From: SerDevian <scriven AT CS DOT ColoState DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Strange speed behaviour (fflush())
Date: Tue, 18 Feb 1997 13:41:27 -0700
Organization: XYZZ Software Co.
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Message-ID: <330A13F6.41C6@CS.ColoState.edu>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

I've noticed something quite unexpected in a program of mine...  Maybe
someone here has some sort of explanation.  (?)

The program is a graphical program that uses the "far" pointer method of
accessing the screen.  This works fine.

The thing that confuses me is that if I add a "fflush(stdout)" before
the main graphical loop, the program speeds up by about 10 fps (80 -> 90
fps).  I figured at first that maybe having something hanging in the
buffer caused something to be lurking in the background, slowing things
down; but that explanation doesn't make sense within this particular
program because I flush stdout before the main loop anyway.  Maybe
fflush() modifies the timer?

The important parts of the source look like this:

void main()
{
  ... Initialize a few tables, load a couple data files ...
  printf("some sort of status\n");
  
  // if I use this line, the graphic loop goes faster...
  // fflush(stdout);

  printf("another status\n");
  fflush(stdout);	// this fflush(stdout) is not the strange one.

  set_graphics_mode();
  start_fps_counter();
  run_graphical_stuff();
  end_fps_counter();
  set_text_mode();
}

The graphical part of the program does absolutely nothing that uses
stdout.

BTW, I'm compiling with -O3, which is probably the cause of the speed
change, but I don't see why gcc would optimize differently with that one
extra line added.
==================
There is one other strange behavior I noticed with optimization... 
(same program)  It probably isn't important, though (due to cache or
pipelining maybe).  In the innermost loop, the program slows down by
about 10-15 fps if I *take out* two lines in the loop.  (they are just
"var += constant;" lines)  I thought I would be clever by optimizing
those two lines out of the inner loop, but it caused slowdown instead. 
Even if I leave things as they were, but comment out the two lines, the
program still slows down...



-- Yamaha / XYZZ          "May farce the with be you."
mailto:scriven AT CS DOT ColoState DOT edu          
mailto:scriven AT VIS DOT ColoState DOT edu
http://www.vis.colostate.edu/~scriven/