For smooth sys-libs/glibc-2.11.1 upgrade, make sure to use >=gcc-4.3-

If you use too old GCC, you may hit bug #292174 (missing cpuid.h) when upgrading glibc to 2.11.1.

Don't worry - in case things go wrong, the error will be caught before the glibc on your system is replaced. Just make sure you have the latest GCC installed:

emerge -uav sys-devel/gcc

And then make sure it is actually used:

gcc-config -l

On my system the output looks like this (the star shows the currently used version):

 [1] i686-pc-linux-gnu-4.4.3 *

And here's an example how to change the active GCC version (using the number from above list):

# gcc-config 1

 * Switching native-compiler to i686-pc-linux-gnu-4.4.3 ...

For more info, see Gentoo GCC Upgrade Guide.