LLVM collaboration?

[Hal Finkel]

----- Original Message -----
> From: "Rafael Espíndola" <rafael.espindola@gmail.com>
> To: "Jan Hubicka" <hubicka@ucw.cz>
> Cc: "Renato Golin" <renato.golin@linaro.org>, "gcc" <gcc@gcc.gnu.org>, "Hal Finkel" <hfinkel@anl.gov>
> Sent: Tuesday, February 11, 2014 3:38:40 PM
> Subject: Re: Fwd: LLVM collaboration?
> 
> > My reading of bfd/plugin.c is that it basically walks the directory
> > and looks
> > for first plugin that returns OK for onload. (that is always the
> > case for
> > GCC/LLVM plugins).  So if I instlal GCC and llvm plugin there it
> > will
> > depend who will end up being first and only that plugin will be
> > used.
> >
> > We need multiple plugin support as suggested by the directory name
> > ;)
> >
> > Also it sems that currently plugin is not used if file is ELF for
> > ar/nm/ranlib
> > (as mentioned by Markus) and also GNU-ld seems to choke on LLVM
> > object files
> > even if it has plugin.
> >
> > This probably needs ot be sanitized.
> 
> CCing Hal Finkel. He got this to work some time ago. Not sure if he
> ever ported the patches to bfd trunk.

I have a patch for binutils 2.24 (attached -- I think this works, I hand isolated it from my BG/Q patchset). I would not consider it to be of upstream quality, but I'd obviously appreciate any assistance on making everything clean and proper ;)

 -Hal

> 
> >> For OS X the situation is a bit different. There instead of a
> >> plugin
> >> the linker loads a library: libLTO.dylib. When doing LTO with a
> >> newer
> >> llvm, one needs to set DYLD_LIBRARY_PATH. I think I proposed
> >> setting
> >> that from clang some time ago, but I don't remember the outcome.
> >>
> >> In theory GCC could implement a libLTO.dylib and set
> >> DYLD_LIBRARY_PATH. The gold/bfd plugin that LLVM uses is basically
> >> a
> >> API mapping the other way, so the job would be inverting it. The
> >> LTO
> >> model ld64 is a bit more strict about knowing all symbol
> >> definitions
> >> and uses (including inline asm), so there would be work to be done
> >> to
> >> cover that, but the simple cases shouldn't be too hard.
> >
> > I would not care that much about symbols in asm definitions to
> > start with.
> > Even if we will force users to non-LTO those object files, it would
> > be an
> > improvement over what we have now.
> >
> > One problem is that we need a volunteer to implement the reverse
> > glue
> > (libLTO->plugin API), since I do not have an OS X box (well, have
> > an old G5,
> > but even that is quite far from me right now)
> >
> > Why complete symbol tables are required? Can't ld64 be changed to
> > ignore
> > unresolved symbols in the first stage just like gold/gnu-ld does?
> 
> I am not sure about this. My *guess* is that it does dead stripping
> computation before asking libLTO for the object file. I noticed the
> issue while trying to LTO firefox some time ago.
> 
> Cheers,
> Rafael
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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