Re: [committed] libstdc++: Define std::numeric_limits<_FloatNN> before C++23

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 4 Oct 2023 at 16:54, Stephan Bergmann <sbergman@redhat.com> wrote:
>
> On 8/17/23 22:32, Jonathan Wakely via Libstdc++ wrote:
> > Tested x86_64-linux. Pushed to trunk.
> >
> > -- >8 --
> >
> > The extended floating-point types such as _Float32 are supported by GCC
> > prior to C++23, you just can't use the standard-conforming names from
> > <stdfloat> to refer to them. This change defines the specializations of
> > std::numeric_limits for those types for older dialects, not only for
> > C++23.
> >
> > libstdc++-v3/ChangeLog:
> >
> >       * include/bits/c++config (__gnu_cxx::__bfloat16_t): Define
> >       whenever __BFLT16_DIG__ is defined, not only for C++23.
> >       * include/std/limits (numeric_limits<bfloat16_t>): Likewise.
> >       (numeric_limits<_Float16>, numeric_limits<_Float32>)
> >       (numeric_limits<_Float64>): Likewise for other extended
> >       floating-point types.
> > ---
> >   libstdc++-v3/include/bits/c++config |   4 +-
> >   libstdc++-v3/include/std/limits     | 194 +++++++++++++++-------------
> >   2 files changed, 103 insertions(+), 95 deletions(-)
> >
> [...]
> > diff --git a/libstdc++-v3/include/std/limits b/libstdc++-v3/include/std/limits
> > index 52b19ef8264..7a59e7520eb 100644
> > --- a/libstdc++-v3/include/std/limits
> > +++ b/libstdc++-v3/include/std/limits
> > @@ -1890,189 +1890,197 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> [...]
> >   __glibcxx_float_n(64)
> >   #endif
> > -#ifdef __STDCPP_FLOAT128_T__
> > +#ifdef __FLT128_DIG__
> >   __glibcxx_float_n(128)
> >   #endif
> >   #undef __glibcxx_float_n
> [...]
>
> The above change (from __STDCPP_FLOAT128_T__ to __FLT128_DIG__) now
> started to cause issues with Clang on Clang 18 trunk:
>
> * Clang does not support a _Float128 type.
>
> * Clang does not predefine __STDCPP_FLOAT128_T__.
>
> * But since
> <https://github.com/llvm/llvm-project/commit/457f582ffe23e951380bc345c4c96ec053c09681>
> "[clang] Predefined macros for float128 support (#67196)", Clang 18
> trunk does predefine __FLT128_DIG__ now.  Which causes
>
> > $ cat test.cc
> > #include <limits>
>
> > $ clang++ -fsyntax-only test.cc
> > In file included from test.cc:1:
> > /home/sbergman/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/14.0.0/../../../../include/c++/14.0.0/limits:1995:1: error: use of undeclared identifier '_Float128'
> >  1995 | __glibcxx_float_n(128)
> >       | ^
> > /home/sbergman/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/14.0.0/../../../../include/c++/14.0.0/limits:1903:27: note: expanded from macro '__glibcxx_float_n'
> >  1903 |     struct numeric_limits<_Float##BITSIZE>                              \
> >       |                           ^
> > <scratch space>:36:1: note: expanded from here
> >    36 | _Float128
> >       | ^
> > 1 error generated.
>
> (I don't know whether or not it is useful for Clang to predefine
> __FLT128_DIG__ when not providing a _Float128 type.  I assume
> <https://www.iso.org/standard/65615.html> "ISO/IEC TS 18661-3:2015", as
> referenced by the C++ standard, might be relevant here, but don't know
> that document.  I added Pranav, the author of the relevant Clang commit,
> in cc here.)


It's completely wrong or Clang to define a macro describing properties
of a non-existent type.

This was reported as
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111687 and closed as
INVALID, it needs to be fixed in Clang.


>