[cfe-dev] Why clang creates ELF section group with only one section in it?

[Dmitri Shubin]

Hello Rafael!

There are actually 2 (related) questions.

On 15.05.2012 01:07, Rafael Espíndola wrote:
>> But then it makes sense to put relocs against this section in the same group
>> also so they can be discarded together (as Sun C++ does).
> I am not sure I understand. The regular way of relocations are
> recorded on ELF is by having a relocation section for each section
> that needs one.
Yes.
So if the section with function code (.text._Z3barIiEiT_) is discarded 
there is no point to have corresponding reloc section.
So it makes sense to put it in the same COMDAT group as .text._Z3barIiEiT_
This is what Sun CC does:

$ CC -V
CC: Sun C++ 5.11 SunOS_i386 145731-02 2011/02/11
usage: CC [ options ] files.  Use 'CC -flags' for details
$ CC -c a.cpp
$ elfdump -g a.o

Group Section:  .group%__1cDbar4Ci_6FTA_i_
      index    flags / section         signature symbol
        [0]   [ COMDAT ]               __1cDbar4Ci_6FTA_i_
        [1]   .text%__1cDbar4Ci_6FTA_i_ [4]
        [2]   .rel.text%__1cDbar4Ci_6FTA_i_ [18]
$ echo __1cDbar4Ci_6FTA_i_ | c++filt
int bar<int>(__type_0)

Compare with clang:

$ /opt/clang/bin/clang++ -c a.cpp
$ elfdump -c a.o | grep ^Section
Section Header[1]:  sh_name: .group
Section Header[2]:  sh_name: .text
Section Header[3]:  sh_name: .rel.text
Section Header[4]:  sh_name: .data
Section Header[5]:  sh_name: .bss
Section Header[6]:  sh_name: .text._Z3barIiEiT_
Section Header[7]:  sh_name: .rel.text._Z3barIiEiT_
Section Header[8]:  sh_name: .note.GNU-stack
Section Header[9]:  sh_name: .eh_frame
Section Header[10]:  sh_name: .rel.eh_frame
Section Header[11]:  sh_name: .shstrtab
Section Header[12]:  sh_name: .symtab
Section Header[13]:  sh_name: .strtab
$ elfdump -g a.o

Group Section:  .group
      index    flags / section         signature symbol
        [0]   [ COMDAT ]               _Z3barIiEiT_
        [1]   .text._Z3barIiEiT_ [6]

This is the first question -- why clang doesn't put both sections to 
COMDAT group?

The second question is about relocation against .eh_frame section which 
Solaris ld is complaining about.

What I'm trying to do:

$ /opt/clang/bin/clang++ -c a.cpp
$ /opt/clang/bin/clang++ -c b.cpp
$ ld -Dsections -r -o all.o a.o b.o
debug:
debug: Solaris Linkers: 5.10-1.1505
debug:
debug: section=[1].group; input from file=a.o
debug: section=[1].group; input from file=a.o; defines COMDAT group: 
signature symbol: _Z3barIiEiT_
debug: section=.group; added to segment=extra (created)
...
debug: section=[6].text._Z3barIiEiT_; input from file=a.o
debug: section=[6].text._Z3barIiEiT_; input from file=a.o; member of 
COMDAT group: signature symbol: _Z3barIiEiT_
debug: section=.text._Z3barIiEiT_; added to segment=text (created)
...
debug: section=[9].eh_frame; input from file=a.o
debug: section=.eh_frame; added to segment=data (created)
...
debug: section=[1].group; input from file=b.o
debug: section=[1].group; input from file=b.o; defines COMDAT group: 
signature symbol: _Z3barIiEiT_
debug: section=[1].group; input from file=b.o; discarded in favor of 
group: signature symbol: _Z3barIiEiT_: file=a.o
...
debug: section=[6].text._Z3barIiEiT_; input from file=b.o
debug: section=[6].text._Z3barIiEiT_; input from file=b.o; member of 
COMDAT group: signature symbol: _Z3barIiEiT_
debug: section=[6].text._Z3barIiEiT_; input from file=b.o; discarded in 
favor of group: signature symbol: _Z3barIiEiT_: file=a.o
...
debug: section=[9].eh_frame; input from file=b.o
debug: section=.eh_frame; added to segment=data
debug:
ld: fatal: relocation error: R_386_32: file b.o: section 
[10].rel.eh_frame: symbol .text._Z3barIiEiT_ (section): symbol has been 
discarded with discarded section: [6].text._Z3barIiEiT_

So the linker successfully discarded .text._Z3barIiEiT_ in b.o in favor 
of the one in a.o

But it got problems when processing relocations for .eh_frame section in b.o

$ elfdump -r -N .rel.eh_frame b.o
Relocation Section:  .rel.eh_frame
     type                       offset             section        symbol
   R_386_32                       0x20             .rel.eh_frame  .text 
(section)
   R_386_32                       0x3c             .rel.eh_frame  
.text._Z3barIiEiT_ (section)

$ dump -r b.o
b.o:
     **** RELOCATION INFORMATION ****
...
.rel.eh_frame:
Offset      Symndx              Type
0x20        2                   1
0x3c        5                   1

$ elfdump -l -s b.o

Symbol Table Section:  .symtab
      index    value      size      type bind oth ver shndx / name
        [0]  0x00000000 0x00000000  NOTY LOCL  D    0 UNDEF
        [1]  0x00000000 0x00000000  FILE LOCL  D    0 ABS            b.cpp
        [2]  0x00000000 0x00000000  SECT LOCL  D    0 .text
        [3]  0x00000000 0x00000000  SECT LOCL  D    0 .data
        [4]  0x00000000 0x00000000  SECT LOCL  D    0 .bss
        [5]  0x00000000 0x00000000  SECT LOCL  D    0 .text._Z3barIiEiT_
        [6]  0x00000000 0x00000000  SECT LOCL  D    0 .note.GNU-stack
        [7]  0x00000000 0x00000000  SECT LOCL  D    0 .eh_frame
        [8]  0x00000000 0x00000000  SECT LOCL  D    0 .group
        [9]  0x00000000 0x00000030  FUNC WEAK  D    0 .text._Z3barIiEiT_ 
_Z3barIiEiT_
       [10]  0x00000000 0x00000022  FUNC GLOB  D    0 .text          _Z3foov
       [11]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF          
_ZN1SD1Ev

The second relocation entry in .rel.eh_frame references section symbol 
[5].text._Z3barIiEiT_ which is LOCAL and thus is discarded with 
discarded section .text._Z3barIiEiT_

Of course, this is my understanding of what linker is saying.