Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!inuxc!iuvax!pur-ee!uiucdcs!uxc.cso.uiuc.edu!ccvaxa!aglew
From: aglew@ccvaxa.UUCP
Newsgroups: comp.arch
Subject: Re: taken -vs- untaken branches, Fo
Message-ID: <28200085@ccvaxa>
Date: 9 Jan 88 19:31:00 GMT
References: <839@ima.ISC.COM>
Lines: 48
Nf-ID: #R:ima.ISC.COM:839:ccvaxa:28200085:000:1899
Nf-From: ccvaxa.UUCP!aglew    Jan  9 13:31:00 1988


..> The Fortran FREQUENCY statement:
..>   John Levine (paraphrased) says that the lesson is that the programmer 
..>   should not provide the frequency hints, the system should.

I would rather that you had said "the system should be able to",
but "the programmer can if he really wants to".

First, a high level directive like FREQUENCY can be generated either
by the programmer or automatically by the language system, so why not
make this functionality available to the programmer?

Second, in many cases the programmer *does* know the frequency of certain
statements, particularly if he's implementing a well known and well analyzed
algorithm. It may be impossible or expensive for the language system to
discover this automatically. Why not make it possible to use all the
available information?

Finally, it is possible that the frequency directive could be extended
in ways that are not immediately automatable, eg:

	/* A computation involving Y and X */
	IF x > 0 THEN
		FREQUENCY y<1 ? 0.25 : 0.75
		...
	:: x <= 0 THEN
		FREQUENCY y<1 ? 0.75 : 0.25
		...
	FI

This might be, for example, used to generate two different copies of code,
if y is known far enough in advance.

I think it is more likely that compilers can be built that *use* this sort
of information, than it is that a system can automatically *discover* this
sort of thing in the near future.



Andy "Krazy" Glew. Gould CSD-Urbana.    1101 E. University, Urbana, IL 61801   
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