[MIPS] Reserve a .gnu_attribute value

[Maciej W. Rozycki]

On Wed, 8 Jul 2015, Moore, Catherine wrote:

> > > > diff --git a/binutils/readelf.c b/binutils/readelf.c index
> > > > 398a165..86cfd26 100644
> > > > --- a/binutils/readelf.c
> > > > +++ b/binutils/readelf.c
> > > > @@ -13197,6 +13197,9 @@ print_mips_fp_abi_value (int val)
> > > >      case Val_GNU_MIPS_ABI_FP_64A:
> > > >        printf (_("Hard float compat (32-bit CPU, 64-bit FPU)\n"));
> > > >        break;
> > > > +    case Val_GNU_MIPS_ABI_FP_NAN2008 :
> > > > +      printf (_("NaN 2008 compatibility\n"));
> > > > +      break;
> > >
> > > No space before ":"
> > >
> > > OK with that change, thanks.
> > 
> >  Shouldn't there be more case values?  `Val_GNU_MIPS_ABI_FP_NAN2008'
> > was ORed with the lower bits, e.g. `Val_GNU_MIPS_ABI_FP_DOUBLE'.
> 
> I'm trying to provide a minimal implementation that will ensure that 
> Val_GNU_MIPS_ABI_FP_NAN2008 doesn't get used for anything else.  There 
> isn't anything in the upstream repository that will generate or use that 
> bit unless it's by mistake.  I thought it would be best to have readelf 
> just indicate that the bit was in use.

 Sure, however as proposed with your change `Val_GNU_MIPS_ABI_FP_NAN2008' 
is merely an enum value, it does not indicate in any way that the whole 
8-15 range has indeed been taken.

 Also 0 is `Val_GNU_MIPS_ABI_FP_ANY' i.e. usually (in code generated by 
recent GCC) means no-fp, so consequently the value of 8 won't normally be 
there.  The usual case will be 9, for `Val_GNU_MIPS_ABI_FP_DOUBLE' and 
2008 NaN; `Val_GNU_MIPS_ABI_FP_SINGLE' and consequently 10 is also a 
reasonable case.  For these values `???' will still be printed.

 FAOD I don't want to be a hindrance for you with this change, I just 
wonder if we can do a better job to cheaply avoid a potential ABI clash 
here.  It's not like we're short of numeric values available, wasting 8 of 
them is I think not a big deal.

 Maybe simply masking the bit/value out in `print_mips_fp_abi_value' and 
merely documenting the range in include/elf/mips.h will be a better way?  
This way the FP ABI will be reported correctly and you'll still have the 
EF_MIPS_NAN2008 ELF file header bit to tell legacy- and 2008-NaN binaries 
apart.  Win-win?

  Maciej