Re: [PATCH] S/390: Fix two issues with the IFUNC optimized mem* routines

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

On Thu, Aug 30, 2012 at 6:09 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> The linker has to optimize the GOT reference into a relative reloc if
>>> you want IFUNC to work properly, sparc does this as does x86.
>>
>> It would only work if ld would be able to get rid of the runtime relocations entirely. In order to
>> do this ld would need to rewrite the code accessing the GOT slots to use pc or got relative
>> addressing. Interesting, but I don't think x86 is already doing this. At least ld didn't in the
>> testcase I'm discussing with H.J.Lu.
>>
>
> I have no plan to edit code sequence for this.
>

Here is the x86 change to convert MOV to LEA.

-- 
H.J.
---
diff --git a/bfd/elf32-i386.c b/bfd/elf32-i386.c
index 7d3652d..7dc6fb7 100644
--- a/bfd/elf32-i386.c
+++ b/bfd/elf32-i386.c
@@ -3470,6 +3470,25 @@ elf_i386_relocate_section (bfd *output_bfd,
 	  if (off >= (bfd_vma) -2)
 	    abort ();

+	  if (h != NULL
+	      && h->def_regular
+	      && info->shared
+	      && SYMBOL_REFERENCES_LOCAL (info, h)
+	      && bfd_get_8 (input_bfd,
+			    contents + rel->r_offset - 2) == 0x8b)
+	    {
+	      /* Convert
+		 movl	foo@GOT(%reg), %reg
+		 to
+		 leal	foo@GOTOFF(%reg), %reg
+	       */
+	      bfd_put_8 (output_bfd, 0x8d,
+			 contents + rel->r_offset - 2);
+	      relocation -= (htab->elf.sgotplt->output_section->vma
+			     + htab->elf.sgotplt->output_offset);
+	      break;
+	    }
+
 	  relocation = htab->elf.sgot->output_section->vma
 		       + htab->elf.sgot->output_offset + off
 		       - htab->elf.sgotplt->output_section->vma
diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
index a29ba8a..c291662 100644
--- a/bfd/elf64-x86-64.c
+++ b/bfd/elf64-x86-64.c
@@ -3460,6 +3460,23 @@ elf_x86_64_relocate_section (bfd *output_bfd,
 	  if (off >= (bfd_vma) -2)
 	    abort ();

+	  if (r_type == R_X86_64_GOTPCREL
+	      && info->shared
+	      && h->def_regular
+	      && SYMBOL_REFERENCES_LOCAL (info, h)
+	      && bfd_get_8 (input_bfd,
+			    contents + rel->r_offset - 2) == 0x8b)
+	    {
+	      /* Convert
+		 movl foo@GOTPCREL(%rip), %reg
+		 to
+		 leal foo(%rip), %reg
+	       */
+	      bfd_put_8 (output_bfd, 0x8d,
+			 contents + rel->r_offset - 2);
+	      break;
+	    }
+
 	  relocation = base_got->output_section->vma
 		       + base_got->output_offset + off;
 	  if (r_type != R_X86_64_GOTPCREL && r_type != R_X86_64_GOTPCREL64)