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<H2>Issue DYNAMIC-EXTENT-FUNCTION Writeup</H2>
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<PRE><B>Forum:</B> CLEANUP<P>
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<B>Issue:</B> <A HREF="iss141.htm">DYNAMIC-EXTENT-FUNCTION</A><P>
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<B>References:</B> Scope and Extent<P>
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Issue <A REL=DEFINITION HREF="../Body/d_dynami.htm#dynamic-extent"><B>DYNAMIC-EXTENT</B></A><P>
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<B>Category:</B> ADDITION<P>
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<B>Edit history:</B> 04-Apr-89, Version 1 by Loosemore<P>
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11-Jun-89, Version 2 by Loosemore<P>
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<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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Proposal <A HREF="iss142.htm">DYNAMIC-EXTENT:NEW-DECLARATION</A>, passed at the March 89 <P>
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meeting, provides a mechanism for declaring that the values of<P>
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variables have only dynamic (rather than indefinite) extent. It<P>
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would be useful to have similar functionality to indicate that<P>
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functional bindings may have only dynamic extent. (For example,<P>
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this would permit compilers to stack-allocate closures.)<P>
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<P>
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<B>Proposal (DYNAMIC-EXTENT-FUNCTION:EXTEND):<P>
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</B><P>
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Extend the <A REL=DEFINITION HREF="../Body/d_dynami.htm#dynamic-extent"><B>DYNAMIC-EXTENT</B></A> declaration to accept arguments that are<P>
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lists of the form (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> <name>) where <name> is a function name,<P>
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as well as symbols.<P>
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<P>
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A (<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> <name>) list appearing in a <A REL=DEFINITION HREF="../Body/d_dynami.htm#dynamic-extent"><B>DYNAMIC-EXTENT</B></A> declaration is<P>
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used to declare that the lexically visible functional binding of <name><P>
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has dynamic extent. Except for the interpretation of <name> as the<P>
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name of a function instead of the name of a variable, such a declaration<P>
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otherwise has semantics that are identical to those already described<P>
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in proposal <A HREF="iss142.htm">DYNAMIC-EXTENT:NEW-DECLARATION</A>.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This permits a programmer to offer advice to an implementation about<P>
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what functions may be stack-allocated for efficiency.<P>
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<P>
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It may be difficult or impossible for a compiler to infer this<P>
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same information statically.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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JonL says that Lucid's compiler can stack-allocate closures, but they<P>
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have no mechanism for programmers to give the compiler permission to<P>
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do so.<P>
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<P>
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HPCL-I has an UPWARD-CLOSURES declaration that pervasively affects<P>
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all closures created within the scope of the declaration.<P>
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<P>
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The Symbolics Genera compiler can often infer when functions can be <P>
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implemented to have dynamic extent. Also, if a function has a<P>
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SYS:DOWNWARD-FUNCTION declaration in front of its body, then the<P>
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function is implemented with dynamic extent regardless of whether<P>
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the compiler thinks all uses are "downward". (This declaration is<P>
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rather peculiar because its scope is actually larger than the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A><P>
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expression containing the declaration; implementationally, it's the<P>
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surrounding function definition.)<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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No cost is forced since implementations are permitted to simply<P>
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ignore the <A REL=DEFINITION HREF="../Body/d_dynami.htm#dynamic-extent"><B>DYNAMIC-EXTENT</B></A> declaration. <P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None. This change is upward compatible.<P>
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<P>
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There may be some hidden costs to debugging using this declaration (or any<P>
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feature which permits the user to access dynamic extent objects without<P>
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the compiler proving that they are appropriate). If the user misdeclares<P>
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something and returns a pointer into the stack (or stores it in the heap),<P>
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an undefined situation may result and the integrity of the Lisp storage<P>
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mechanism may be compromised. Debugging these situations may be tricky,<P>
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but users who have asked for this feature have indicated a willingness<P>
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to deal with such costs. Nevertheless, the perils should be clearly<P>
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documented and casual users should not be encouraged to use this<P>
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declaration.<P>
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<P>
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<B>Cost of Non-Adoption:<P>
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</B><P>
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Some portable code would be forced to run more slowly (due to<P>
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GC overhead), or to use non-portable language features.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The cost of non-adoption is avoided.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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This declaration allows a fairly low level optimization to work<P>
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by asking the user to <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> only very high level information.<P>
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The alternatives (sharpsign conditionals, some of which may<P>
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lead to more bit-picky abstractions) are far less aesthetic.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Loosemore supports <A HREF="iss141.htm">DYNAMIC-EXTENT-FUNCTION:EXTEND</A>.<P>
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<P>
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This proposal does not attempt to address the issue of specifying<P>
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dynamic extent for anonymous closures (which is really a special case<P>
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of the more general problem of specifying dynamic extent for unnamed<P>
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objects of any type). It's possible, although often awkward, to<P>
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restructure the program to give the object a name and explicitly<P>
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identify its extent.<P>
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<P>
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One possible solution to the problem of dynamic extent for anonymous<P>
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lambdas would be to clarify that a reference to a closed-over variable<P>
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or function appearing lexically within a <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> form is enough to<P>
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cause its value to be "saved" when the <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> form is executed,<P>
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regardless of whether or not that reference is actually executed when<P>
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the resulting function is called. Then, if all of the closed-over<P>
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functions and variables referenced within a closure are declared to<P>
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have dynamic extent, the closure could be assumed to have dynamic<P>
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extent as well. (More precisely, its maximum extent would be the<P>
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intersection of the extents of the closed-over functions and<P>
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variables.)<P>
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-------<P>
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<P>
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</PRE>
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