cpuid2cpuflags -- CPU_FLAGS_* generator
(c) 2017 Michał Górny
2-clause BSD license


Usage
~~~~~

The program attempts to obtain the identification and capabilities
of the currently used CPU, and print the matching set of CPU_FLAGS_*
flags for Gentoo. To use it, just run it:

  $ cpuid2cpuflags
  CPU_FLAGS_X86: 3dnow 3dnowext mmx mmxext sse sse2 sse3

There are no command-line options. Please note that the program
identifies the apparent CPU capabilities using available CPU calls
or system interfaces, *not* the capabilities indicated by compiler
flags.

The flag definitions match the flags described in Gentoo profiles/desc
at the time of program release. If additional flags are introduced
in the future, they will be added in a future program release.

The output format is compatible both with Portage (package.use)
and Paludis (use.conf/options.conf). If you find it useful to
generate/update it automatically, you can use a dedicated file:

  $ mkdir /etc/portage/package.use   # if not used yet
  $ echo "*/* $(cpuid2cpuflags)" > /etc/portage/package.use/00cpuflags


Implementation details
~~~~~~~~~~~~~~~~~~~~~~

X86 (incl. x86-64)
------------------

On x86 platforms, cpuid2cpuflags issues the CPUID instruction to obtain
processor capabilities. This should work reliably across different
systems and kernels, unless the system somehow blocks this instruction.
If this is the case, please report a bug.


ARM and AArch64
---------------

On ARM platforms, the userspace processes are not allowed to obtain
processor information directly. Instead, the program is relying
on kernel identification of the CPU provided via the system interfaces.
Currently, only Linux is supported.

On Linux, two interfaces are used: uname() to identify the CPU family,
and getauxval(AT_HWCAP*...) to obtain detailed feature flags.

The textual value obtained from uname (armv* or aarch64) is used
to enable appropriate ARM version flags and some feature flags. It is
also used to determine whether the kernel is 64- or 32-bit since that
affects the interpretation of AT_HWCAP* flags.

Afterwards, the remaining feature flags are enabled based on either flag
bits provided by AT_HWCAP*, or implicitly based on the subarchitecture
(i.e. currently a number of features is always set on AArch64).

It should be noted that the program strongly depends on correct
identification of the CPU in the kernel. If you find the results
incorrect, please report a bug but I can't promise I'll be able to find
a good workaround.