Found this announcement
SPECIAL NOTE for Ubuntu x86 12.04 users: By default OpenCV is now built with "-O2" optimization flag >instead of "-O3" on 32-bit Linux. The compiler in the 32-bit version of Ubuntu 12.04 produces incorrect >code with "-O3", so it's strongly recommended not to use this flag.
You are welcome to try out the latest and greatest OpenCV!
Best regards, OpenCV development team.
I notice that -O2 is still used as default in the CMake files for v3.0 beta Is the issue with using -O3 not resolved for newer versions of gcc than what is supported by Ubuntu 12.04? Is there a significant performance penalty associated with having to use -O2 ?