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Re: C++ support for decimal floating point
On Wed, 2009-09-23 at 10:29 +0200, Richard Guenther wrote:
> On Wed, Sep 23, 2009 at 2:38 AM, Janis Johnson <janis187@us.ibm.com> wrote:
> > I've been implementing ISO/IEC TR 24733, "an extension for the
> > programming language C++ to support decimal floating-point arithmetic",
> > in GCC. It might be ready as an experimental feature for 4.5, but I
> > would particularly like to get in the compiler changes that are needed
> > for it.
> >
> > Most of the support for the TR is in new header files in libstdc++ that
> > depend on compiler support for decimal float scalar types. Most of that
> > compiler functionality was already available in G++ via mode attributes.
> > I've made a couple of small fixes and have a couple more to submit, and
> > when those are in I'll starting running dfp tests for C++ as well as C.
> > The suitable tests have already been moved from gcc.dg to c-c++-common.
> >
> > In order to provide interoperability with C, people on the C++ ABI
> > mailing list suggested that a C++ compiler should recognize the new
> > decimal classes defined in the TR and pass arguments of those types the
> > same as scalar decimal float types for a particular target. I had this
> > working in an ugly way using a langhook, but that broke with LTO. I'm
> > looking for the right places to record that an argument or return value
> > should be passed as if it were a different type, but could use some
> > advice about that.
>
> How do we (do we?) handle std::complex<> there? My first shot would
> be to make sure the aggregate type has the proper mode, but I guess
> most target ABIs would already pass them in registers, no?
std::complex<> is not interoperable with GCC's complex extension, which
is generally viewed as "unfortunate".
The class types for std::decimal::decimal32 and friends do have the
proper modes. I suppose I could special-case aggregates of those modes
but the plan was to pass these particular classes (and typedefs of
them) the same as scalars, rather than _any_ class with those modes.
I'll bring this up again on the C++ ABI mailing list.
Perhaps most target ABIs pass single-member aggregates using the
mode of the aggregate, but not all. In particular, not the 32-bit
ELF ABI for Power.
Janis