Obscure .eh_frame debug info question.

[Tom Horsley]

I was just wondering if there is anyone out there in redhat
land who can shed more light on the use of dwarf register 16
for a virtual "return address" in .eh_frame info, probably
hand generated .eh_frame info in the vicinity of the
pthreads library mutex locking code.

It is briefly mentioned in one place in the x86_64 abi
with no clue as to how it is used. Google finds zillions
of patches where it was inserted in the library, but
no actual explanation.

The .eh_frame dumps of the pthread library via readelf
make no sense to me at all.

And it will clearly take me at least 15 years to trace through
the code in gdb and figure out what it actually does, so
if anyone out there knows, please tell me :-).