Re: darwin LTO broken under Xcode 3.2.6/4.0

[Jack Howarth <howarth@bromo.med.uc.edu>]

On Sun, Mar 13, 2011 at 12:39:26PM +0100, Jan Hubicka wrote:
> >    With release of Xcode 3.2.6/4.0 this week, an unfortunate change was made to
> > the darwin assembler which effectively breaks LTO support for darwin. The design
> > of LTO on darwin was based on the fact that mach-o object files tolerated additional
> > sections as long as they didin't contain symbols. With Xcode 3.2.6/4.0, the assembler
> > appears to be strictly counting sections and objecting when these exceed 255. This
> > breaks huge sections of the lto testsuite and prevents larger projects like xplor-nih
> > to compile if Xcode 3.2.6/4.0 is installed. I am afraid that unless Apple reverts this
> > change, our only recourse would be to resort to an elf object container for the lto
> > sections within the mach-o files (introducing an undesired dependency on libelf for
> > FSF gcc on darwin). My understanding was that the lto design did not allow the number
> > of sections required in the lto files to be reduced.
> 
> If the problem is not fixed, we could always pack all the LTO sections into one section containing
> our own subsections.
> 
> Honza

Jan,
  If this could be done without resorting to other container types (like elf), it might be
the wisest approach for the long run. I've read through the mach-o documentation and it
seems rather vague on the section limits. Even if Apple fixes Xcode (which likley won't
happen for 6-9 months at best), we always we have to worry that they will break this 
'feature' somewhere else in their tool chain. Better to follow the strictest possible reading
of mach-o object format to protect ourselves from overzealous Apple interns.
> >               Jack