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Path: utzoo!dciem!nrcaer!cognos!jimp
From: jimp@cognos.uucp (Jim Patterson)
Newsgroups: comp.lang.c
Subject: Re: Optimization vs. the programmer
Message-ID: <541@cognos.uucp>
Date: Fri, 10-Apr-87 12:53:14 EST
Article-I.D.: cognos.541
Posted: Fri Apr 10 12:53:14 1987
Date-Received: Sat, 11-Apr-87 18:46:29 EST
References: <479@danews.ATT.COM>
Reply-To: jimp@cognos.UUCP (Jim Patterson)
Organization: Cognos Incorporated, Ottawa, Canada
Lines: 38

In article <479@danews.ATT.COM> lvc@danews.UUCP writes:
>There is a certain computer (that I still use) that
>requires the statement:
>
>	n = n;
>
>in order to force the hardware to work right for certain
>values of n.
>If I had an optimizing compiler that was smart (?$!?)
>enough to excise this I couldn't make the machine work
>right.

I suggest that the compiler should do more - it should perform
its calculations so that the necessary operations (e.g. "n = n")
are done automatically, as necessary, to ensure that the results
are correct.  If I am porting software to this certain computer,
I certainly would not want to go into the code and insert dummy
assignments wherever necessary (wherever that is) just to make
otherwise valid algorithms work.  This is a task that the
compiler should do for me.  Otherwise, it is not correctly 
implementing the abstract machine model for the C language.

Conversely, if I am using a machine where these dummy assignments
aren't necessary, then I don't care if the compiler removes them
except that in general I want the code to run as fast as possible,
but correctly of course.

C is a (reasonably) portable language for sofware development.
As such, it depends on an abstract machine model to define its
operations.  The role of any C compiler is to implement that model
as closely as is possible.  To this end, it should not do any
optimizations that comprimise its conformance to the model.  Hence,
on your computer it should not likely optimize out "n = n" if it
may affect the results of an operation (unless it otherwise
guarantees that "n" is properly represented, which I think is the
preferrable approach).  Conversely, any optimizations it can do which
DO NOT ALTER the results that are predicted by the abstract model
can reasonably be performed.