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

On Thu, Aug 20, 2026 at 2:36 PM Jonathan Wakely <[email protected]> wrote:

> 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.

We will get inconsistent results, depending on how you format them:
 * if (b)float16 is passed as argument to format call, then ye we will
convert
   them to float, and get correct result
 * if you call formatter<(b)float16> direclty (for example becaue you are
writting
  formatter for a class with a member), you will get correct output

> So the extra specializations in gcc-14 and gcc-15 are not doing any harm,
> and
> it doesn't seem worth backporting it.
>
They are keeping us consistently wrong; if we remove them, we will be
inconsistent.
and wrong in the usual case.

>
>
> > 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
> >
> >
>
>