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- From: hrubin@pop.stat.purdue.edu (Herman Rubin)
- Newsgroups: comp.arch,comp.lang.misc
- Subject: Re: Goals, Cost, and Flexibility (was Re: "Training" of programmers)
- Message-ID: <BuFK3y.3H5@mentor.cc.purdue.edu>
- Date: 11 Sep 92 20:11:57 GMT
- References: <1992Sep4.151001.9886@sei.cmu.edu> <Bu5qD3.GAM@mentor.cc.purdue.edu> <1992Sep11.184744.19329@CS.ORST.EDU>
- Sender: news@mentor.cc.purdue.edu (USENET News)
- Organization: Purdue University Statistics Department
- Lines: 73
-
- In article <1992Sep11.184744.19329@CS.ORST.EDU> crowl@jade.CS.ORST.EDU (Lawrence Crowl) writes:
- >In article <Bu5qD3.GAM@mentor.cc.purdue.edu>
- >hrubin@pop.stat.purdue.edu (Herman Rubin) writes:
- >>And then the hardware designers, being told that something is useless, drop
- >>it from the instruction set?
-
- >You misunderstand what has happened in the industry. The hardware designers
- >were told that something was rarely used, and therefore was not cost
- >effective. As good engineers, they dropped the feature.
-
- >>Designing flexibility in both hardware and software is not that expensive;
- >>putting in something left out may be.
-
- >My experience is just the opposite. Designing flexibility in either hardware
- >or software is very expensive. Given the time to program a simple utility
- >with one option versus one with twenty supports this claim. The time to build
- >an shifter of constant shift versus one of variable shift also supports this
- >claim. I invite you to provide _evidence_ in support of your claim.
-
- >I grant you, putting something in later will be expensive.
-
- >>A simple example is that of square root. The cost of putting in square root
- >>on any machine which has "classical" division hardware is essentially zero
- >>at the design and construction stage, but an algorithm to do it is otherwise
- >>is necessarily slow and costly.
-
- >Only costly when making simplistic comparisons. If the lack of square root
- >hardware increases my run time from 10 seconds to 10.0001 seconds (which is
- >probably realistic for many if not most program runs), it is _not_ costly.
-
- If the increase is that small, not even very many divisions are likely to
- be done. If one is only interested in the characteristic roots and a
- few characteristic vectors, the most commonly used method at this time,
- the QR algorithm, would spend most of its time in computing square roots.
- At best, it spends a fair amount in that operation.
-
- >>Bit vectors do not necessarily start on a word, or even a byte, boundary,
-
- >Yes, but if only one in a million don't, supporting non-aligned bit vectors
- >probably isn't cost effective.
-
- "Natural" programming on the CYBER 205 would have at least an appreciable
- portion of bit vectors unaligned, if not most. Now there are ways of
- reprogramming to avoid some of this, but very definitely not all. This
- programming requires more than a knowledge of HLLs.
-
- >>and again, it is easier to deal with the problem at hardware design time
- >>rather than in software.
-
- >This claim is strongly supported by the wide abundance of hardware-implemented
- >compilers and spreadsheet packages. :-) My experience indicates that the
- >cost to deliver a working solution to _almost_any_ problem is cheaper in
- >software than hardware. Consider the cost of building a working multiplier
- >versus the cost of writing an iterative adding subroutine. I invite you to
- >provide _evidence_ that your claim is true.
-
- But the building of the multiplier is done once, and the adding subroutine is
- run billions of times. Ask a designer how much more it would cost to put in
- fixed multiplication with the accuracy of floating at the time it was being
- built. Every floating point unit essentially contains a fixed point unit
- inaccessible to the user.
-
- >What you meant to say is "its easier for me if the hardware designers gave me
- >exactly what I wanted".
-
- It is also easier for the number theorists, signal processing people, and
- anyone who does not rely on cheap inefficient packages, which frequently are
- so bad that the roundoff, undetected, gives highly erroneous results.
- --
- Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907-1399
- Phone: (317)494-6054
- hrubin@pop.stat.purdue.edu (Internet, bitnet)
- {purdue,pur-ee}!pop.stat!hrubin(UUCP)
-