[gcc r16-9569] libstdc++: Format std::float16_t and std::bfloat16_t using respective std::to_chars overloads [PR126
Tomasz Kaminski
tkaminsk@gcc.gnu.org
Thu Aug 20 16:31:26 GMT 2026
https://gcc.gnu.org/g:9d7f5369bd888a1c62376db5065cea49007a7eb8
commit r16-9569-g9d7f5369bd888a1c62376db5065cea49007a7eb8
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date: Thu Aug 13 09:51:56 2026 +0200
libstdc++: Format std::float16_t and std::bfloat16_t using respective std::to_chars overloads [PR126731]
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 than float for above.
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 reverted parts of r14-3305-g6cf214b4fc97f5 (revereted partially
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.
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.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
(cherry picked from commit 4b6ce501c4f1db06c8084170a68796622266b671)
Diff:
---
libstdc++-v3/include/std/format | 44 ----------------------
.../testsuite/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 ee7eea58f34a..a90a0b03597d 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2941,28 +2941,6 @@ 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>
@@ -3052,28 +3030,6 @@ 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 82e696fa797c..7b231b070060 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
More information about the Libstdc++-cvs
mailing list