compiler option O0 and O2 generate different results

Hi all,

I am optimizing my code. When I use O2 compiler option, the code runs repeatedly two times, the results are different in each repeat. But when I change to O0 compiler option, the results are the same. What might be the reason?

BTW, I am using CentOS 6.5 and the command line is as follows:

icpc -O2 -DALIGN_OPT=16 -DSSE -ipo main_turbo_decoding_cfunc.cpp turbo_decoding_cfunc.cpp -o turbo_decoding -lrt

Thank you!