В свежих stage3 уже gcc не собирается

  Title                     CPU_FLAGS_X86 introduction                                                                                                                                                 
  Author                    Michał Górny <mgorny@gentoo.org>                                                                                                                                           
  Posted                    2015-01-28                                                                                                                                                                 
  Revision                  2                                                                                                                                                                          
                                                                                                                                                                                                       
The USE flags corresponding to the instruction sets and other features                                                                                                                                 
specific to the x86 (amd64) architecture are being moved into a separate                                                                                                                               
USE flag group called CPU_FLAGS_X86.                                                                                                                                                                   
                                                                                                                                                                                                       
In order not to lose CPU-specific optimizations, users will be required                                                                                                                                
to update their make.conf (and package.use) file. For example, if                                                                                                                                      
the following USE flags were present:                                                                                                                                                                  
                                                                                                                                                                                                       
  USE="mmx mmxext sse sse2 sse3"                                                                                                                                                                       
                                                                                                                                                                                                       
Those flags need to be copied into:                                                                                                                                                                    
                                                                                                                                                                                                       
  CPU_FLAGS_X86="mmx mmxext sse sse2 sse3"                                                                                                                                                             
                                                                                                                                                                                                       
Please note that the same CPU_FLAGS_X86 variable is used both on x86                                                                                                                                   
and amd64 systems.                                                                                                                                                                                     
                                                                                                                                                                                                       
When in doubt, you can consult the flag descriptions using one of                                                                                                                                      
the commonly available tools, e.g. `equery uses` from gentoolkit:

  $ equery uses media-video/ffmpeg

Most of the flag names match /proc/cpuinfo names, with the notable
exception of SSE3 which is called 'pni' in /proc/cpuinfo (please also
do not confuse it with distinct SSSE3).

To help users enable the correct USE flags, we are providing a Python
script that generates the correct value using /proc/cpuinfo. It can be
found in the app-portage/cpuinfo2cpuflags package:

  $ emerge -1v app-portage/cpuinfo2cpuflags
  $ cpuinfo2cpuflags-x86

In order to ensure safe migration and maintain compatibility with
external repositories, it is recommended to preserve the old USE
settings for a period of one year or until no package of interest is