Re: [RFA] Compact EH Patch

[Richard Henderson <rth at redhat dot com>]

On 10/06/2015 10:14 AM, Moore, Catherine wrote:
> > > +  switch (pr_index) {
> > > +  case 0:
> > > +      p = read_encoded_value (context, bases->eh_encoding, p,
> > &personality);
> > > +      fs->personality = (_Unwind_Personality_Fn) personality;
> > > +      break;
> > > +  case 1:
> > > +      fs->personality = __gnu_compact_pr1;
> > > +      break;
> > > +  case 2:
> > > +      fs->personality = __gnu_compact_pr2;
> > > +      break;
> > > +  default:
> > > +      fs->personality = NULL;
> > > +  }
> >
> > This is the aspect of this spec about which I am least keen.  The existing
> > method whereby the personality function is explicit in each object file means
> > that we've got automatic version control on data that is private to the object
> > file.
> >
> > That is, if we should ever change the format of the LSDA -- as in fact you are
> > doing here -- then all we need do is change __gcc_personality_v0 to
> > __gcc_personality_v1, and all is well.  One can mix and match object files
> > from different compiler versions and all that is required for correctness is
> > that the runtime libraries must continue to provide all previous versions.
> >
> > What you're doing here doesn't allow the format of index {1,2,3} to *ever*
> > change.  You've fixed it forever, barring abandoning those indices and always
> > using index 0.
> >
> > I know that the ARM EH format made exactly the same mistake, but let's see
> > if we can find some way of not replicating it, eh?
>
> I'm not 100% sure what you are suggesting here.  Would you like to see __gnu_compact_pr2_v0,  etc?
> Are you also saying that you want to see a undefined reference to the personality routine in the object file?

I'm commenting on what I see as a defect in the spec, but not making a specific 
recommendation.

Using __gnu_compact_pr2_v0 etc doesn't help, because you're still using just 2 
bits to reference it.  The only real solution is a symbolic reference from the 
object file, which you get from pr_index == 0.


r~