Re: Fw: Re: Java interpreter on ARM GNU/Linux

[Andrew Haley <aph-gcc@littlepinkcloud.COM>]

Daniel Jacobowitz writes:
 > On Fri, Jul 13, 2007 at 11:08:15AM +0100, Andrew Haley wrote:
 > >  > > I can work around some of the failures, but I can't really be
 > >  > > bothered: the real fix for this is EABI.
 > >  > 
 > >  > Unfortunately, the EABI won't fix this either.  In its native form it
 > >  > supported neither forced unwinding nor _Unwind_Backtrace; we added
 > >  > forced unwinding for the benefit of NPTL (though we're still talking
 > >  > to ARM intermittently about its semantics).  But no one's tried to
 > >  > make _Unwind_Backtrace work yet, and it's not clear how to.  To me
 > >  > anyway.
 > > 
 > > Interesting.  Presumably there is enough information in the unwinder
 > > data to do it?  If it's technically impossible to do a simple
 > > backtrace then I have to wonder what the point of the EABI is.
 > 
 > The data is there, but it's not structured the right way.  In
 > .eh_frame unwinding, there's a standard format to define saved
 > registers and an augmentation to define the language-specific data and
 > the personality routine which understands that language-specific data.
 > In ARM EABI unwinding, the personality routine is responsible for
 > reading the saved register data as well as the language specific
 > data.  So the core unwinder (_Unwind_* in libgcc) is limited in what
 > it can do by the flags accepted by the personality routines.
 > 
 > There are several standard formats defined by the EHABI.  GCC only
 > ever uses those.  But we might not have our own implementations of the
 > personality routines, so we can't ABI-conformantly pass them invalid
 > flags.  We play a little loose with this for forced unwinding and let
 > ourselves sleep at night since we proposed standardization language
 > for them to ARM :-)

OK.  Can you send me a copy of your proposed standardization language?

 > However, for the standard table formats and personality routines, we
 > could arrange to do backtraces anyway.  It should be just a matter of
 > a day's work or so and a little more code in the unwinder, and it
 > would work with all GCC-compiled code with unwind info.

Thanks.  I'm busy for the next week or so, but I might be able to get
back to it after then, depending on hardware availability.

Andrew.