Path: utzoo!utgpu!water!watmath!clyde!rutgers!paul.rutgers.edu!segall
From: segall@clash.rutgers.edu (Ed Segall)
Newsgroups: comp.arch
Subject: Re: FREQUENCY statements: fill a much-needed gap
Message-ID: <2861@clash.rutgers.edu>
Date: 12 Jan 88 20:16:12 GMT
References: <839@ima.ISC.COM> <28200085@ccvaxa> <1267@winchester.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 50

> 4) Features like FREQUENCY are a crutch to fill the gap between
> what a human knows and what the compiler can do.  C's "register"
> declaration was such a crutch, appropriate at the time, since it
> made a big difference, and optimizing compilers were not as widely
> available as they are today.  Many current compilers almost ignore
> "register" in favor of their own allocation.
> 
> FREQUENCY is like that, but not as useful, since it isn't spanning
> much of gap, as current leading-edge software technology is quite
> capable of gathering and using the needed information.
> It is a feature that "fills a much-needed gap" (in the words of Ken T.)


Aaarrrggghh!


Yes, it would be really nice to have a compiler figure everything out
in advance, and I wish the greatest encouragement to those who can get
a compiler to figure out which branch is most likely to be taken.
Yes, it it true that profiling (with appropriate input data, for
programs with data-dependent profiles) is the best way to do this.
However, predicting the most likely branch is not the only reason to
include a directive in a language to tell the compiler which branch to
optimize.  Perhaps FREQUENCY is a misnomer.  May I propose
MAKE_FASTEST_BRANCH, instead? This is only half-serious, of course,
and I hope the choice of a name doesn't turn into a "this name is
better than that name" contest, but there is an important semantic
difference between the two. 

I think the missle example showed it best (to paraphrase: you don't
want to optimize for the most common case, when no missles are coming
in.  You'd be better off optimizing the very uncommon case when they
are).  So, in mine humble opinion, it is a Very Good Thing to have a
compiler figure out (guess, usually) which branch is going to be
executed most frequently, and to use that information, but it is a
Very Bad Thing to ignore the need for communicating as much useful
information as possible to the compiler.  Sure, most such features
would go unused most of the time, often when used they would be wrong,
and the code clutter would be horrendous if they are used extensively,
(sounds like a candidate for Hypertext-style programming)  but please
don't say they shouldn't be developed and used.




--Ed
-- 

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