From: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
Newsgroups: comp.os.msdos.djgpp
Subject: Loop unrolling: Don't bother
Date: 28 Feb 1997 03:51:15 GMT
Organization: The National Capital FreeNet
Lines: 24
Message-ID: <5f5knj$cho@freenet-news.carleton.ca>
Reply-To: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
NNTP-Posting-Host: freenet5.carleton.ca
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp



I have determined that loop unrolling produces very little speed gain.


A loop such as

register int i,j;
for (i=0; i<15; i++) {
  /* simple arithmetic */
  for (j=0; j<21; j++) {
    /* Function call and some math */
  }
  /* More math */
}

compiled with -O3 and a manually-unrolled loop (using macros) also
compiled with -O3 don't run at appreciably different rates.

--
    .*.  Where feelings are concerned, answers are rarely simple [GeneDeWeese]
 -()  <  When I go to the theater, I always go straight to the "bag and mix"
    `*'  bulk candy section...because variety is the spice of life... [me]
Paul Derbyshire ao950 AT freenet DOT carleton DOT ca, http://chat.carleton.ca/~pderbysh