Issue ALLOW-LOCAL-INLINE Writeup

Forum:		Compiler

Issue:		ALLOW-LOCAL-INLINE

References:	CLtL p. 156, 159

Related issues: PROCLAIM-INLINE-WHERE

Category:	CLARIFICATION

Edit History:   21 Sept. 88 V1 by David Gray

                27 Oct.  88 V2 by David Gray - new proposal

		 9 Nov.  88 V3 by David Gray - expanded problem

			description and discussion sections.

		30 Dec.  88 V4 by Sandra Loosemore -- suggestions from Pitman

 

Problem Description:


  The proposal PROCLAIM-INLINE-WHERE:BEFORE (which was accepted by X3J13

  on 10/12/88) clarifies the use of INLINE proclamations, but there

  remains a similar problem with the use of a local (DECLARE (INLINE

  ...)):  how can the compiler expand the function inline if it didn't

  know that the necessary information should have been saved when the

  function was compiled?


  Note that an INLINE proclamation does two things:


    (1) It tells the compiler to do extra things when it sees the

        function -definition-, to make it possible to code the function

	inline.


    (2) It tells the compiler to code -calls- to the function inline.


  In order for local INLINE declarations to be useful, we need part 1

  without part 2.

 

Proposal ALLOW-LOCAL-INLINE:INLINE-NOTINLINE


  Clarify that to define a function FOO which is not INLINE by default

  but for which (DECLARE (INLINE FOO)) will make FOO be locally inlined,

  the proper definition sequence is:

    

   (PROCLAIM '(INLINE foo))

   (DEFUN foo ...)

   (PROCLAIM '(NOTINLINE foo))


  The INLINE proclamation preceding the DEFUN ensures that compiler will

  save the information necessary for inline expansion, and the NOTINLINE

  proclamation following the DEFUN prevents it from being expanded

  inline everywhere.  


  Note that while implementations are never required to perform inline

  expansion of function calls, many implementations that do support

  inline expansion will not be able to respond to local INLINE requests

  if this technique is not followed.


 Rationale:


  Local INLINE declarations are of little use without some way of

  alerting the compiler to the possibility of inline expansion before

  the function is compiled.  This seems the simplest solution since it

  just clarifies existing practice instead of adding a new feature to

  the language.


  A compiler could use some heuristic to save the definitions of functions

  that are short enough to look like good candidates for inline expansion,

  but then the user is never sure what to expect.  It is possible that a

  compiler could simply save all definitions (assuming availability

  of adequate storage space) but we shouldn't require that.


 Test Cases/Examples: 


  Given the following input to COMPILE-FILE, does F1 get expanded inline

  in F2, and does F3 get expanded inline in F4?


    (defun f1 (a) (+ a 100))

    (defun f2 (b) (declare (inline f1)) (f1 b))

    (proclaim '(inline f3))

    (defun f3 (a) (+ a 100))

    (proclaim '(notinline f3))

    (defun f4 (b) (f3 b))			;F3 is not inline.

    (defun f5 (c) (declare (inline f3)) (f3 c)) ;F3 is locally inline.

    (defun f6 (c) (f3 c)) 			;The local effect is not 

						;  persistent.

 

 Current Practice:

 

  In the example above, using Symbolics, Lucid, or Explorer, F1 is not

  expanded in F2, but F3 is expanded in F5.


 Cost to implementors:

 

  None, since this is a clarification in accordance with current

  practice.


 Cost to users:

  

  None.


 Benefits:


  Users will be able to use (DECLARE (INLINE ...)) with greater assurance

  that it will really do something.


 Costs of Non-Adoption: 


  Users will not know how to reliably request inline expansion on a

  local basis.  This technique is not obvious, and even the need

  for it likely to be apparent only to people who understand something

  about how the compiler does inline expansion.


 Discussion:


  Version 1 of this issue included proposal

  ALLOW-LOCAL-INLINE:PROCLAIM-ALLOW-INLINE to make an addition to the

  language:

    (PROCLAIM '(ALLOW-INLINE foo))

  This was met with a lack of enthusiasm since it was pointed out that

  the same effect could be obtained by using a combination of INLINE and

  NOTINLINE.


  This is may not be completely true, however, since people's thinking

  about NOTINLINE has evolved in the direction of a declaration that

  tells the compiler "assume nothing about this function".  Thus, a

  NOTINLINE proclamation might suppress some optimizations that would

  have occurred if there had never been an INLINE and NOTINLINE.


  Ideally, it would be nice to have multiple levels of control instead

  of just INLINE or NOTINLINE -- something like:

    * always inline

    * maybe inline (let the compiler decide)

    * just save the definition for possible local inline

    * default

    * never inline


  Pitman has said that he generally approves of the direction of this

  proposal, but he has also expressed concerns about how the persistance

  of INLINE proclamations may cause confusion when functions are redefined

  in an incremental development environment.