Re: Prevent all uses of DFP when unsupported (PR c/91985)

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Nov 27, 2019 at 05:48:21PM +0000, Joseph Myers wrote:
> > > On 26/11/19 00:57 +0000, Joseph Myers wrote:
> > >>On Mon, 25 Nov 2019, Rainer Orth wrote:
> > >>
> > >>> and a few more, all DFP related.  They used to be emitted by g++ for
> > >>> __fundamental_type_info in libsupc++/fundamental_type_info.cc and lived
> > >>> in the CXXABI_1.3.4 version.  However, since Solaris *does* lack DFP
> > >>> support, that's no longer the case.  I'm uncertain how best to deal with
> > >>> this, however.
> > >>
> > >>As I understand it, _GLIBCXX_USE_DECIMAL_FLOAT should already have been
> > >>undefined for this target, and so std::decimal::decimal32 etc. should not
> > >>have been usable (both the header not working without that define, and the
> > >>mode attributes in the header being rejected by the front end when DFP is
> > >>unsupported).  I.e. such defines in libsupc++ would never have been usable
> > >>on this target, so I think they are something it should be safe to remove
> > >>from the ABI baseline.
> > >
> > > If it's actually impossible that any real program could have depended
> > > on those symbols, then I agree.
> > 
> > this is exactly what I've got no way of telling, that's why I was asking
> > for guidance.  Just removing the DFP symbols from the baselines works,
> > of course.
> 
> I don't think any real program could have used those symbols; it would 
> have required using __typeof (__builtin_fabsd32 (0)) or similar to access 
> types that weren't normally available for those targets (and by accessing 
> the types using builtins like that, you're getting a completely undefined 
> function-calling ABI for them anyway).

I think various tools we use to check ABI will be unhappy about removal
of symbols.  Can't we on targets that do support aliases and don't support
decimal float e.g. alias the DFP rtti symbols to void rtti symbols?

	Jakub