PATCH: 2 stage BFD linker for LTO plugin

[Richard Guenther]

On Wed, Jan 19, 2011 at 1:27 AM, Ian Lance Taylor <iant@google.com> wrote:
> After some discussion on IRC, here is another approach to resolving the
> issue with static linking and LTO.
>
> In this approach, the linker keeps track of all archives found after the
> first file claimed by the plugin.  If the plugin adds any object files,
> and the object files refer to any symbols which are not yet defined,
> then the linker will scan all the saved archives, in order, for a
> definition of the symbol.  If a definition is found, the linker will
> pull in the appropriate object from the archive.  If that object, in
> turn, has any undefined symbols, the linker will pull in the appropriate
> object from that archive or any later ones, and so forth.  The linker
> will honor --start-group/--end-group while rescanning.
>
> This should ensure that any previously unseen undefined symbols
> introduced by the compiler are handled correctly.  I think it is
> appropriate to do this unconditionally when using plugins, as there is
> no other reasonable way to handle undefined symbols in a file added by a
> plugin.
>
> I've appended a patch to gold which implements this approach.  The patch
> is reasonably efficient and introduces no unnecessary file I/O.  With
> this patch to gold, and no change to gcc, the problematic -static test
> cases which I know about pass.  Also all the current lto.exp tests pass.
> All those tests also pass if I edit the gcc LTO plugin to ignore the
> -pass-through option, as that option is not necessary when the linker
> implements this approach.
>
> As this patch does not require any changes to gcc, and fixes some cases
> which are clearly bugs, I plan to commit this patch to binutils mainline
> and to binutils 2.21 branch after a few days if I don't hear any
> comments.

Nice.  Can we on the GCC side somehow identify Gold versions
that support this and avoid -pass-through handling in that case?
I'm not sure if with your patch the add_input_library or
add_input_file plugin hooks are completely useless (and thus
gold could simply ignore those at all).

Richard.

> Ian
>
>
> 2011-01-18  Ian Lance Taylor  <iant@google.com>
>
>        * plugin.cc (class Plugin_rescan): Define new class.
>        (Plugin_manager::claim_file): Set any_claimed_.
>        (Plugin_manager::save_archive): New function.
>        (Plugin_manager::save_input_group): New function.
>        (Plugin_manager::all_symbols_read): Create Plugin_rescan task if
>        necessary.
>        (Plugin_manager::new_undefined_symbol): New function.
>        (Plugin_manager::rescan): New function.
>        (Plugin_manager::rescannable_defines): New function.
>        (Plugin_manager::add_input_file): Set any_added_.
>        * plugin.h (class Plugin_manager): define new fields rescannable_,
>        undefined_symbols_, any_claimed_, and any_added_.  Declare
>        Plugin_rescan as friend.  Declare new functions.
>        (Plugin_manager::Rescannable): Define type.
>        (Plugin_manager::Rescannable_list): Define type.
>        (Plugin_manager::Undefined_symbol_list): Define type.
>        (Plugin_manager::Plugin_manager): Initialize new fields.
>        * archive.cc (Archive::defines_symbol): New function.
>        (Add_archive_symbols::run): Pass archive to plugins if any.
>        * archive.h (class Archive): Declare defines_symbol.
>        * readsyms.cc (Input_group::~Input_group): New function.
>        (Finish_group::run): Pass input_group to plugins if any.
>        * readsyms.h (class Input_group): Declare destructor.
>        * symtab.cc (add_from_object): Pass undefined symbol to plugins if
>        any.
>
>
>