__register_frame_info and unwinding shared libraries

[Andrew Haley <aph at redhat dot com>]

This is about DWARF 2 unwinding through shared libraries.

As far as I can see, on current x86 GNU/Linux targets we don't call
__register_frame_info() or __register_frame_info_bases() from the
library startup code in crtbegin.  There is code to do it, but it's
disabled.

When _Unwind_Find_FDE() is called during unwinding, it first calls
_Unwind_Find_registered_FDE(), which promptly returns because nothing
has ever been registered.  Then, _Unwind_Find_FDE() calls
dl_iterate_phdr (_Unwind_IteratePhdrCallback, ...) to iterate over
every shared library looking for a PC address.

This is a linear search, so if you have many shared libraries open
this can be pretty slow.

So, now for my question: why do we not call __register_frame_info() or
__register_frame_info_bases() ?  We'd avoid a great many trips through
dl_iterate_phdr () and _Unwind_IteratePhdrCallback().

Thanks,
Andrew.