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From: john@aplcomm.jhuapl.edu (John Hayes)
Newsgroups: comp.lang.forth
Subject: Re:  BASIS 14... comments
Message-ID: <1990Dec4.100730.10629@aplcen.apl.jhu.edu>
Date: 4 Dec 90 10:07:30 GMT
References: <9011302145.AA09747@ucbvax.Berkeley.EDU>
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Reply-To: john@aplcomm.jhuapl.edu (John Hayes)
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Mitch Bradley writes:
> By the way, now that we have COMPILE, (compile-comma), the words
> POSTPONE , [COMPILE] , and COMPILE are all unnecessary, as the desired
> effects may be achieved with  ['] foo EXECUTE   and  ['] foo COMPILE,

   This is true but the results of such an implementation would be
inefficient on native code systems.  The problem is that a postfix
COMPILE, (compile-comma) operator has less information available
to it than a prefix POSTPONE operator.  I offer two examples:

1)  Some native code systems have a compiler vocabulary with
compile-time versions of many primitives (e.g. DUP).  This compile
time DUP, compiles top-of-stack duplication code and may apply
peephole optimization.  An ordinary version of DUP appears in the
user visible vocabulary; ['] returns this 'ordinary' DUP.
   ['] DUP COMPILE,
compiles a subroutine call to DUP; the COMPILE, (compile-comma) does
not have enough information to inline the DUP.

2)  Some native code systems store information in a definition's header
to determine whether that definition should be inlined or or called as
a subroutine.  The compiler's internal equivalent of COMPILE, (compile-comma)
given the execution address of a definition, computes the address of the
data in the header.  A user accessible COMPILE, (compile-comma) may not assume
that the execution address belongs to a definition with a header;
START: allows the construction of headerless code.  Therefore, COMPILE,
(compile-comma), must always compile a subroutine call.  Again, COMPILE,
(compile-comma) does not have the necessary information.

   I assert that POSTPONE is the fundamental operator.
POSTPONE is used for more often than COMPILE, (compile-comma) and
[COMPILE] in programs.  Therefore, it should be as efficient as possible.
By being prefix, it has the opportunity to use the name following it
in the input stream to extract the necessary information.
COMPILE, and [COMPILE] can be defined:
   : COMPILE,  POSTPONE LITERAL POSTPONE EXECUTE ;
   : [COMPILE]  ' COMPILE, ; IMMEDIATE

John R. Hayes			john@aplcomm.jhuapl.edu
Applied Physics Laboratory
Johns Hopkins University


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