Re: x86-64 large data sections updated

["H. J. Lu" <hjl at lucon dot org>]

On Wed, Jun 15, 2005 at 12:13:15AM +0200, Jan Hubicka wrote:
> Index: bfd/elf64-x86-64.c
> ===================================================================
> RCS file: /cvs/src/src/bfd/elf64-x86-64.c,v
> retrieving revision 1.95
> diff -c -3 -p -r1.95 elf64-x86-64.c
> *** bfd/elf64-x86-64.c	17 May 2005 16:43:02 -0000	1.95
> --- bfd/elf64-x86-64.c	14 Jun 2005 21:50:20 -0000
> *************** elf64_x86_64_check_relocs (bfd *abfd, st
> *** 975,980 ****
> --- 991,1028 ----
>     return TRUE;
>   }
>   
> + /* Hook called by the linker routine which adds symbols from an object
> +    file.  We use it to put .comm items in .lbss, and not .bss.  */
> + 
> + static bfd_boolean
> + elf64_x86_64_add_symbol_hook (bfd *abfd, struct bfd_link_info *info ATTRIBUTE_UNUSED,
> + 			      Elf_Internal_Sym *sym, const char **namep ATTRIBUTE_UNUSED,
> + 			      flagword *flagsp ATTRIBUTE_UNUSED,
> + 			      asection **secp, bfd_vma *valp)
> + {
> +   if (sym->st_shndx == SHN_X86_64_LCOMMON)
> +     {
> + 
> +       asection *lcomm = bfd_get_section_by_name (abfd, ".lbss");
> + 
> +       if (lcomm == NULL)
> + 	{
> + 	  lcomm = bfd_make_section (abfd, ".lbss");
> + 	  if (lcomm == NULL
> + 	      || !bfd_set_section_flags (abfd, lcomm, (SEC_ALLOC
> + 						       | SEC_IS_COMMON
> + 						       | SEC_LINKER_CREATED)))
> + 	    return FALSE;
> + 	}
> + 
> +       *secp = lcomm;
> +       *valp = sym->st_size;
> +     }
> + 
> +   return TRUE;
> + }
> + 
> + 

There are 2 problems:

1. You can't use .lbss section index in symbol table for
SHN_X86_64_LCOMMON. They are different. I don't think you can
set SEC_IS_COMMON for .lbss.
2. I don't think like creating a real section for common symbol. I know
MIPS does it this way. But it doesn't make it right. There is
SHN_X86_64_LCOMMON. It should be used in symbol table.


>   /* Return the section that should be marked against GC for a given
>      relocation.	*/
>   
> *************** elf64_x86_64_section_from_shdr (bfd *abf
> *** 2839,2844 ****
> --- 2912,2985 ----
>     return TRUE;
>   }
>   
> + /* Given a BFD section, try to locate the corresponding ELF section
> +    index.  This is used by both the 32-bit and the 64-bit ABI.
> +    Actually, it's not clear to me that the 64-bit ABI supports these,
> +    but for non-PIC objects we will certainly want support for at least
> +    the .scommon section.  */
> + 
> + static bfd_boolean
> + elf64_x86_64_elf_section_from_bfd_section (bfd *abfd ATTRIBUTE_UNUSED,
> + 					asection *sec, int *retval)
> + {
> +   if (strcmp (bfd_get_section_name (abfd, sec), ".lcommon") == 0
> +       || strcmp (bfd_get_section_name (abfd, sec), ".lbss") == 0)
> +     {
> +       *retval = SHN_X86_64_LCOMMON;
> +       return TRUE;
> +     }
> +   return FALSE;
> + }

There is no such a section .lcommon. SHN_X86_64_LCOMMON is not a
special section index which doesn't have a real section.

> Index: ld/scripttempl/elf.sc
> ===================================================================
> RCS file: /cvs/src/src/ld/scripttempl/elf.sc,v
> retrieving revision 1.59
> diff -c -3 -p -r1.59 elf.sc
> *** ld/scripttempl/elf.sc	10 Jun 2005 00:39:56 -0000	1.59
> --- ld/scripttempl/elf.sc	14 Jun 2005 21:50:21 -0000
> ***************
> *** 78,83 ****
> --- 78,85 ----
>   #  .debug_info	.gnu.linkonce.wi.foo
>   #  .tdata	.gnu.linkonce.td.foo
>   #  .tbss	.gnu.linkonce.tb.foo
> + #  .ldata	.gnu.linkonce.l.foo
> + #  .lbss	.gnu.linkonce.lb.foo

You missed .lrodata/.gnu.linkonce.lr.foo.

