If you have a new Intel core Core i3/i5/i7 CPUs, like for example:
$ grep -m1 -A3 "vendor_id" /proc/cpuinfo vendor_id : GenuineIntel cpu family : 6 model : 58 model name : Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
then the recent GCC version, starting with the version 4.7 brings a new support for optimization flags.
$ gcc-config -l [1] x86_64-pc-linux-gnu-4.6.3 [2] x86_64-pc-linux-gnu-4.7.2 *
For the IvyBridge chipset
$ dmesg | grep Ivy [ 0.179101] Performance Events: PEBS fmt1+, 16-deep LBR, IvyBridge events, Intel PMU driver. [ 0.922097] pci 0000:00:00.0: Intel Ivybridge Chipset
a new march option can be used:
File: /etc/portage/make.conf CFLAGS="-march=core-avx-i -O2 -pipe"
More Information can be found on Gentoo wiki and on the Intel pages.