About addition of .symtab and .strtab sections in simple-object-elf.c

About addition of .symtab and .strtab sections in simple-object-elf.c

[Rishi Raj]

Hi Everyone,
I am working on the GSOC project "Bypass Assembler when generating LTO
object files." My mentors and I have decided to work on the ELF files
first, so I will add .symtab along with the symbol __gnu_lto_slim to
the ELF file as a first step.
When I was going through the simple-object-elf.c:
simple_object_elf_write_to_file(....) I found out that it writes the
following:
/* Write out a complete ELF file.
   Ehdr
   initial dummy Shdr
   user-created Shdrs
   .shstrtab Shdr
   user-created section data
   .shstrtab data  */
and .symtab is missing here. To add the missing symtab I have thought
of these two possible implementations.
1) Add it in simple-object-elf.c (based on -fbypass-asm flag).
2) We can add .symtab section in lto-object.cc along with other LTO sections.
I am a bit skeptical about the second one as .symtab with other lto
sections might be confusing. Any comments regarding which one should I
proceed with will be helpful.
--
Thanks & Regards
Rishi Raj

Re: About addition of .symtab and .strtab sections in simple-object-elf.c

[Jan Hubicka]

> Hi Everyone,
Hello,
> I am working on the GSOC project "Bypass Assembler when generating LTO
> object files." My mentors and I have decided to work on the ELF files
> first, so I will add .symtab along with the symbol __gnu_lto_slim to
> the ELF file as a first step.
> When I was going through the simple-object-elf.c:
> simple_object_elf_write_to_file(....) I found out that it writes the
> following:
> /* Write out a complete ELF file.
>    Ehdr
>    initial dummy Shdr
>    user-created Shdrs
>    .shstrtab Shdr
>    user-created section data
>    .shstrtab data  */
> and .symtab is missing here. To add the missing symtab I have thought
> of these two possible implementations.
> 1) Add it in simple-object-elf.c (based on -fbypass-asm flag).
> 2) We can add .symtab section in lto-object.cc along with other LTO sections.
> I am a bit skeptical about the second one as .symtab with other lto
> sections might be confusing. Any comments regarding which one should I
> proceed with will be helpful.

I think we need to take into consideration that eventually we want to
support also coff/xcoff and macho so writting symtab section directly
out of lto-object.cc (which is supposed to be file format agnostic) is
hard.  On the other hand libiberty knows nothing about internals of LTO
implementatoin.
I think we want to extend simple-object.c by (simple at first at
least) API to add symbols to the symbol table and interface it to
simple-object-elf.c via simple_object_functions. That way
lto-object.cc (which knows the symbol names but no particular file
format) can call generic simple-object.cc API which will in turn call
simple-object-elf.c to create the symtab section.

Honza
> --
> Thanks & Regards
> Rishi Raj