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CPU_FLAGS_X86 are being introduced to group together USE flags managing CPU instruction sets.

By Mgorny / January 31, 2015

Hello, everyone.

The USE flags corresponding to the instruction sets and other features specific to the x86 (including x86-64) 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:

   /etc/portage/make.conf - The original make.conf flags
USE="mmx mmxext sse sse2 sse3"

Those flags need to be copied into:

   /etc/portage/make.conf - The new make.conf flags
CPU_FLAGS_X86="mmx mmxext sse sse2 sse3"

Please note that the same CPU_FLAGS_X86 variable is used both on 32-bit and 64-bit x86 (amd64) systems.

When in doubt, you can consult the flag descriptions using one of the commonly available tools, e.g. equery uses from gentoolkit:

user $ 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:

root # emerge -1v app-portage/cpuinfo2cpuflags
user $ 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 still using them.