[PATCH][LTO] Fixup gold/linker-plugin symbol resolution

[Richard Guenther]

It seems that the symbol resolution gold and/or the linker-plugin
provides for commons is completely broken.  It also seems that the
linker-plugin code is now unmaintained (or rather that noone
understanding where the resolution comes from -- I only can
see a single assignment to it, entry->resolution = LDPR_UNKNOWN;,
wants to or is able to look into this bug).

Thus, the following papers over the problem.  A different patch
would just dump the gold symbol resolution and would reset it
all to LDPR_UNKNOWN at the very same spot.

Well.  I'm going to bootstrap and test the following variant
(but of course it ain't excercised in that cycle, we cannot
have -fuse-linker-plugin testsuite entries as we do not have
a testsuite support for detecting linker-plugin support).

Anyway.  Here goes the patch - any preferences regarding the
fix?  Any help from gold / linker-plugin people?  Please?
Where does the resolution come from?  Why is there no
prevailing definition for commons?  Shouldn't the largest
entry win here?

Thanks,
Richard.

2010-03-31  Richard Guenther  <rguenther@suse.de>

	PR lto/42757
	* lto-symtab.c (lto_symtab_resolve_symbols): Fixup broken
	gold/linker-plugin symbol resolution for commons.

Index: gcc/lto-symtab.c
===================================================================
*** gcc/lto-symtab.c	(revision 157854)
--- gcc/lto-symtab.c	(working copy)
*************** lto_symtab_resolve_symbols (void **slot)
*** 410,418 ****
  
    e = (lto_symtab_entry_t) *slot;
  
!   /* If the chain is already resolved there is nothing else to do.  */
!   if (e->resolution != LDPR_UNKNOWN)
!     return;
  
    /* Find the single non-replaceable prevailing symbol and
       diagnose ODR violations.  */
--- 410,441 ----
  
    e = (lto_symtab_entry_t) *slot;
  
!   /* If the chain is already resolved there is nothing else to do.
!      In theory.  But gold and/or the linker-plugin is seriously broken
!      with respect to commons, so try to fixup here.  See PR42757.  */
!   if (e->resolution != LDPR_UNKNOWN
!       && TREE_CODE (e->decl) == VAR_DECL)
!     {
!       bool has_can_prevailing_entry = false;
!       for (; e; e = e->next)
! 	{
! 	  if (e->resolution == LDPR_PREVAILING_DEF_IRONLY
! 	      || e->resolution == LDPR_PREVAILING_DEF)
! 	    return;
! 	  if (lto_symtab_resolve_can_prevail_p (e))
! 	    has_can_prevailing_entry = true;
! 	}
!       if (has_can_prevailing_entry)
! 	for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
! 	  if (lto_symtab_resolve_can_prevail_p (e))
! 	    {
! 	      if (TREE_READONLY (e->decl))
! 		e->resolution = LDPR_PREVAILING_DEF_IRONLY;
! 	      else
! 		e->resolution = LDPR_PREVAILING_DEF;
! 	    }
!       return;
!     }
  
    /* Find the single non-replaceable prevailing symbol and
       diagnose ODR violations.  */