[PATCH] libstdc++: Correct preprocessing checks for floatX_t and bfloat_16 formatting

[Tomasz Kamiński]

Floating points types _Float16, _Float32, _Float64, and bfloat16,
can be formatted only if std::to_chars overloads for such types
where provided. Currently this is only the case for architectures
where float and double are 32-bits and 64-bits IEEE floating points types.

This patch updates the preprocessing checks for formatters
for above types to check _GLIBCXX_FLOAT_IS_IEEE_BINARY32
and _GLIBCXX_DOUBLE_IS_IEEE_BINARY64. Making them non-formattable
on non-IEEE architectures.

Remove a potential UB, where we could produce basic_format_arg
with _M_type set to _Arg_fp32 or _Arg_fp64, that was later not
handled by `_M_visit`.

libstdc++-v3/ChangeLog:

	* include/std/format (formatter<_Float16, _CharT>): Define only if
	_GLIBCXX_FLOAT_IS_IEEE_BINARY32 macro is defined.
	(formatter<_Float16, _CharT>): As above.
	(formatter<__gnu_cxx::__bfloat16_t, _CharT>): As above.
	(formatter<_Float64, _CharT>): Define only if
	_GLIBCXX_DOUBLE_IS_IEEE_BINARY64 is defined.
	(basic_format_arg::_S_to_arg_type): Normalize _Float32 and _Float64
	only to float and double respectivelly.
	(basic_format_arg::_S_to_enum): Remove handling of _Float32 and _Float64.
---
Tested on x86_64-linux. OK for trunk?

 libstdc++-v3/include/std/format | 32 ++++++++------------------------
 1 file changed, 8 insertions(+), 24 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index e7e0d2d142b..1809f040077 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2276,7 +2276,7 @@ namespace __format
     };
 #endif
 
-#ifdef __STDCPP_FLOAT16_T__
+#if defined(__STDCPP_FLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for _Float16.
   template<__format::__char _CharT>
     struct formatter<_Float16, _CharT>
@@ -2298,7 +2298,7 @@ namespace __format
     };
 #endif
 
-#if defined(__FLT32_DIG__)
+#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for _Float32.
   template<__format::__char _CharT>
     struct formatter<_Float32, _CharT>
@@ -2320,7 +2320,7 @@ namespace __format
     };
 #endif
 
-#if defined(__FLT64_DIG__)
+#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
   // Reuse __formatter_fp<C>::format<double, Out> for _Float64.
   template<__format::__char _CharT>
     struct formatter<_Float64, _CharT>
@@ -2364,7 +2364,7 @@ namespace __format
     };
 #endif
 
-#ifdef __STDCPP_BFLOAT16_T__
+#if defined(__STDCPP_BFLOAT16_T__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
   // Reuse __formatter_fp<C>::format<float, Out> for bfloat16_t.
   template<__format::__char _CharT>
     struct formatter<__gnu_cxx::__bfloat16_t, _CharT>
@@ -3443,22 +3443,16 @@ namespace __format
 	    return type_identity<float>();
 #endif
 
-#ifdef __FLT32_DIG__
+#if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
 	  else if constexpr (is_same_v<_Td, _Float32>)
-# ifdef _GLIBCXX_FLOAT_IS_IEEE_BINARY32
 	    return type_identity<float>();
-# else
-	    return type_identity<_Float32>();
-# endif
 #endif
-#ifdef __FLT64_DIG__
+
+#if defined(__FLT64_DIG__) && defined(_GLIBCXX_DOUBLE_IS_IEEE_BINARY64)
 	  else if constexpr (is_same_v<_Td, _Float64>)
-# ifdef _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
 	    return type_identity<double>();
-# else
-	    return type_identity<_Float64>();
-# endif
 #endif
+
 #if _GLIBCXX_FORMAT_F128
 # if __FLT128_DIG__
 	  else if constexpr (is_same_v<_Td, _Float128>)
@@ -3538,16 +3532,6 @@ namespace __format
 	    return _Arg_u128;
 #endif
 
-	  // N.B. some of these types will never actually be used here,
-	  // because they get normalized to a standard floating-point type.
-#if defined __FLT32_DIG__ && ! _GLIBCXX_FLOAT_IS_IEEE_BINARY32
-	  else if constexpr (is_same_v<_Tp, _Float32>)
-	    return _Arg_f32;
-#endif
-#if defined __FLT64_DIG__ && ! _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
-	  else if constexpr (is_same_v<_Tp, _Float64>)
-	    return _Arg_f64;
-#endif
 #if _GLIBCXX_FORMAT_F128 == 2
 	  else if constexpr (is_same_v<_Tp, __format::__float128_t>)
 	    return _Arg_f128;
-- 
2.48.1