Compiling GNU/Linux with -O3 optimization

-O3 has several disadvantages:

1. First of all it often produces slower code than -O2 or -Os. Sometimes it produces longer code due to loop unrolling which may be in fact slower due to worse cache performance of code.
2. As it was said it sometimes produces wrong code. It may be either due to error in optimalization or error in code (like ignoring strict aliasing). As kernel code sometimes is and sometimes have to be 'smart' I'd say it is possible that some kernel developer made some error. I experienced various strange problems, like crashing of userspace utilities, when I compiled kernel with gcc 4.5 which at that point was stable. I still use gcc 4.4 for kernel and several selected userspace utilities due to various bugs. The same may apply for -O3.
3. I don't think it offers much benefit for the Linux kernel. The kernel does not do heavy computations and in places it does, it is optimized with assembly. -O3 flag will not change the cost of context switching or speed of I/O. I don't think something like <0.1% speedup of overall performance is worth it.

It's used in Gentoo, and I didn't notice anything unusual.

Note that large chunks of the toolchain (glibc in particular) flat out don't compile if you change optimization levels. The build system is setup to ignore your -O preferences for these sections on most sane distros.

Simply put, certain fundamental library and OS features depend on the code actually doing what it says, not what would be faster in many cases. -fgcse-after-reload in particular (enabled by -O3) can cause odd issues.