Re: Using DW_TAG_imported_unit for eliminating discarded section DWARF info

[Tom Tromey <tromey at redhat dot com>]

>>>>> "Senthil" == Senthil Kumar Selvaraj <senthil_kumar.selvaraj@atmel.com> writes:

Senthil>   The DWARF 4 standard (section E.3.3) suggests using
Senthil>   DW_TAG_imported_unit (together with section groups) to
Senthil>   generate per function DWARF information in a way that's
Senthil>   suitable for the linker to discard unused functions. My
Senthil>   understanding of the standard is that there is one CU
Senthil>   containing a DW_TAG_imported_unit generated for each
Senthil>   subprogram, with the DW_AT_import attribute pointing to the
Senthil>   "main" CU that contains file level declarations and
Senthil>   definitions.

Yeah, I agree, DWARF says that is possible.

Senthil>   However, I don't see gdb supporting usage of the tag for this
Senthil>   purpose - I only see it being used for referring *to* other
Senthil>   units from a DW_TAG_compile_unit, not the other way
Senthil>   around. Is my finding correct, or am I missing something?

We implemented the current scheme because it fits what "dwz" emits.  At
the time nothing else emitted DW_TAG_imported_unit, and I did not want
to implement something I had no reasonable way to test.

Senthil>   FWIW, I managed to hack GCC into generating the following
Senthil>   DWARF output if -ffunction-sections is turned on. gdb treats
Senthil>   the CUs at offset 0x31 and 0x66 as full fledged CUs though,
Senthil>   and eventually crashes when trying to resolve main
Senthil>   (psymtab->symtab = NULL for the CU containing main).

I think it's fine to implement this if you want.

Tom