Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!sun-barr!newstop!sun!imagen!qmsseq!pipkins
From: pipkins@qmsseq.imagen.com (Jeff Pipkins)
Newsgroups: comp.sys.ibm.pc.programmer
Subject: Re: No Aliasing Compile Option
Keywords: Microsoft 6.0 C update woes
Message-ID: <134@qmsseq.imagen.com>
Date: 24 May 90 22:29:26 GMT
References: <4886@daffy.cs.wisc.edu> <26547195.F69@tct.uucp> <1990May19.141401.4350@ux1.cso.uiuc.edu> <265861D7.3293@tct.uucp>
Reply-To: pipkins@qmsseq.UUCP (Jeff Pipkins)
Organization: QMS Inc., Mobile, Alabama
Lines: 34

My two cents worth (an inflated price, at that):

1. Because of the design of the C language, it is not possible for a C
compiler to completely detect all aliasing situations ("not possible" means
without spending potentially infinite amounts of time simulating the code,
of course).  Other languages have been designed specifically so that
the compiler can detect aliasing.  Unfortunately, this requires constraints
on the language that are so severe that the language becomes unsuitable
for professional use.  The "cost" is too high.

2. It is not "wrong" to use aliasing, either morally or legally, and it is
not generally considered bad practice.  Since I used the word "morally",
this is obviously my opinion.

3. Why is this compiler option there, anyway?  Because the compiler writer
says to himself (while writing the compiler), "DAMN!  If I could just make
this one little ASSumption, I could produce tighter code!  Wouldn't that
be nifty?!!"  But he knows that it is an UNSAFE optimization.  But he
still can't resist.  It would be so neat.  So he adds it as an option.
POINT: Not only should the option not be used to compile benchmarks, IT SHOULD
NOT EVEN BE AN AVAILABLE OPTION FOR THE COMPILER IN THE FIRST PLACE!
Unsafe optimizations are just that -- unsafe.  The option is there because
the compiler writer couldn't resist the temptation, and now he is passing
the temptation on to the user of the compiler by encouraging him to use
an unsafe option.  Tell me, how do you guarantee that the option doesn't
break your code?  Tell me how you have insured it is safe -- by looking
at it?  How do you test for that?  If it doesn't run correctly, does it
matter that it runs faster?  The only thing that should be done with
unsafe optimizations is to omit them.

<End of soapbox>

Jeff Pipkins
pipkins@imagen.com