fake symbols to aid debugging

[Alan Modra <amodra at bigpond dot net dot au>]

PowerPC64 (and other architechures) use linker stubs for various things,
most notably to make calls to functions in shared objects.  Typical
disassembly looks like:

the stub
  7ea138:       3d 82 00 01     addis   r12,r2,1
  7ea13c:       f8 41 00 28     std     r2,40(r1)
  7ea140:       e9 6c 11 88     ld      r11,4488(r12)
  7ea144:       e8 4c 11 90     ld      r2,4496(r12)
  7ea148:       7d 69 03 a6     mtctr   r11
  7ea14c:       e9 6c 11 98     ld      r11,4504(r12)
  7ea150:       4e 80 04 20     bctr

call to stub
  87d6f0:       e8 7e 80 90     ld      r3,-32624(r30)
  87d6f4:       4b f6 ca 45     bl      7ea138 <._init+0xe9b8>
  87d6f8:       e8 41 00 28     ld      r2,40(r1)

Note the meaningless symbol given as target for the "bl".  Now, if you
know enough about the ABI, it's possible to figure out which function is
being called, but that's a pain.  In the case of PowerPC64 you need to
a) Find the value of r2 by looking at the calling function's opd entry.
   (with multiple GOT/TOC r2 is not fixed).
b) Perform the calculation done by the stub to find the plt entry
   address. 
c) Search the relocs to find the one for this particular plt entry.
   The symbol on the reloc gives the function name.

I waste enough time doing this that I figure it's worth doing something
about it.  My first idea, already implemented, was to have the linker
emit extra symbols to identify the stubs.  This works well but bloats
the symbol table and isn't on by default.  A better idea would be to
create the stub symbols on the fly.  With that in mind, I propose to
add two new bfd functions

long bfd_get_fake_symtab_upper_bound (bfd *abfd);
long bfd_canonicalize_fake_symtab (bfd *abfd, asymbol **buf);

analogous to bfd_get_symtab_upper_bound and bfd_canonicalize_symtab.

Comments?

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre