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,
sseare only available on x86,
altivecis only available on
visis only available on sparc v9.
Disabling unavailable soft dependencies. A simple hypothetical example — say
mips, but has an optional dependency (controlled by the
libbar, which doesn't work on
mips. Then by adding the
fooappcould be made available to
mipsusers with the unresolvable dependency forcibly disabled.
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
use.mask should be handled via the relevant arch team.
See noblah USE Flags for more discussion.