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Not to be confused with CFLAGS.

CPU_FLAGS_* is a USE_EXPAND variable containing instruction set and other CPU-specific features. Currently Gentoo supports CPU_FLAGS_X86 (for amd64 and x86 architectures), CPU_FLAGS_ARM (for arm and arm64 architectures), and CPU_FLAGS_PPC (for ppc and ppc64 architectures).

See also
For more information see the CPU_FLAGS_* section in the Gentoo Handbook.

Difference between CFLAGS and CPU_FLAGS_*

A common question is "what's the difference between CFLAGS and e.g. CPU_FLAGS_X86?"

CPU_FLAGS_* is an example of a USE_EXPAND. It enables specific options in ebuilds which are passed onto the build system. For example, CPU_FLAGS_X86_SSE2, if defined for a package, will enable handwritten ASM. These options enable specific code which already exists within the package.

CFLAGS, on the other hand, are simply used to tell the compiler it is allowed to try to generate code using such instructions if it is able. It does not mean it will be successful in doing so. e.g. -msse2 in CFLAGS does not mean the compiler will be clever enough to generate SSE2 for a certain function. These options just permit the compiler to generate certain code with certain instructions.

It is therefore important to configure CPU_FLAGS_* appropriately to get the best performance out of packages.


These variables need to be set as CPU_FLAGS_X86 (CPU_FLAGS_ARM, CPU_FLAGS_PPC) variable in a file in /etc/portage/package.use/:

FILE /etc/portage/package.use/my-cpu-flagsSetting CPU_FLAGS_X86
# This is just an example, please run the 'cpuid2cpuflags' tool to get an appropriate value for each system!
*/* cpu_flags_x86: mmx mmxext sse sse2 sse3

When in doubt, consult the flag descriptions using one of the commonly available tools, e.g. equery uses from app-portage/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).

Using cpuid2cpuflags

app-portage/cpuid2cpuflags helps users determine the correct CPU_FLAGS_ USE_EXPAND variables for their CPU architecture.


root #emerge --ask app-portage/cpuid2cpuflags


user $cpuid2cpuflags
CPU_FLAGS_X86: mmx mmxext sse sse2 sse3

Example to apply globally:

root #echo "*/* $(cpuid2cpuflags)" > /etc/portage/package.use/00cpu-flags

External resources