Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!vsi1!altos86!elxsi!beatnix!corbett
From: corbett@beatnix.UUCP (Bob Corbett)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran 8x:  pointers and optimization
Message-ID: <2812@elxsi.UUCP>
Date: 7 Jul 89 18:53:20 GMT
References: <664hallidayd@yvax.byu.edu>
Sender: news@elxsi.UUCP
Reply-To: uunet!elxsi!corbett (Bob Corbett)
Organization: ELXSI, San Jose
Lines: 36

-In article <664hallidayd@yvax.byu.edu> hallidayd@yvax.byu.edu writes:
->In message <2773@elxsi.UUCP> Bob Corbett (corbett@beatnix.UUCP) makes the
->statements
->>              Thus, adding pointers does not make optimization harder than
->>the IDENTIFY statement would.  However, pointers offer more functionality
->>than the IDENTITY statement.
->
->The first statement has already been adressed (the ALIASing provided by the
->ALIAS/IDENTIFY pair cannot be reIDENTIFYed in a subroutine, while that provided
->by POINTERs can).  The second statement, that POINTERs offer more functionality
->than the IDENTIFY is false if we are considering only the ALIASing of array
->sections (POINTERs do provide the added functionality of recursive data
->structures, which the ALIAS/IDANTIFY pair was never intended to do).  There is
->no way for the POINTER syntax to ALIAS arbitrary, functionally related, array
->elements as can be acomplished with ALIAS/IDENTIFY---try ALIASing the diagonal
->of an array with the present proposed POINTER syntax, it can't be done (as at
->least one other poster has stated).

     I concede that if the functionality pointers provide beyond ALIAS/IDENTIFY
is used, it will make alias analysis harder, though not much harder.  But
please note that if pointers are used to provide the functionality of
ALIAS/IDENTIFY, it does not make alias analysis noticeably harder.  The
example of a procedure call modifying the association of a pointer can occur
only if the pointer is external or the pointer is passed as a pointer
argument.  Neither case can arise when pointers are used to emulate
ALIAS/IDENTIFY.  Thus, the added functionality of pointers costs only
when it is used.

     I had missed the fact that pointers cannot be used to ALIAS the diagonal
of an array.  I regard this lack as a serious problem.  Clearly, X3J3 should
extend the notation for array sections to add this capability.

					  Yours very truly,
					  Bob Corbett
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