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From: gl8f@astsun7.astro.Virginia.EDU (Greg Lindahl)
Newsgroups: comp.arch
Subject: Re: Kneejerk answers to Herman
Message-ID: <1991Feb9.004131.5977@murdoch.acc.Virginia.EDU>
Date: 9 Feb 91 00:41:31 GMT
References: <27AF17B9.72E2@tct.uucp> <5275@mentor.cc.purdue.edu> <27B19A39.321E@tct.uucp>
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Organization: Department of Astronomy, University of Virginia
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In article <27B19A39.321E@tct.uucp> chip@tct.uucp (Chip Salzenberg) writes:

>Machines have been designed for efficient execution in the most common
>cases.  If the most common cases are compiled C and Fortran programs,
>optimizing the hardware for those cases is only natural.
>
>Remember, Herman, your instruction mix is radically atypical.

Actually, C and Fortran programs show a wide variety of "common
cases". Algorithms change, and so does the optimal hardware.

Example: 2 years ago my "typical" hydrodynamics code almost never did
a square root. Now it does quite a lot of them. Had you designed your
instruction set back then, and your competition covered their asses by
slowing themselves down a little bit to provide good performance in a
wider variety of situations.

The only thing that bothers me more than Herman repeating himself
repeatedly are people who give the knee-jerk answer "we know what
optimal is", without realizing that many applications are atypical,
and that they change over time. Yes, numerical hydrodynamics isn't a
big fraction of the total money spent on computers, but you can come
up with other examples if you think a bit. If you try to sell your
risc chip without integer multiply to a company that has 10 million
lines of Fortran code all written using 2d fortran arrays with
adjustable bounds, you could be in deep shit.

Optimize the common case, but cover your ass.

[ No, I have no idea if what Herman wants is a useful way to hedge
your bets, but I'm tired of seeing it flamed about in such a
content-less fashion. ]