-O3 is indeed riskier. All experienced embedded systems developers know this: embedded compilers tend to be much buggier than compilers for desktop platforms. But desktop compilers are buggy too. Over the last couple years my group has reported 190 bugs to compiler development teams. A lot of these bugs turn up only at higher optimization levels. If you search on my email address "regehr@cs.utah.edu" as bug reporter in either GCC or LLVM's bugzilla, you can see plenty of examples.
If he's still teaching it, I recommend his advanced embedded systems class to anyone at utah.edu who wants to gain practical experience with such compiler errors.
It was actually this class that motivated the whole compiler bug-finding project. The quality of the average embedded compiler is appalling, students trip on codegen bugs all the time.
Of course as many people are pointing out in this thread, most of the time the compiler is not to blame when changing optimization options changes program behavior.
Embedded compilers are much worse. You have to pick and choose which stable version of gcc-4.x you can safely use. My AVR projects have been broken by compiler changes.
But it's much more than the optimizer. Even code generation at -O0 can be broken by assumptions about alignment, insn size, etc. This usually happens when you're using a very new or very old part and the gcc developers make assumptions based on their limited dev board setups.
All appreciation should be paid to those gcc developers as it is a very difficult job they do for free. Thanks!
