Newsgroups: comp.arch
Path: utzoo!utgpu!jarvis.csri.toronto.edu!godzilla.ele.toronto.edu!glenn
From: glenn@godzilla.ele.toronto.edu (Glenn Mackintosh)
Subject: Re: FREQUENCY statements: fill a much-needed gap
Message-ID: <1988Jan13.232930.7936@jarvis.csri.toronto.edu>
Organization: EECG, University of Toronto
References: <839@ima.ISC.COM> <28200085@ccvaxa> <1267@winchester.UUCP> <6107@oberon.USC.EDU>
Date: Wed, 13-Jan-88 23:29:29 EST

In article <6107@oberon.USC.EDU> blarson@skat.usc.edu (Bob Larson) writes:
>Programmers often don't realy know how their code will be used.  This
>means that not only will their intuitive guesses on how frequently
>sections of code will be used will be bad, but also they may not be
>able to pick a reasonable set of cases to profile.  (Would you like to
>see various versions of a product based on what profiles were fead to
>the optomizer?)
>
>Also, there are times where the most common case is not the one that
>needs optimization.  Would you want a missle tracking system optimized
>for the case where there are no incoming missles?  (Many such
>real-time applications are coded in assembler, but more and more are
>now in high-level languages.)

I'm not really sure which side your arguing for here.  Maybe you were trying
to say what I'm about to and I missed it.  The first paragraph sounds like
an argument against things like the FREQUENCY statement but the second
paragraph seems like a good argument for them.  In the above missile example
the programmers knowledge of which direction MUST be optimized is probably
far more important and more correct than any specific group of test cases
could be.  Even a group of simulated attacks would likely not cover all the
cases and in fact they might tend to cancel each other out.  Therefore good
use can be made of profiling but you may still need to force it to optimize
in a specific direction.

My personal opinion is that hints from the programmer can be useful and
should be taken into account by the compiler.  The optimizer should weigh
this against things it discovers from profiling or other techniques and
possibly choose to go the opposite way.  The example above illustrates that
it can be important to be able to override the compiler though.


			Glenn Mackintosh (@ University of Toronto)

Standard disclaimer applies.