[committed] libstdc++: Fix detection of std::format support for __float128 [PR107693]
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Commit Message
Tested x86_64-linux and x86_64-w64-mingw32. Pushed to trunk.
-- >8 --
std::format gives linker errors on targets that define __float128 but
do not support using it with std::to_chars. This improves the handling
of 128-bit flaoting-point types so they are disabled if unsupportable.
libstdc++-v3/ChangeLog:
PR libstdc++/107693
* include/std/format (_GLIBCXX_FORMAT_F128): Define to 2 when
basic_format_arg needs to use its _M_f128 member.
(__extended_floating_point, __floating_point): Replace with ...
(__formattable_floating_point): New concept.
* testsuite/std/format/functions/format.cc: Check whether
__float128 is supported. Also test _Float128.
---
libstdc++-v3/include/std/format | 77 ++++++++++---------
.../testsuite/std/format/functions/format.cc | 20 ++++-
2 files changed, 59 insertions(+), 38 deletions(-)
Comments
On Tue, Nov 15, 2022 at 02:31:19PM +0000, Jonathan Wakely via Gcc-patches wrote:
> Tested x86_64-linux and x86_64-w64-mingw32. Pushed to trunk.
>
> -- >8 --
>
> std::format gives linker errors on targets that define __float128 but
> do not support using it with std::to_chars. This improves the handling
> of 128-bit flaoting-point types so they are disabled if unsupportable.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/107693
> * include/std/format (_GLIBCXX_FORMAT_F128): Define to 2 when
> basic_format_arg needs to use its _M_f128 member.
> (__extended_floating_point, __floating_point): Replace with ...
> (__formattable_floating_point): New concept.
> * testsuite/std/format/functions/format.cc: Check whether
> __float128 is supported. Also test _Float128.
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> +#elif __FLT128_DIG__ && defined(__GLIBC_PREREQ) // see floating_to_chars.cc
I'd just use here
#elif __FLT128_DIG__ && defined(_GLIBCXX_HAVE_FLOAT128_MATH)
instead.
The reason for defined(__GLIBC_PREREQ) in floating_{to,from}_chars.cc
is that I didn't want to make the ABI of linux libstdc++.so.6 dependent
on whether gcc was built against glibc 2.26+ or older glibc.
So, the symbols exist in libstdc++.so.6 even for older glibcs, but it will
actually only work properly (without losing precision; otherwise it will
just go through long double) if at runtime one uses glibc 2.26+.
But in the headers, defined(_GLIBCXX_HAVE_FLOAT128_MATH) is used everywhere
else (which is true only when compiling against glibc 2.26+).
Jakub
On Tue, 15 Nov 2022 at 14:42, Jakub Jelinek wrote:
>
> On Tue, Nov 15, 2022 at 02:31:19PM +0000, Jonathan Wakely via Gcc-patches wrote:
> > Tested x86_64-linux and x86_64-w64-mingw32. Pushed to trunk.
> >
> > -- >8 --
> >
> > std::format gives linker errors on targets that define __float128 but
> > do not support using it with std::to_chars. This improves the handling
> > of 128-bit flaoting-point types so they are disabled if unsupportable.
> >
> > libstdc++-v3/ChangeLog:
> >
> > PR libstdc++/107693
> > * include/std/format (_GLIBCXX_FORMAT_F128): Define to 2 when
> > basic_format_arg needs to use its _M_f128 member.
> > (__extended_floating_point, __floating_point): Replace with ...
> > (__formattable_floating_point): New concept.
> > * testsuite/std/format/functions/format.cc: Check whether
> > __float128 is supported. Also test _Float128.
>
> > --- a/libstdc++-v3/include/std/format
> > +++ b/libstdc++-v3/include/std/format
>
> > +#elif __FLT128_DIG__ && defined(__GLIBC_PREREQ) // see floating_to_chars.cc
>
> I'd just use here
> #elif __FLT128_DIG__ && defined(_GLIBCXX_HAVE_FLOAT128_MATH)
> instead.
>
> The reason for defined(__GLIBC_PREREQ) in floating_{to,from}_chars.cc
> is that I didn't want to make the ABI of linux libstdc++.so.6 dependent
> on whether gcc was built against glibc 2.26+ or older glibc.