>   #
>   #  Each of these can also have corresponding .rel.* and .rela.* sections.
>   
> *************** if test -z "${SDATA_GOT}"; then
> *** 157,162 ****
> --- 159,189 ----
>       SDATA_GOT=" "
>     fi
>   fi
> + if test -z "${NO_LARGE_DATA}"; then
> +   LBSS=".lbss         ${RELOCATING-0} :
> +   {
> +     ${RELOCATING+PROVIDE (__lbss_start = .);}
> +     ${RELOCATING+PROVIDE (___lbss_start = .);}
> +     *(.dynlbss)
> +     *(.lbss${RELOCATING+ .lbss.* .gnu.linkonce.lb.*})
> +     *(.lcommon)
> +     ${RELOCATING+PROVIDE (__lbss_end = .);}
> +     ${RELOCATING+PROVIDE (___lbss_end = .);}
> +   }"
> +   LDATA="
> +   .ldata        ${RELOCATING-0} : 
> +   {
> +     ${RELOCATING+${LDATA_START_SYMBOLS}}
> +     *(.ldata${RELOCATING+ .ldata.* .gnu.linkonce.l.*})
> +   }"
> +   LRODATA=".lrodata       ${RELOCATING-0} : { *(.lrodata${RELOCATING+ .lrodata.* .gnu.linkonce.lr.*}) }"
> +   REL_LDATA=".rel.ldata    ${RELOCATING-0} : { *(.rel.ldata${RELOCATING+ .rel.ldata.* .rel.gnu.linkonce.l.*}) }
> +   .rela.ldata   ${RELOCATING-0} : { *(.rela.ldata${RELOCATING+ .rela.ldata.* .rela.gnu.linkonce.l.*}) }"
> +   REL_LBSS=".rel.lbss     ${RELOCATING-0} : { *(.rel.lbss${RELOCATING+ .rel.lbss.* .rel.gnu.linkonce.lb.*}) }
> +   .rela.lbss    ${RELOCATING-0} : { *(.rela.lbss${RELOCATING+ .rela.lbss.* .rela.gnu.linkonce.lb.*}) }"

You need REL_LRODATA.

> + else
> +   NO_LARGE_DATA=" "
> + fi
>   test -n "$SEPARATE_GOTPLT" && SEPARATE_GOTPLT=" "
>   CTOR=".ctors        ${CONSTRUCTING-0} : 
>     {
> *************** eval $COMBRELOCCAT <<EOF
> *** 274,279 ****
> --- 301,310 ----
>     ${REL_SBSS2}
>     .rel.bss      ${RELOCATING-0} : { *(.rel.bss${RELOCATING+ .rel.bss.* .rel.gnu.linkonce.b.*}) }
>     .rela.bss     ${RELOCATING-0} : { *(.rela.bss${RELOCATING+ .rela.bss.* .rela.gnu.linkonce.b.*}) }
> +   ${REL_LDATA}
> +   ${REL_LBSS}
> +   ${REL_LDATA2}
> +   ${REL_LBSS2}

What are REL_LDATA2 and REL_LBSS2? You need REL_RODATA.

>   EOF
>   if [ -n "$COMBRELOC" ]; then
>   cat <<EOF
> *************** cat <<EOF
> *** 401,406 ****
> --- 432,441 ----
>      ${RELOCATING+. = ALIGN(. != 0 ? ${ALIGNMENT} : 1);}
>     }
>     ${OTHER_BSS_SECTIONS}
> +   ${LBSS}
> +   ${LDATA}
> +   ${RELOCATING+. = ALIGN(${ALIGNMENT});}
> +   ${LRODATA}
>     ${RELOCATING+. = ALIGN(${ALIGNMENT});}
>     ${RELOCATING+_end = .;}
>     ${RELOCATING+${OTHER_BSS_END_SYMBOLS}}

I think aligment can use some tuning. You don't need to pad the .bss
section if there is a large data section after it. Also LRODATA may
need to be properly page aligned.


H.J.