Re: [5/6][PATCH] Perform second link stage and ignore now-obsolete linker -pass-through= option.

["Rafael Ávila de Espíndola" <respindola@mozilla.com>]

On 2011-02-25 19:46, Dave Korn wrote:
> 
>     Hi list,
> 
>   This is the second main functional change, and it's really the only way to
> resolve the problems caused by discrepancies between the initial set of
> symbols returned by the plugin based on the LTO symtabs in the IR files, and
> the actual set of symbols used and defined by the eventual object file(s)
> added to the link after LTRANS.  It's largely the same approach as HJ's
> 2-stage link, except that it closes and reopens the existing input BFDs in
> place, rather than adding a second set of input statements.  This results in
> some different behaviour between HJ's linker and this patched version.
> 
> 
> ld/ChangeLog:
> 
> 2011-02-20  Dave Korn  <...
> 
> 	PR ld/12365
> 	* ldcref.c (cref_hash_table_free): New function.
> 	* ld.h (cref_hash_table_free): Add prototype.
> 	* ldlang.c (lang_gc_sections): Dont de-exclude claimed file sections.
> 	(set_exclude): New function.
> 	(reopen_inputs): Likewise.  Walk list of input objects, excluding
> 	claimed (IR-only) files and archive members, then re-walk list, closing
> 	and re-opening and re-adding the symbols from objects and libs.
> 	(lang_process): After opening plugin-supplied objects and scanning
> 	their library dependencies, tear down existing link, cref and
> 	already-linked-section hashes, erase link_info input bfds list, finally
> 	call reopen_inputs.
> 	* plugin.c (plugin_opt_plugin_arg): Discard any instances of the
> 	now-obsolete "-pass-through=" option if found.
> 
>   I know Ian would rather avoid this approach, but I don't think that there's
> any other way to do LTO in LD without either rearchitecting BFD quite a bit.
> We can't do symbol resolution without adding symbols from the LTO symtabs into
> the linker hash table, but then after LTRANS we may have a different final set
> of symbols present.  There's no way to excise individual symbols from the
> linker hash table in BFD, so you have to achieve the same effect by tearing
> down the symbol table and rebuilding it without adding the unwanted symbols at
> all second time round.  That means you have to close and reopen all the BFDs,
> because BFD backends cache pointers to hash table entries in the bfd's private
> data, which are now stale once you've built a new symbol table.  I can't see
> any way around having to re-do at least this amount of work, but without it
> we'll get bogus symbols in our final outputs, which is a real correctness issue.

Ian,

Assuming this is ok for bfd, would a patch implementing the same logic
be ok for gold? It would be bad to have different interfaces in bfd ld
and gold.

>     cheers,
>       DaveK
> 

Cheers,
Rafael