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From: schow@bcarh185.bnr.ca (Stanley T.H. Chow)
Newsgroups: comp.arch,comp.lang.misc
Subject: Re: Compiler Costs
Message-ID: <3229@bnr-rsc.UUCP>
Date: 6 Jul 90 14:14:15 GMT
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Reply-To: bcarh185!schow@bnr-rsc.UUCP (Stanley T.H. Chow)
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In article <1797@apctrc.UUCP> zrra07@backus (Randall R. Appleton) writes:
>I have a simple question:  Does anyone know what sort of speed-up
>on gets by going from a good implementation of some algorithm in a
>third generation language (C, Fortran) and a good optimizing compiler
>to hand-coded assembly?
>
>In other words, if I take my average well written program, compiled
>with a good optimizing compiler, and re-write it in assembler, what sort
>of speedup should I expect to see?
>
>Thanks,
>Randy

The short answer is: Depends.

OKay, you ask, on what does it depend?

Well, it depends on just about every single adjective you used in
your question:
  How good is "a good implementation"?
  Which algoritm is the "some algorithm"?
  Which "good optimizing compiler"?
  Whose average? Over which set of programs?
  "Well written" for what purpose?
  Who will do the rewriting?
  For which computer?

Basically, performance is like anything else, the variation between
people (using identical tools) is larger than variation between tools.
So the "optimum" stratagy is to get the best person for the job.

You really will have to specify more details to get any meaningful
answer to your question. For example, There are some Fortran compilers
that are extremely good; for many programs, the code is just about
optimal.

The meaning of "well written" is very context dependent. Some people
insist on clear, understandable code; others want it fast, still others
want it to be robust and bullet-proof. Which properties does your
program have and which properties do you want to preserve in assembler?

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