Re: On inlining in C++

[Scott Robert Ladd <coyote at coyotegulch dot com>]

Joe Buck wrote:
> For -O3, try what you want, and maybe I could recommend its use someday,
> but not for 3.2 or 3.3 and C++.  For evidence why, I suggest looking at a
> recent speed comparison between Gentoo and Debian versions of a number of
> apps: why did Gentoo come out slower despite its custom optimizations?  My
> theory: -O3.

I ran into precisely the behavior you describe above: -O3 produces 
slower code than -O2 in several cases.

For example, on my C benchmark of Huffman compression (lots of bit 
manipulations and small loops), run on a 2.8GHz Pentium 4 hardware with 
gcc 3.2.2:

options  time (secs)
-------  -----------
n/a      44.4
-Os      28.4
-O1      26.3
-O2      30.2
-O3      30.7

Using -funroll-all-loops and other options can bring the run-time down 
to under 24 seconds. I'm still investigating how various "inline" 
settings affect the outcome; on preliminary tests, -finline-functions is 
definitely a loser.

I've been distracted by paying work (silly me!), but I'll try to get my 
results into coherent form in the coming days. I probably need to run 
tests for GCC 3.4, too...

--
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing