[Bug default/27557] Use threading at sub-file level

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27557

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
An interesting question is whether find_dups can be parallelized.

The common resource that is used, is the dup_htab. For an actual insertion, the
htab as a whole needs to be locked.

For comparing, comparing DIEs with different checksums can be done in parallel. 
So, we could fold the ref_checksum to a byte, and use that as in index into an
array of 256 locks.  The problem however is the optimistic marking of DIEs as
duplicates when following references.  Marking those optimistically would also
require acquiring the lock for the corresponding checksum, which opens the door
to deadlock problems. We could mitigate for that by detecting cycles upfront,
and avoiding parallel processing of those, but that would again reduce
effectivity.

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