Fw: Re: Java interpreter on ARM GNU/Linux

Fri Jul 13 12:51:00 GMT 2007

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.