Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: Heroic constant folding (was micro-optimizing loops etc.)
Message-ID: <619@taumet.com>
Date: 4 Mar 91 16:18:24 GMT
References: <10191@dog.ee.lbl.gov> <14522@ganymede.inmos.co.uk> <10450@dog.ee.lbl.gov> <1991Mar2.010049.21044@grebyn.com>
Organization: Taumetric Corporation, San Diego
Lines: 38

ckp@grebyn.com (Checkpoint Technologies) writes:

>int fib_array[1024];

>void init_fib(void)
>{
	... dynamic initialization of fib_array ...
>}

>int main(int argc, char **argv)
>{
>init_fib();
>...

>...into...

>int fib_array[1024] = {1, 1, 2, 3, 5, 8, 13, 21, /* you get the idea */

I certainly would NOT want the compiler to do this, as it changes the
meaning of the program.

1. fib_array is a global array which might be set and used elsewhere in the
program before init_fib() is called.  Replacing the runtime initialization
with a static initialization could well result in completely different
program behavior. 

2. init_fib() is a global function which might be called elsewhere.  If
the compiler deletes the function, the program will fail to link.

You could argue for replacing the loop in init_fib() with a series of
constant assignments.  ('Loop unrolling' is a standard optimization
technique on parallelizing computers with lots of memory.)  This would
preserve the program's semantics, but might result in a larger program.
Depending on instruction caching and look-ahead, the unrolled loop might
execute more slowly than the original loop, apart from being larger.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com