Profiles use.mask file

The use.mask file can be used to mark USE flags as unavailable on a particular profile. This can be useful for various reasons:

• Masking hardware-specific feature flags. For example, mmx and sse are only available on x86, altivec is only available on ppc and vis is only available on sparc v9.
• Disabling unavailable soft dependencies. A simple hypothetical example — say fooapp works on mips, but has an optional dependency (controlled by the bar flag) upon libbar, which doesn't work on mips. Then by adding the bar flag to profiles/arch/mips/use.mask, fooapp could be made available to mips users with the unresolvable dependency forcibly disabled.

Note that use.mask is a per-flag thing, not per package's use of a given flag. This is one of the reasons that USE flags must have a specific well defined purpose.

Updates to use.mask should be handled via the relevant arch team. Any additions are sorted chronologically, starting at the top of the file (underneath any comment header blocks).

See noblah USE flags for more discussion.