[committed] libstdc++: Define std::numeric_limits<_FloatNN> before C++23
Stephan Bergmann
sbergman@redhat.com
Wed Oct 4 15:54:38 GMT 2023
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.)
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