Re: and then

[usenet.ufl.edu!travis.csd.harris.com!ada1.ssd.csd.harris.com!danr@gatech. edu (Dan Rittersdorf)]

In article <1993Apr7.162133.3564@nosc.mil> sampson@nosc.mil (Charles H. Sampson
) writes:
>
>     Unfortunately, that style can conflict with efficiency considerations. 
>Ada compiler writers appear to have abdicated their responsibility and make
>no attempt to determine when boolean operations can be optimized as short-
>circuit.  (Somebody contradict me, please.)  They apparently feel that if
>the programmer wants short-circuit he can ask for it.  In addition, there
>are cases when it is unreasonable to expect such an optimization; when the
>second operand contains a function reference, for example.
>
>                                   Charlie


   Charlie,

      Some vendors, especially those that have concentrated on excellent
   optimization, like (blatant plug alert) Harris, do make this 
   optimization when it can be determined that the operands of
   the boolean operator have no side effects.  Someone else also stated
   that the LRM requires the evaluation of the operands, but I think
   several people have done a good job of arguing that one to the
   ground, so I won't.

      I find it odd, though, that you feel a vendor has some *responsibility*
   to provide this optimization.  Each possible optimization has a cost and 
   a benefit -- to the vendor and the user, and each vendor decides what 
   optimizations they can afford to provide, given the benefit (profit)
   that can be obtained.  That's business.  If a vendor decides that benefits
   of providing short circuit boolean optimizations don't outweigh their
   investment, they may well decide not to provide it.  They have no 
   responsibility to provide it -- just the option.  If you want it, 
   let your vendor know.  They can't know what the customer wants if the
   customer doesn't tell them.  If they're unresponsive, remember that 
   they still have to weigh the benefit to their entire user community
   against the cost to the same.  Sometimes one user's request can't be
   granted if the impact to the entire user community would be deemed
   too great (for example, if the requested optimization were to require
   adding a new, time consuming pass to the compiler which would increase
   everybody's compile times).

      Note: I'm not implying that the short circuit optimization would
   require a cost of that magnitude -- those are separate examples...)

      The language provides two forms of the operators, and one requires
   short circuiting.  If that's what you want, say so; compilers don't have
   dwim switches yet.  If you don't care, use the general form of the 
   operator, but in doing so, you inform the compiler that short circuiting is 
   NOT required, so don't be surprised if you don't get it.

      I hope this doesn't sound like a flamer -- if so, my apologies.  I just
   wanted to provide a vendor perspective on the issue.  And you did ask for
   contradictions...  :-)

-- 
-danr
______________________________________________________________________________
  Dan Rittersdorf                             danr@ada1.ssd.csd.harris.com
  Harris Corporation, Computer Systems Division, Fort Lauderdale, FL 33309
  Michigan Address:                     233 Pippin Drive, Sparta, MI 49345
______________________________________________________________________________