RE: Group identifier of a comdat group

["Jessica Han" <jessica at cup dot hp dot com>]

Thanks for the response.
I am using the GNU ELF convention for link-once sections, since all COMDAT
sections marked by gnu.linkonce.
(http://gcc.gnu.org/ml/gcc/2001-12/msg00975.html). When the compiler sees a
section is marked by LINK_ONCE, it puts out the group information in
assembly. A COMDAT section is marked by something like:
 .section .gnu.linkonce.t.foo,"axG",@progbits,foo_group

Then the assembler can generate the COMDAT using the group support already
in binutils. A COMDAT group contains not only all sections have the same
group "foo_group" in assembly file, but also sections generated by the
assembler based on any of those member sections.

For two sections like
.section .text.foo,"axG",@progbits,foo_group
.section .rodata.foo,"aG",@progbits,foo_group
I can get a new section called foo_group with the two member section, and
sh_link of the section header points to .symtab, sh_info is the index of
section foo_group. However, .symtab section doesn't have section names.

----Original Message-----
From: Alan Modra [mailto:amodra@bigpond.net.au]
Sent: Sunday, June 02, 2002 9:20 PM
To: Jessica Han
Cc: binutils@sources.redhat.com; gcc@gcc.gnu.org; reva@cup.hp.com
Subject: Re: Group identifier of a comdat group


On Fri, May 31, 2002 at 10:08:01AM -0700, Jessica Han wrote:
>   I am trying to implement the COMDAT sections support for GCC on IA64,
and
> I am having a problem finding the group identifier of a COMDAT group.

Let's step back a little from the details of the implementation for a
moment.  Are you using the existing binutils support for ELF groups,
that is, gcc will emit something like:

 .section .text.foo,"axG",@progbits,foo_group
.
.
 .section .rodata.foo,"aG",@progbits,foo_group

to set up a group, foo_group, with two members, .text.foo and
.data.foo?

I didn't implement GRP_COMDAT at the time I did the rest of the group
support mainly because it wasn't relevant to the immediate problem I
was trying to solve at the time.  Also, there were too many options
for specifying a link-once type group.  :)

Should we follow the GNU ELF convention for link-once sections?
ie. use something like

 .section .text.foo,"axG",@progbits,.gnu.linkonce.foo_group

to specify that the group as a whole is link-once?  Must the member
sections all be named .gnu.linkonce.*?  (I don't think they need to
be, but if they all are, then should we just automatically add the
GRP_COMDAT flag?)

A second alternative is to follow the COFF scheme, using

 .linkonce foo_group

I think that's cleaner than the magic .gnu.linkonce.* section names.
As a side issue, I think it's a pity that existing ELF link once
sections need special names; I'd rather see a SHF_LINKONCE flag in
sh_flags.

Then there are other options like an extra section letter, say

 .section .text.foo,"axGC",...

or an extra field, say

 .section .text.foo,"axG",@progbits,foo_group,comdat

Once we get these issues sorted out, it shouldn't be too hard to
implement in the linker.

--
Alan Modra
IBM OzLabs - Linux Technology Centre