE (DYNAMIC-EXTENT X))

    (PRINT X)

    NIL)

  PRINT does not "save" any part of its input.

  This prints (1 2 3)


- - - - - - - -


  (DO ((L (LIST-ALL-PACKAGES) (CDR L)))

      ((NULL L))

    (DECLARE (DYNAMIC-EXTENT L))

    (PRINT (CAR L)))

  prints all packages; none of the newly-allocated list structures are saved.

- - - - - - - -

  (DEFUN ADD (&REST X) (DECLARE (DYNAMIC-EXTENT X)) (APPLY #'+ X))

  (ADD 1 2 3) => 6


I.e., useful way to declare that &REST lists have dynamic extent

- - - - - - - -

  (DEFUN ZAP (X Y Z)

    (DO ((L (LIST X Y Z) (CDR L)))

	((NULL L))

      (DECLARE (DYNAMIC-EXTENT L))

      (PRIN1 (CAR L))))

  (ZAP 1 2 3)

  prints 123


- - - - - - - -

  (DEFUN ZAP (N M)

    ;; Computes (RANDOM (+ M 1)) at relative speed of roughly O(N).

    ;; It may be slow, but with a good compiler at least it

    ;; doesn't waste much heap storage.  :-)

    (LET ((A (MAKE-ARRAY N)))

      (DECLARE (DYNAMIC-EXTENT A))

      (DOTIMES (I N) 

	(DECLARE (DYNAMIC-EXTENT I))

	(SETF (AREF A I) (RANDOM (+ I 1))))

      (AREF A M)))

  (< (ZAP 5 3) 3) => T


- - - - - - - -

The following are in error, since the value of X is used outside of its

extent:


       (LENGTH (LIST (LET ((X (LIST 1 2 3))) (DECLARE (DYNAMIC-EXTENT X)) X)))


  (PROGN (LET ((X (LIST 1 2 3))) (DECLARE (DYNAMIC-EXTENT X)) X)

         NIL)


- - - - - - - -


Rationale:


  This permits a programmer to offer advice to an implementation about

  what may be stack-allocated for efficiency.


  It may be difficult or impossible for a compiler to infer this

  same information statically.


  Since a number of implementations offer this capability and there

  is demand from users for access to the capability, this ``codifies

  existing practice.''


  Because this approach is purely lexical, it does not interact badly with

  other programs in the way that the macro WITH-DYNAMIC-EXTENT (see issue

  by same name) would.


Current Practice:


  Symbolics Genera and Symbolics Cloe offer stack allocation, though not

  in this strategy.


  [KMP thinks that] Lucid supports the proposal.


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:


  A previous version of this proposal suggested primitives STACK-LET

  and STACK-LET*. Consensus was that the more general declaration facility

  would be more popular.


  Moon came up with a description of something called a "proper part" which

  Steele formalized into the idea of an "otherwise inaccessible part". The

  two are essentially interchangeable, but Steele's description was more

  rigorous.


  KMP: ... it still raises the question of whether we should define 

       per-function for every CL function whether any of the arguments is

       permitted to be "saved" so that CL programs don't get any funny

       surprises. If we don't, it ends up being implementor's discretion how

       to resolve cases ... and everyone might not agree that all cases are

       ... obvious ...


  JonL: PDP10 MacLisp had a similar problem w.r.t pdlnums.  That is why

	"identity" functions were so troublsome for it -- in order to

        return a guaranteed safe value, it typically had to copy it's

	pdlnum argument, thereby making some cases of "fast arithmetic" 

	code much worse than interpreted code!  [Remember PRINT in MacLisp?

	it returns T rather than it's argument for just this reason.]


	It is necessary for an optimizing compiler to know something about

	what happens to the data it passes along to "system" functions; for

	example, it could assume that GET doesn't clobber the list given

	to it, nor does it retain pointers to any part of it [what was the

	terminology in the revised proposal?  "saved"? and "proper part"?]

	The issue LISP-SYMBOL-REDEFINITION might help here, in that an

	implementation's compilers could depend upon it's own internal

	database.  But it wouldn't hurt at all to have some of these

	requirements "up front" in the standard.


  It was generally agreed that we would also like to consider a proposal

  on dynamic extent functions at the next meeting. (Sandra said she would

  prepare one, and has already done so. See issue DYNAMIC-EXTENT-FUNCTION.)