Re: PowerpC pointer_equality_needed optimisation

["H.J. Lu" <hjl dot tools at gmail dot com>]

Hi Alan,

Is this related to

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32219

Does the testcase work with PIE on PPC?

Thanks.

H.J.
On Fri, Feb 22, 2008 at 5:52 PM, Alan Modra <amodra@bigpond.net.au> wrote:
> This implements Jakub's pointer_equality_needed optimisation for
>  PowerPC.  x86 and x86_64 maintainers might like to copy the warning
>  I added for weak symbols in ppc_elf_finish_dynamic_symbol.
>
>         * elf32-ppc.c (ppc_elf_copy_indirect_symbol): Copy
>         pointer_equality_needed.
>         (ppc_elf_check_relocs): Split out non-branch relocs from others
>         that might emit dynamic relocs.  Set pointer_equality_needed
>         for their symbols.  Don't set non_got_ref on branch reloc symbols.
>         (ppc_elf_hash_symbol): New function.
>         (elf_backend_hash_symbol): Define.
>         (ppc_elf_finish_dynamic_symbol): Handle pointer_equality_needed.
>         Error if pointer_equality_needed on weak plt symbol.
>
>  Index: bfd/elf32-ppc.c
>  ===================================================================
>  RCS file: /cvs/src/src/bfd/elf32-ppc.c,v
>  retrieving revision 1.229
>  diff -u -p -r1.229 elf32-ppc.c
>  --- bfd/elf32-ppc.c     20 Feb 2008 17:42:35 -0000      1.229
>  +++ bfd/elf32-ppc.c     23 Feb 2008 01:41:44 -0000
>  @@ -2696,6 +2696,7 @@ ppc_elf_copy_indirect_symbol (struct bfd
>    edir->elf.ref_regular |= eind->elf.ref_regular;
>    edir->elf.ref_regular_nonweak |= eind->elf.ref_regular_nonweak;
>    edir->elf.needs_plt |= eind->elf.needs_plt;
>  +  edir->elf.pointer_equality_needed |= eind->elf.pointer_equality_needed;
>
>    /* If we were called to copy over info for a weak sym, that's all.  */
>    if (eind->elf.root.type != bfd_link_hash_indirect)
>  @@ -3388,6 +3389,26 @@ ppc_elf_check_relocs (bfd *abfd,
>             info->flags |= DF_STATIC_TLS;
>           goto dodyn;
>
>  +       case R_PPC_ADDR32:
>  +       case R_PPC_ADDR16:
>  +       case R_PPC_ADDR16_LO:
>  +       case R_PPC_ADDR16_HI:
>  +       case R_PPC_ADDR16_HA:
>  +       case R_PPC_UADDR32:
>  +       case R_PPC_UADDR16:
>  +         if (h != NULL && !info->shared)
>  +           {
>  +             /* We may need a plt entry if the symbol turns out to be
>  +                a function defined in a dynamic object.  */
>  +             if (!update_plt_info (abfd, h, NULL, 0))
>  +               return FALSE;
>  +
>  +             /* We may need a copy reloc too.  */
>  +             h->non_got_ref = 1;
>  +             h->pointer_equality_needed = 1;
>  +           }
>  +         goto dodyn;
>  +
>         case R_PPC_REL32:
>           if (h == NULL
>               && got2 != NULL
>  @@ -3432,17 +3453,10 @@ ppc_elf_check_relocs (bfd *abfd,
>             }
>           /* fall through */
>
>  -       case R_PPC_ADDR32:
>         case R_PPC_ADDR24:
>  -       case R_PPC_ADDR16:
>  -       case R_PPC_ADDR16_LO:
>  -       case R_PPC_ADDR16_HI:
>  -       case R_PPC_ADDR16_HA:
>         case R_PPC_ADDR14:
>         case R_PPC_ADDR14_BRTAKEN:
>         case R_PPC_ADDR14_BRNTAKEN:
>  -       case R_PPC_UADDR32:
>  -       case R_PPC_UADDR16:
>         dodyn1:
>           if (h != NULL && !info->shared)
>             {
>  @@ -3450,9 +3464,6 @@ ppc_elf_check_relocs (bfd *abfd,
>                  a function defined in a dynamic object.  */
>               if (!update_plt_info (abfd, h, NULL, 0))
>                 return FALSE;
>  -
>  -             /* We may need a copy reloc too.  */
>  -             h->non_got_ref = 1;
>             }
>
>         dodyn:
>  @@ -5186,6 +5197,20 @@ ppc_elf_size_dynamic_sections (bfd *outp
>
>    return TRUE;
>   }
>  +
>  +/* Return TRUE if symbol should be hashed in the `.gnu.hash' section.  */
>  +
>  +static bfd_boolean
>  +ppc_elf_hash_symbol (struct elf_link_hash_entry *h)
>  +{
>  +  if (h->plt.plist != NULL
>  +      && !h->def_regular
>  +      && (!h->pointer_equality_needed
>  +         || !h->ref_regular_nonweak))
>  +    return FALSE;
>  +
>  +  return _bfd_elf_hash_symbol (h);
>  +}
>
>   #define ARRAY_SIZE(a) (sizeof (a) / sizeof ((a)[0]))
>
>  @@ -7127,15 +7152,28 @@ ppc_elf_finish_dynamic_symbol (bfd *outp
>
>             if (!h->def_regular)
>               {
>  -               /* Mark the symbol as undefined, rather than as defined in
>  -                  the .plt section.  Leave the value alone.  */
>  +               /* Mark the symbol as undefined, rather than as
>  +                  defined in the .plt section.  Leave the value if
>  +                  there were any relocations where pointer equality
>  +                  matters (this is a clue for the dynamic linker, to
>  +                  make function pointer comparisons work between an
>  +                  application and shared library), otherwise set it
>  +                  to zero.  */
>                 sym->st_shndx = SHN_UNDEF;
>  -               /* If the symbol is weak, we do need to clear the value.
>  -                  Otherwise, the PLT entry would provide a definition for
>  -                  the symbol even if the symbol wasn't defined anywhere,
>  -                  and so the symbol would never be NULL.  */
>  -               if (!h->ref_regular_nonweak)
>  +               if (!h->pointer_equality_needed)
>                   sym->st_value = 0;
>  +               else if (!h->ref_regular_nonweak)
>  +                 {
>  +                   /* Choose your poison.  We must have either text
>  +                      dynamic relocations, broken function pointer
>  +                      comparisons, or broken tests for a NULL
>  +                      function pointer.  */
>  +                   (*_bfd_error_handler)
>  +                     (_("weak reference to %s in non-pic code"
>  +                        " will break function pointer comparisons"),
>  +                      h->root.root.string);
>  +                   sym->st_value = 0;
>  +                 }
>               }
>             doneone = TRUE;
>           }
>  @@ -7687,6 +7725,7 @@ ppc_elf_finish_dynamic_sections (bfd *ou
>   #define elf_backend_adjust_dynamic_symbol      ppc_elf_adjust_dynamic_symbol
>   #define elf_backend_add_symbol_hook            ppc_elf_add_symbol_hook
>   #define elf_backend_size_dynamic_sections      ppc_elf_size_dynamic_sections
>  +#define elf_backend_hash_symbol                        ppc_elf_hash_symbol
>   #define elf_backend_finish_dynamic_symbol      ppc_elf_finish_dynamic_symbol
>   #define elf_backend_finish_dynamic_sections    ppc_elf_finish_dynamic_sections
>   #define elf_backend_fake_sections              ppc_elf_fake_sections
>
>  --
>  Alan Modra
>  Australia Development Lab, IBM
>