[Bug c/38777] New: gcc optimize flag causes different program behavior

[dov dot grobgeld at gmail dot com]

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. 

The tests were conducted with the gcc-4.3.2 under Fedora which has the internal
version gcc-4.3.2-7.src.rpm.

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
4. make
5. ./lesson40
6. perl -pi~ -e 's/-g/-O2/' Makefile
7. make clean
8. make
9. ./lesson40

Actual results:

The second run of lesson40 yields chaotic motion.

Expected results:

The second run of lesson40 should give identical result to the first run.


-- 
           Summary: gcc optimize flag causes different program behavior
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dov dot grobgeld at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38777