Red Hat Bugzilla – Bug 478704
gcc optimize flag causes different program behaviour
Last modified: 2009-12-18 02:50:44 EST
Description of problem:
I downloaded a opengl source tutorial from http://nehe.gamedev.net/data/lessons/linuxsdl/lesson40.tar.gz and compiled it twice. The first time with the default Makefile as provided in the tarball. The program then worked as described in the tutorial, a simulation of a string fell down the ground and came to a standstill. I then edited the Makefile and replaced -g with -O2 and recompiled. When running the program the string never settled but instead I got chaotic motion.
It thus appears that there is an optimization bug that causes numeric instability of this particualar program.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. wget http://nehe.gamedev.net/data/lessons/linuxsdl/lesson40.tar.gz
2. tar -xf lesson40.tar.gz
3. cd lesson40_linux_sdl
6. perl -pi~ -e 's/-g/-O2/' Makefile
7. make clean
The second run of lesson40 yields chaotic motion.
The second run of lesson40 should give identical result to the first run.
I gave a look at this test case on every Fedora we support but on every particular one it looked the same -- rope settling down pixel by pixel. Can you still reproduce it? What about F-11/Rawhide? Can you make a OGV from the motion?
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I just tested it with Fedora 12 and it has been solved.