Re: [PATCH] libstdc++: Format std::float16_t and std::bfloat16_t using respective std::to_chars overloads [PR126731]

On Thu, 13 Aug 2026 at 10:36 +0200, Tomasz Kamiński wrote:
>This patch removes the partial specialization for float16_t and bfloat16_t
>that formatted the values by casting to float. In consequence an implicit
>specialization of generic floating-point specialization is used, and
>to_chars overloads for their types are used.
>As explained in r13-3591-g0ae26533b3e268 (that adds corresponding to_chars
>overload), the shortest string (produced when no precision is specified)
>is shorter for above, than float.
>
>As formatter specializations were defined under the same condition as
>corresponding to_chars overloads, the __formattable_float constrain on
>formatter floating-point specialization makes it enable in same cases.
>
>This reverts parts of r14-3305-g6cf214b4fc97f5 (revereted partially

"reverted"

>in r14-3329-g27d0cfcb2b33de), leading to (necessary for correctness)
>increase of the number template specializations. It also depends on
>preserving values of above types (instead of float) in basic_format_arg
>introduced r16-616-g9c9a7316adb996.

Ah, so I added these specializations in r14-3305-g6cf214b4fc97f5 to
support printing extended floating-point types in C++20, but that
turned out to not work. So I adjusted the conditions for enabling them
in r14-3329-g27d0cfcb2b33de, so that they are only enabled for C++23.
But really we should have just removed the specializations again,
because the __formattable_float specialization formats them properly
since r16-616-g9c9a7316adb996.

>The specializations for float32_t and float64_t are left unchanged,
>as their to_chars overloads are also implemented in terms of casting
>to float/double respectivelly.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/126731
>	* include/std/format (std::formatter<_Float16, _CharT>)
>	(std::formatter<__format::__bflt16_t, _CharT>): Remove
>	partial specializations.
>	* testsuite/std/format/formatter/ext_float.cc: Test output
>	for closest value to 1/10.
>---
>As far as I can tell this case will not be affected by P3505R4
>"Fix the default floating-point representation in std::format",
>as float(0.1f16) is 1.998p-4, while 0.1 f is 1.99999ap-4, so
>only later could use 0.1 as shortest string.
>
>Testing on x86_64-linux. *format* passed in all standard modes,
>-m32, debug and assertions. OK for trunk and 16?
>(We cannot backport it to eariel standards due dependency described
>above).

Yes OK for trunk and gcc-16.

If I understand correctly, I think we could remove them in gcc-14 and
gcc-15, because they're redundant (the __formattable_float
specialization can format them).  But we would still get incorrect
results, because basic_format_arg would convert them to float. So the
extra specializations in gcc-14 and gcc-15 are not doing any harm, and
it doesn't seem worth backporting it.


> libstdc++-v3/include/std/format               | 44 -------------------
> .../std/format/formatter/ext_float.cc         | 22 ++++++----
> 2 files changed, 13 insertions(+), 53 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
>index 729cb89ec60..8d530e883c3 100644
>--- a/libstdc++-v3/include/std/format
>+++ b/libstdc++-v3/include/std/format
>@@ -3003,28 +3003,6 @@ namespace __format
>     };
> #endif
>
>-#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>
>-    {
>-      formatter() = default;
>-
>-      [[__gnu__::__always_inline__]]
>-      constexpr typename basic_format_parse_context<_CharT>::iterator
>-      parse(basic_format_parse_context<_CharT>& __pc)
>-      { return _M_f.parse(__pc); }
>-
>-      template<typename _Out>
>-	typename basic_format_context<_Out, _CharT>::iterator
>-	format(_Float16 __u, basic_format_context<_Out, _CharT>& __fc) const
>-	{ return _M_f.format((float)__u, __fc); }
>-
>-    private:
>-      __format::__formatter_fp<_CharT> _M_f;
>-    };
>-#endif
>-
> #if defined(__FLT32_DIG__) && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
>   // Reuse __formatter_fp<C>::format<float, Out> for _Float32.
>   template<__format::__char _CharT>
>@@ -3114,28 +3092,6 @@ namespace __format
>     };
> #endif
>
>-#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<__format::__bflt16_t, _CharT>
>-    {
>-      formatter() = default;
>-
>-      [[__gnu__::__always_inline__]]
>-      constexpr typename basic_format_parse_context<_CharT>::iterator
>-      parse(basic_format_parse_context<_CharT>& __pc)
>-      { return _M_f.parse(__pc); }
>-
>-      template<typename _Out>
>-	typename basic_format_context<_Out, _CharT>::iterator
>-	format(__gnu_cxx::__bfloat16_t __u,
>-	       basic_format_context<_Out, _CharT>& __fc) const
>-	{ return _M_f.format((float)__u, __fc); }
>-
>-    private:
>-      __format::__formatter_fp<_CharT> _M_f;
>-    };
>-#endif
> #endif // __cpp_lib_to_chars
>
>   /** Format a pointer.
>diff --git a/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc b/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>index 82e696fa797..7b231b07006 100644
>--- a/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>+++ b/libstdc++-v3/testsuite/std/format/formatter/ext_float.cc
>@@ -4,10 +4,14 @@
> #include <testsuite_hooks.h>
>
> template<typename T>
>-bool format_float()
>+void
>+verify_output()
> {
>-    auto s = std::format("{:#} != {:<+7.3f}", (T)-0.0, (T)0.5);
>-    return s == "-0. != +0.500 ";
>+  auto s = std::format("{:#} != {:<+7.3f}", T(-0.0), T(0.5));
>+  VERIFY( s == "-0. != +0.500 ");
>+
>+  s = std::format("{}", T(1)/T(10));
>+  VERIFY( s == "0.1");
> }
>
> #if __cplusplus > 202002L
>@@ -23,7 +27,7 @@ test_float16()
> {
> #if __FLT16_DIG__
>   if constexpr (formattable<_Float16>)
>-    VERIFY( format_float<_Float16>() );
>+    verify_output<_Float16>();
>   else
>     std::puts("Cannot format _Float16 on this target");
> #endif
>@@ -34,7 +38,7 @@ test_float32()
> {
> #if __FLT32_DIG__
>   if constexpr (formattable<_Float32>)
>-    VERIFY( format_float<_Float32>() );
>+    verify_output<_Float32>();
>   else
>     std::puts("Cannot format _Float32 on this target");
> #endif
>@@ -45,7 +49,7 @@ test_float64()
> {
> #if __FLT64_DIG__
>   if constexpr (formattable<_Float64>)
>-    VERIFY( format_float<_Float64>() );
>+    verify_output<_Float64>();
>   else
>     std::puts("Cannot format _Float64 on this target");
> #endif
>@@ -56,13 +60,13 @@ test_float128()
> {
> #ifdef __SIZEOF_FLOAT128__
>   if constexpr (formattable<__float128>)
>-    VERIFY( format_float<__float128>() );
>+    verify_output<__float128>();
>   else
>     std::puts("Cannot format __float128 on this target");
> #endif
> #if __FLT128_DIG__
>   if constexpr (formattable<_Float128>)
>-    VERIFY( format_float<_Float128>() );
>+    verify_output<_Float128>();
>   else
>     std::puts("Cannot format _Float128 on this target");
> #endif
>@@ -75,7 +79,7 @@ test_bfloat16()
>   using bfloat16_t = decltype(0.0bf16);
>
>   if constexpr (formattable<bfloat16_t>)
>-    VERIFY( format_float<bfloat16_t>() );
>+    verify_output<bfloat16_t>();
>   else
>     std::puts("Cannot format bfloat16_t on this target");
> #endif
>-- 
>2.55.0
>
>