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).
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.
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:
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
emerge --ask app-portage/cpuid2cpuflags
CPU_FLAGS_X86: mmx mmxext sse sse2 sse3
Example to apply globally:
echo "*/* $(cpuid2cpuflags)" > /etc/portage/package.use/00cpu-flags
- News item: new CPU_FLAGS_PPC USE_EXPAND