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Re: Relocation to discarded sections from DWARF

Hi Andrew,

I am currently trying to extend bfd to help me with a problem I
encounter when using -ffunction-sections to gcc, and --gc-sections to
the linker.

When creating DWARF debugging information in an ELF references from the
debugging information to the code are usually created as relocations.
When sections are removed due to --gc-sections this can sometimes result
in invalid debugging information being created.

Hmm - it seems to me that this problem would go away if gcc emitted the debug information in function specific sections as well. Ie when -ffunction-sections is enabled, instead of gcc generating just one .debug_line section containing all of the line number information it could create a series of .debug_line.{func_name} sections. Then in the linker when --gc-sections discards the relocations would be noticed and that section discarded as well. Simples.

So, am I right to think what I am trying to achieve is a similar to the
handling of eh_frame/stabs?


Next, I tried to solve this by hooking into the backend-data
elf_backend_discard_info function for my target.

This strikes me as being a bad move. Discarding irrelevant line data information is a generic (to ELF/DWARF) issue, not a target specific issue. Instead I think that the code ought to go into bfd/elflink.c and/or bfd/dwarf2.c.

Looking through bfd_elf_discard_info, and the other users of
elf_backend_discard_info I can't see any examples of how to remove
relocations / content from the input section at this point. Is this the
right stage to be doing such things?

Look at the linker relaxing code used by various backends. (Eg: bfd/elf-m10300.c:mn10300_elf_relax_delete_bytes and mn10300_elf_relax_section). Relocations are usually "removed" by changing their type to BFD_RELOC_NONE.

Cheers Nick

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