Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!cornell!rochester!ur-tut!sunybcs!boulder!hao!oddjob!mimsy!eneevax!umd5!brl-adm!adm!TLIMONCE%DREW.BITNET@CUNYVM.CUNY.EDU
From: TLIMONCE%DREW.BITNET@CUNYVM.CUNY.EDU
Newsgroups: comp.lang.c
Subject: Re: a couple quickies / foo(&i)
Message-ID: <11189@brl-adm.ARPA>
Date: 9 Jan 88 05:36:16 GMT
Sender: news@brl-adm.ARPA
Lines: 48

A while ago I wrote:
>    for (i=0; i==100; i++) foo(&i);

Yes, I did mean:
    for (i=0; i<100; i++) foo(&i);

Sorry for the typo and thanks for the answers (and interesting insights
to what both would do.

I must have posted that one late at night because I totally forgot that
for() is expanded to while() and that made the answer obvious.

Now, let's look at a not-so-obvious use of noalias:

noalias int i;
for (i=0; i<100; i++) {...anything...}

Because I told the compiler noalias it knows that I will not be doing
anything funny to i.  A compiler can rightly ignore "noalias" since
the default is the "safe" method.  But look at what a 1990's compiler
could do here.  If the compiler notices that I don't modify i in the loop
and if the target machine has a compare_to_zero function
that is many times faster than compare_to_an_integer the code can
be re-written to be:

noalias int i, _hidden_i;
i=0;
_hidden_i=100;
while (_hidden_i) {...anything...; i++; --_hidden_i;}

 ...which could be very-much faster on the target.  This level of
optimization is something that has never before been do-able until
now.  Of course, with a _lot_ of extra programming (motion tracing, etc)
the compiler may have surmised that this was possible.

(I bet that a good motion tracer could find huge numbers of
variables that could be ALSO go as noalias as the programmer
didn't recognize.  ...but a good motion tracer will not be
seen in many compilers until the late (?) 1990's)

There you have it folks.  A real purpose for noalias that most people
didn't even think of.
                              T. Limoncelli
  Drew U/Box 1060/Madison NJ 07940     Bitnet: tlimonce@drew.BITNET
  Disclaimer: These are my views, not my employer or Drew Univesity.
       "Any typos should be reported to my mother, who checked
      all my papers until I got a spelling checker in 8th grade."
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