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

[Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>]

Hi Joseph,

> Code that directly uses _Decimal* types on architectures not
> supporting DFP is properly diagnosed ("error: decimal floating-point
> not supported for this target"), via a call to
> targetm.decimal_float_supported_p, if the _Decimal32, _Decimal64 or
> _Decimal128 keywords are used to access it.  Use via mode attributes
> is also diagnosed ("unable to emulate 'SD'"); so is use of the
> FLOAT_CONST_DECIMAL64 pragma.  However, it is possible to access those
> types via typeof applied to constants or built-in functions without
> such an error.  I expect that there are ways to get an ICE from this;
> certainly it uses a completely undefined ABI.
>
> This patch arranges for the types not to exist in the compiler at all
> when DFP is not supported.  As is done with unsupported _FloatN /
> _FloatNx types, the global tree nodes are left as NULL_TREE, and the
> built-in function machinery is made to use error_mark_node for them in
> that case in builtin-types.def, so that the built-in functions are
> unavailable.  Code handling constants is adjusted to give an error,
> and other code that might not work with the global tree nodes being
> NULL_TREE is also updated.
>
> Bootstrapped with no regressions for x86_64-pc-linux-gnu.  Also tested
> with no regressions for cross to aarch64-linux-gnu, as a configuration
> without DFP support.  OK to commit (the changes that aren't C front-end 
> changes)?

AFAICS this caused

+FAIL: libstdc++-abi/abi_check

on Solaris.  In libstdc++.log I find

# of added symbols:              0
# of missing symbols:            9
# of undesignated symbols:       0
# of incompatible symbols:       9

9 missing symbols
0
_ZTIDf
typeinfo for decimal32
version status: unversioned
type: object
type size: 8
status: subtracted
[...]

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.

	Rainer

-- 
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Rainer Orth, Center for Biotechnology, Bielefeld University