CPU_FLAGS_*

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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.

Configuration

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

FILE /etc/portage/make.confSetting CPU_FLAGS_X86

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.

Emerge

root #emerge --ask app-portage/cpuid2cpuflags

Invocation

user $cpuid2cpuflags

CPU_FLAGS_X86: mmx mmxext sse sse2 sse3

Example to apply globally:

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

Migration from deprecated USE flags

In order not to lose CPU-specific optimizations, users will be required to update the make.conf (and potentially package.use) file(s). For example, if the following USE flags were present:

FILE /etc/portage/make.conf

USE="mmx mmxext sse sse2 sse3"

They should be moved to the relevant CPU_FLAGS_* variable.

Saving deprecated USE flags

Previous USE flags corresponding to the instruction sets and other features specific to the x86/amd64, ppc/ppc64 and arm/arm64 architectures are being moved into separate USE_EXPAND variables.

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.

External resources

News item: new CPU_FLAGS_PPC USE_EXPAND