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.)
