Re: .eh_frame optimization question

[Jie Zhang <jzhang918 at gmail dot com>]

Alan Modra wrote:
> On Mon, Dec 25, 2006 at 10:59:43PM +0800, Jie Zhang wrote:
> > I'm looking at a bug of bfin port ld. With ELF FD-PIC ABI, bfin ld
> > will generate a relocation of type R_BFIN_FUNCDESC against symbol
> > __gxx_personality_v0 in .eh_frame section when compiling C++ program.
> > The number of such relocations are counted in size_dynamic_sections ()
> > and the size of the corresponding section are calculated at that time.
> > __gxx_personality_v0 is used in the CIE entry of the .eh_frame.
> > However, it's possible that .eh_frame optimization discards all FDE
> > entries which use the CIE entry, so the CIE entry can be eliminated,
> > too. Thus when bfinfdpic_relocate_section () is going to generating
> > the relocation for each section, it will find some relocations are
> > lost.
> >
> > So my question is: Is it possible to add a backend hook, which can be
> > used by port, like bfin, to tell ld not optimize away CIE entry even
> > there is no FDE use it? Or is there a better way to fix it?
>
> Other ports handle this situation by emitting an R_*_NONE reloc,
> typically reloc number 0.  Can you do the same?
>
> _bfd_elf_section_offset will return -1 or -2 to indicate that a
> dynamic reloc is not needed.
Thanks for your reply. Actually, bfin port handles this situation as 
other ports. But some specific .eh_frame sections are not handled in 
this way. So I spent some time to find out why. The cause is that these 
specific .eh_frame sections are empty after optimization and are removed 
by setting SEC_EXCLUDE. Thus ld doesn't get a chance to change the 
dynamic relocations of the removed .eh_frame entries to R_*_NONE type. 
This patch fixes it. Any comment?


Jie
	* elf-eh-frame.c (_bfd_elf_discard_section_eh_frame): Don't remove
	empty .eh_frame section.

Index: elf-eh-frame.c
===================================================================
RCS file: /cvs/src/src/bfd/elf-eh-frame.c,v
retrieving revision 1.53
diff -u -p -r1.53 elf-eh-frame.c
--- elf-eh-frame.c	21 Nov 2006 11:25:16 -0000	1.53
+++ elf-eh-frame.c	3 Jan 2007 07:25:57 -0000
@@ -927,8 +927,15 @@ _bfd_elf_discard_section_eh_frame
   /* Resize the sec as needed.  */
   sec->rawsize = sec->size;
   sec->size = offset;
-  if (sec->size == 0)
-    sec->flags |= SEC_EXCLUDE;
+
+  /* Previously, there was the following code to remove empty .eh_frame section.
+     
+     if (sec->size == 0)
+       sec->flags |= SEC_EXCLUDE;
+
+     But we cannot remove empty .eh_frame section. Otherwise, the dynamic
+     relocations of the .eh_frame section would have no chance to be changed
+     to R_*_NONE relocation.  */
 
   free (ehbuf);
   if (ecies)