Re: [PATCH] PR ld/17975: Useless FILE entry in symbol table for linker generated symbols

[Alan Modra <amodra at gmail dot com>]

On Sun, Feb 15, 2015 at 05:15:51PM -0800, H.J. Lu wrote:
> On Sun, Feb 15, 2015 at 5:04 PM, Alan Modra <amodra@gmail.com> wrote:
> > On Sun, Feb 15, 2015 at 10:46:02AM -0800, H.J. Lu wrote:
> >> We output a NULL STT_FILE symbol for forced local symbols so that they
> >> are not associated with the STT_FILE symbol for real local symbols. This
> >> patch makes sure that the NULL STT_FILE symbol is placed before forced
> >> local symbols.  OK for trunk?  I will update other expected outputs
> >> after this patch has been checked in.
> >
> > Huh, silly me for not noticing that forced local symbols can currently
> > be associated with the wrong file.  Hmm, they do of course belong with
> > a particular file.  So...
> >
> > We could keep the file association, and do away with the NULL STT_FILE
> > file symbol:
> > - In elf_link_input_bfd, replace all calls to elf_link_output_sym
> >   with another function that buffers up local symbols to an array
> >   attached to the input bfd (I think outsymbols should be free for
> >   this use).
> > - In bfd_elf_final_link, output the first all-zero symbol and section
> >   symbols, then follow that with a pass over globals emitting the
> >   forced-local symbols not associated with an input file (ie. those we
> >   currently output on the second pass), and buffering up other
> >   forced-local symbols to their input bfd.  Then loop over input bfds
> >   emitting the buffered local symbols.  An input bfd with locals but
> >   no initial STT_FILE symbol needs one to be added.  An input bfd with
> >   more than one STT_FILE symbol (ie. specifying more than one source
> >   file) needs to have another STT_FILE symbol added before the
> >   forced-local globals.
> > - Finally, output global symbols.
> >
> > How does that sound?  Let me know if you're interested in implementing
> > this.  If not, I'll find time this week.
> >
> 
> There are a couple issues:
> 
> 1. If the forced local symbols aren't sorted by input file, we may
> generate many redundant STT_FILE symbols.

No, because the above scheme buffers forced-local symbols to the
input file that defines them.

> 2. When the forced local symbols are sorted by input file, we still
> generate one STT_FILE symbol per input file with forced local
> symbols.  We may wind with many STT_FILE symbols.  You can
> try it on glibc and libstdc++.

Yes, you will get an extra STT_FILE symbol for objects that have more
than one STT_FILE and forced-local globals.

> In any case, how useful are they?  Are they really needed?

That's open to debate. :)  If you have debug info, no.

> FWIW, gold doesn't generate any those  STT_FILE symbols.

Yes, well, gold is lacking in many of the corner cases that bfd ld
supports.

-- 
Alan Modra
Australia Development Lab, IBM