Re: [PATCH] LoongArch: Use copy relocation for %pc_lo12 against external symbol

[liuzhensong <liuzhensong@loongson.cn>]

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在 2022/8/31 下午9:22, Xi Ruoyao 写道:
> We'd like to use PC-relative addressing instead of GOT for external
> symbols in main executable images.  This can improve code locality.
> Doing so will need to implement copy relocation.
>
> Despite there was a comment saying "Glibc does not support copy
> relocation yet", I've run a very simple test and it seems copy
> relocation is handled by ld.so without a problem:
>
> 	pcalau12i       $a1, %pc_hi20(stdout)
> 	ld.d            $a1, $a1, %pc_lo12(stdout)
> 	pcalau12i       $a0, %pc_hi20(msg)
> 	addi.d          $a0, $a0, %pc_lo12(msg)
> 	pcalau12i       $ra, %pc_hi20(fputs)
> 	jirl            $ra, $ra, %pc_lo12(fputs)
>
> With this patch, R_LARCH_COPY is correctly emitted for "stdout" in
> Glibc, and the test program runs and outputs "Hello world" in "msg"
> successfully.
> ---
>   bfd/elfnn-loongarch.c | 93 ++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 84 insertions(+), 9 deletions(-)
>
> diff --git a/bfd/elfnn-loongarch.c b/bfd/elfnn-loongarch.c
> index ed42b8b6770..0cf24bd24d8 100644
> --- a/bfd/elfnn-loongarch.c
> +++ b/bfd/elfnn-loongarch.c
> @@ -743,14 +743,26 @@ loongarch_elf_check_relocs (bfd *abfd, struct bfd_link_info *info,
>   	    h->non_got_ref = 1;
>   	  break;
>   
> -	case R_LARCH_PCALA_HI20:
> +	case R_LARCH_PCALA_LO12:
>   	  if (h != NULL)
>   	    {
> -	      /* For pcalau12i + jirl.  */
> -	      h->needs_plt = 1;
> -	      if (h->plt.refcount < 0)
> -		h->plt.refcount = 0;
> -	      h->plt.refcount++;
> +	      /* Check if it's a jirl instruction.  */
> +	      bfd_byte *contents = elf_section_data (sec)->this_hdr.contents;
> +	      uint32_t insn = 0;
> +
> +	      if (contents || bfd_malloc_and_get_section (abfd, sec, &contents))
> +		memcpy(&insn, contents + rel->r_offset, sizeof(insn));
> +
> +	      if ((insn & 0xfc000000) == 0x4c000000)
> +		{
> +		  /* For pcalau12i + jirl.  */
> +		  h->needs_plt = 1;
> +		  if (h->plt.refcount < 0)
> +		    h->plt.refcount = 0;
> +		  h->plt.refcount++;
> +		}
> +	      else
> +		need_dynreloc = 1;
>   
>   	      h->non_got_ref = 1;
>   	      h->pointer_equality_needed = 1;
> @@ -949,7 +961,9 @@ loongarch_elf_adjust_dynamic_symbol (struct bfd_link_info *info,
>   				     struct elf_link_hash_entry *h)
>   {
>     struct loongarch_elf_link_hash_table *htab;
> +  struct loongarch_elf_link_hash_entry *hent;
>     bfd *dynobj;
> +  asection *s, *srel;
>   
>     htab = loongarch_elf_hash_table (info);
>     BFD_ASSERT (htab != NULL);
> @@ -999,9 +1013,52 @@ loongarch_elf_adjust_dynamic_symbol (struct bfd_link_info *info,
>         return true;
>       }
>   
> -  /* R_LARCH_COPY is not adept glibc, not to generate.  */
> -  /* Can not print anything, because make check ld.  */
> -  return true;
> +  /* If copy relocations is disabled via -z nocopyreloc, or we don't find any
> +     dynamic relocs in read-only sections, avoid the copy reloc.  */
> +  if (info->nocopyreloc || !_bfd_elf_readonly_dynrelocs (h))
> +    h->non_got_ref = 0;
> +
> +  if (!h->non_got_ref)
> +    return true;
> +
> +  /* We must allocate the symbol in our .dynbss section, which will
> +     become part of the .bss section of the executable.  There will be
> +     an entry for this symbol in the .dynsym section.  The dynamic
> +     object will contain position independent code, so all references
> +     from the dynamic object to this symbol will go through the global
> +     offset table.  The dynamic linker will use the .dynsym entry to
> +     determine the address it must put in the global offset table, so
> +     both the dynamic object and the regular object will refer to the
> +     same memory location for the variable.  */
> +
> +  /* Generate a copy relocation to tell the dynamic linker to copy the
> +     initial value out of the dynamic object and into the runtime process
> +     image.  We need to remember the offset into the .rel.bss section we
> +     are going to use.  */
> +  hent = (struct loongarch_elf_link_hash_entry *) h;
> +  if (hent->tls_type & ~GOT_NORMAL)
> +    {
> +      s = htab->sdyntdata;
> +      srel = htab->elf.srelbss;
> +    }
> +  else if (h->root.u.def.section->flags & SEC_READONLY)
> +    {
> +      s = htab->elf.sdynrelro;
> +      srel = htab->elf.sreldynrelro;
> +    }
> +  else
> +    {
> +      s = htab->elf.sdynbss;
> +      srel = htab->elf.srelbss;
> +    }
> +
> +  if ((h->root.u.def.section->flags & SEC_ALLOC) != 0 && h->size != 0)
> +    {
> +      srel->size += sizeof (ElfNN_External_Rela);
> +      h->needs_copy = 1;
> +    }
> +
> +  return _bfd_elf_adjust_dynamic_copy (info, h, s);
>   }
>   
>   /* Allocate space in .plt, .got and associated reloc sections for
> @@ -3702,6 +3759,24 @@ loongarch_elf_finish_dynamic_symbol (bfd *output_bfd,
>         loongarch_elf_append_rela (output_bfd, srela, &rela);
>       }
>   
> +  if (h->needs_copy)
> +    {
> +      Elf_Internal_Rela rela;
> +      asection *s;
> +
> +      /* This symbols needs a copy reloc.  Set it up.  */
> +      BFD_ASSERT (h->dynindx != -1);
> +
> +      rela.r_offset = sec_addr (h->root.u.def.section) + h->root.u.def.value;
> +      rela.r_info = ELFNN_R_INFO (h->dynindx, R_LARCH_COPY);
> +      rela.r_addend = 0;
> +      if (h->root.u.def.section == htab->elf.sdynrelro)
> +	s = htab->elf.sreldynrelro;
> +      else
> +	s = htab->elf.srelbss;
> +      loongarch_elf_append_rela (output_bfd, s, &rela);
> +    }
> +
>     /* Mark some specially defined symbols as absolute.  */
>     if (h == htab->elf.hdynamic || h == htab->elf.hgot || h == htab->elf.hplt)
>       sym->st_shndx = SHN_ABS;

The R_LARCH_COPY is supported in older versions,we removed this feature 
and are not going to support.