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From: ingoldsb@ctycal.UUCP (Terry Ingoldsby)
Newsgroups: comp.arch,comp.lang.misc
Subject: Re: Compiler Costs
Summary: Programmer time is worth something
Message-ID: <460@ctycal.UUCP>
Date: 9 Jul 90 18:37:20 GMT
References: <1797@apctrc.UUCP>
Organization: The City of Calgary, Ab
Lines: 45

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?

This seems to be following along in the thread of compiler optimization,
where various people are suggesting that optimizers have or have not
replaced human ingenuity in generating efficient code.  This discussion
loses sight of two things:
  1) How hard (long) does the human try?
    - Some years ago I wrote some critical code for an application, part
      of which ran on a TMS32010 and part on an 8086 (it was a multi-
      processor system).  There were two sections of the program that
      had to be *very* fast.  I worked almost a week to write at most
      three pages of code.  When I was done, I doubt that any optimizer
      could have beat that code.  I tried the implications of using
      different registers for different purposes, and so on until I
      had what was the fastest possible implementation.  But at what
      cost?  I wrote about 150 assembler instructions in 5 days.
      That is 30 instructions/day, or 1 instruction every 15 minutes.

  2) Does portability mean anything to you?
    - The code I wrote was very non-portable.  If the processors had
      been changed, even to other members of the same family (eg.
      TMS32020 or 80286) then all my optimizations would have been
      for naught.  The new processors would have required a different
      optimization strategy.  The old code would have run, but not
      optimally.

To me the argument of which is better is, man or machine, depends on
what you are trying to accomplish.  If time and money are not a
problem, then an expert human can probably beat a compiler.  If
budgets or portability have any importance the compiler wins hands
down.

P.S.
 To answer your question, I probably saved 50% runtime on the tight
portions of my code.  


-- 
  Terry Ingoldsby                ctycal!ingoldsb@calgary.UUCP
  Land Information Services                 or
  The City of Calgary       ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb