elf_gc_sections and relocatable sections

[Earl Chew <earl_chew at agilent dot com>]

I'm seeking some guidance here with binutils ld 2.15.91.0.2 and
why elf_gc_sections() disables garbage collection for relocateable
output.

I've previously been using binutils ld 2.10.1 in a VxWorks
environment where we build relocateable executables that can
be downloaded to a VxWorks target.

I posted:

http://sources.redhat.com/ml/binutils/2001-07/msg00260.html

and followed up with a private email exchange with
Richard Henderson.

I established that -gc-sections wasn't really incompatible
with -r, provided that there is one undefined symbol introduced
at the command line (-Umain) to establish a starting point for
the link. I made minor changes to 2.10.1 to enable --gc-sections
and -r on ELF and we've used this successfully for at least the
last two years.

Now at vesion 2.15.91.0.2 I see that things have changed since
2.10.1.

The code fragment at elf_gc_sections() is key to resurrecting my
patch with 2.15.91.0.2:

>   if (!get_elf_backend_data (abfd)->can_gc_sections
>       || info->relocatable
>       || info->emitrelocations
>       || info->shared
>       || !is_elf_hash_table (info->hash))
>     {
>       (*_bfd_error_handler)(_("Warning: gc-sections option ignored"));
>       return TRUE;
>     }

Can anyone shed any light as to why elf_gc_sections() cannot
proceed if info->relocatable or info->emitrelocations is set?

Is there something in the elf garbage collection algorithm or
implementation that causes it to fail under these circumstances?

ld already contains a guard to prevent -r and --gc-sections, so
in a sense this part of the guard in elf_gc_sections() duplicates
that check.

Earl