[PATCH,plugins,head+2.21] Handle COMDAT groups on ELF.

[Dave Korn]

    Hi again,

  One of the advantages of the two-stage-link tear-down-and-rebuild approach
was that we got COMDAT group selection correct for free, as a consequence of
resetting the already-linked table.  Since we're not doing that now, we have
to handle COMDAT in the linker plugin code by creating synthetic .group
sections in the dummy IR BFD, but making sure that these sections don't
actually win in group selection decisions.

  That's what this patch implements, and on x86_64-linux, with this, we now
get a completely clean GCC testrun (entire c-family and fortran), which brings
ld.bfd into parity with ld.gold.  :-)

bfd/ChangeLog:

2011-05-07  Dave Korn  <dave.korn.cygwin@...

	* elflink.c (elf_link_add_object_symbols): Don't mark symbols undef
	when the kept section comes from a plugin BFD that we expect to be
	replaced later.
	(_bfd_elf_section_already_linked): When there is a duplicate section,
	but the initial kept section comes from a plugin BFD and the newer
	duplicate section does not, keep the newer one and discard the initial
	kept section.

ld/ChangeLog:

	* plugin.c (struct group_sec_hash_table): New derived hash table type.
	(struct group_sec_hash_entry): Derived entry type for the above.
	(group_sec_hash_newfunc): Derived ctor for the table entries creates
	an ELF comdat group section with the given signature.
	(init_group_sec_hash_table): Derived ctor for the table.
	(group_sec_hash_lookup): Derived lookup function.
	(group_sec_hash_traverse): Derived traverse function.
	(group_sec_hash_table_free): Derived dtor.
	(struct add_syms_info): Helper struct to wrap args to callback.
	(get_elf_comdat_section): New helper function for ELF targets.
	(get_comdat_section): Likewise for all targets.
	(asymbol_from_plugin_symbol): Take an add_syms_info rather than a
	bfd pointer argument and adjust usage accordingly.  Generate comdat
	sections or groups for defined symbols with non-NULL comdat_key.
	(add_symbols):  Wrap parameters to pass down in an add_syms_info, and
	handle ELF comdat group section creation.

  Tested on x86_64-unknown-linux-gnu as mentioned.  I'm going to start running
a GCC testsuite on i686-pc-cygwin, but that could take a couple of days for
the final results to come in.  If this patch is OK, I'll wait until that's got
far enough to be sure it's working before I commit.

  Any comments on the implementation, while we're waiting?  Or OK as it is?

    cheers,
      DaveK

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