[PATCH] libstdc++: Format std::float16_t and std::bfloat16_t using respective std::to_chars overloads [PR126731]
Tomasz Kaminski
tkaminsk@redhat.com
Thu Aug 20 16:32:34 GMT 2026
On Thu, Aug 20, 2026 at 5:29 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Thu, 20 Aug 2026 at 14:53, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> >
> >
> >
> > On Thu, Aug 20, 2026 at 3:51 PM Tomasz Kaminski <tkaminsk@redhat.com>
> wrote:
> >>
> >>
> >>
> >> On Thu, Aug 20, 2026 at 2:36 PM Jonathan Wakely <jwakely@redhat.com>
> 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.
>
Backported it to GCC-16 just now.
> >>>
> >>> 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
> >
> > I meant incorrect output in this case.
> >>
> >> * 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.
>
> Ah yes, so definitely worth keeping them on the older branches then.
>
>
> >>>
> >>>
> >>>
> >>> > 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
> >>> >
> >>> >
> >>>
>
>
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