[Bug c++/106652] [C++23] P1467 - Extended floating-point types and standard names

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106652

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org,
                   |                            |redi at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
On targets that do have __float128, I believe we want to mangle it as before
and also handle usual arithmetic conversions etc. the same we did before unless
the
C++23 extended floating-point types are involved.
Which is why I've introduced the float128t_type_node hack, where non-C++ can
continue to do what it did before but for C++ __float128 will be a new distinct
type.
For mangling of std::float{16,32,64,128}_t I'm using the Itanium ABI
_Float{16,32,64,128} mangling, i.e. DF{16,32,64,128}_
This collides with the apparently never really used mangling of
FIXED_POINT_TYPEs 
(fixed points are really only supported in C on a few platforms, not in C++,
and at some point they leaked into the C++ FE through 0r and similar literals,
but that has been fixed shortly afterwards).
The patch only introduces _Float{16,32,64,128}, not _Float{32,64,128}x that C
also supports, so I've removed __FLT{32,64,128}X_* predefined macros.

The patch is still incomplete and I'm getting stuck on it (except I can surely
provide testsuite coverage for what is already implemented):

1) there is no bf16/BF16 constant suffix nor underlying type for
std::bfloat16_t
   for now; I think we need to come to agreement on how the underlying type
   would be called (__bf16 like aarch64/arm/i386 currently have their extension
   type?) and how to mangle it (all 3 currently mangle it as u6__bf16) and
   if we choose a different keyword for it whether it is distinct from __bf16

2) I haven't implemented the [conv.double] addition:
   "with a greater or equal conversion rank ([conv.rank]). A prvalue of
standard 
   floating-point type can be converted to a prvalue of another standard
   floating-point type" - not really sure where it should be done
   (but the new cp_compare_floating_point_conversion_ranks function can be
   used to compare ranks and subranks)

3) for the [expr.static.cast] addition, I wonder if there is anything to do,
   I'd expect it would just work as is

4) for the [expr.arith.conv] changes, I think I've implement those in
   cp_common_type, except for the
   "Otherwise, the expression is ill-formed."
   part where I just return error_mark_node, but cp_common_type doesn't
   emit any diagnostics whatsoever, so I wonder if it should be done
   somewhere in the callers, or if the function and its wrappers should
   get tsubst_flags_t complain argument or what.

5) I've skipped the [over.ics.rank] changes, I'm afraid it is another thing
   I'm not really familiar with

6) the library part is unimplemented altogether, the __FLT* macros can be used
   to implement numerical limits, but e.g. for the <cmath>/<complex> stuff
   not really sure how far can we get for std::float128_t if not on glibc or
   on old glibc (guess the others at least when they match float/double
   which can be tested through preprocessor macros can be handled by casts
   to those types)