Title: CPU_FLAGS_X86 introduction
Author: Michał Górny <firstname.lastname@example.org>
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 tool
that generates the correct value for the local CPU. It can be found
in the app-portage/cpuid2cpuflags package:
$ emerge -1v app-portage/cpuid2cpuflags
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
still using them.