> So, the symbols exist in libstdc++.so.6 even for older glibcs, but it will
> actually only work properly (without losing precision; otherwise it will
> just go through long double) if at runtime one uses glibc 2.26+.
Yes, and my intention was that std::format would also support
__float128, with the same imprecise behaviour. But could just limit
std::format support to when std::to_chars works properly.
> But in the headers, defined(_GLIBCXX_HAVE_FLOAT128_MATH) is used everywhere
> else (which is true only when compiling against glibc 2.26+).
@@ -1213,40 +1213,35 @@ namespace __format
_Spec<_CharT> _M_spec{};
};
+ // Decide how 128-bit floating-point types should be formatted (or not).
+ // When supported, the typedef __format::__float128_t is the type that
+ // format arguments should be converted to for storage in basic_format_arg.
+ // Define the macro _GLIBCXX_FORMAT_F128 to say they're supported.
+ // _GLIBCXX_FORMAT_F128=1 means __float128, _Float128 etc. will be formatted
+ // by converting them to long double (or __ieee128 for powerpc64le).
+ // _GLIBCXX_FORMAT_F128=2 means basic_format_arg needs to enable explicit
+ // support for _Float128, rather than formatting it as another type.
+#undef _GLIBCXX_FORMAT_F128
+
#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
-# define _GLIBCXX_FORMAT_F128 1
+
+ // Format 128-bit floating-point types using __ieee128.
using __float128_t = __ieee128;
-#elif defined _GLIBCXX_LDOUBLE_IS_IEEE_BINARY128
# define _GLIBCXX_FORMAT_F128 1
+
+#elif defined _GLIBCXX_LDOUBLE_IS_IEEE_BINARY128
+
+ // Format 128-bit floating-point types using long double.
using __float128_t = long double;
-#elif __FLT128_DIG__
-# define _GLIBCXX_FORMAT_F128 2
+# define _GLIBCXX_FORMAT_F128 1
+
+#elif __FLT128_DIG__ && defined(__GLIBC_PREREQ) // see floating_to_chars.cc
+
+ // Format 128-bit floating-point types using _Float128.
using __float128_t = _Float128;
-#else
-# undef _GLIBCXX_FORMAT_F128
-#endif
+# define _GLIBCXX_FORMAT_F128 2
-#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
- template<typename _Tp>
- concept __extended_floating_point = __is_same(_Tp, _Float128)
- || __is_same(_Tp, __ibm128)
- || __is_same(_Tp, __ieee128);
-#elif _GLIBCXX_FORMAT_F128
- template<typename _Tp>
- concept __extended_floating_point = __is_same(_Tp, __float128_t);
-#else
- template<typename _Tp>
- concept __extended_floating_point = false;
-#endif
-
- template<typename _Tp>
- concept __floating_point = std::floating_point<_Tp>
- || __extended_floating_point<_Tp>;
-
- using std::to_chars;
-
-#if _GLIBCXX_FORMAT_F128 == 2 \
- && (__cplusplus == 202002L || !defined(_GLIBCXX_HAVE_FLOAT128_MATH))
+# if __cplusplus == 202002L || !defined(_GLIBCXX_HAVE_FLOAT128_MATH)
// These overloads exist in the library, but are not declared for C++20.
// Make them available as std::__format::to_chars.
to_chars_result
@@ -1260,8 +1255,16 @@ namespace __format
to_chars_result
to_chars(char*, char*, _Float128, chars_format, int) noexcept
__asm("_ZSt8to_charsPcS_DF128_St12chars_formati");
+# endif
#endif
+ using std::to_chars;
+
+ // We can format a floating-point type iff it is usable with to_chars.
+ template<typename _Tp>
+ concept __formattable_float = requires (_Tp __t, char* __p)
+ { __format::to_chars(__p, __p, __t, chars_format::scientific, 6); };
+
template<__char _CharT>
struct __formatter_fp
{
@@ -1984,7 +1987,7 @@ namespace __format
#endif
/// Format a floating-point value.
- template<__format::__floating_point _Tp, __format::__char _CharT>
+ template<__format::__formattable_float _Tp, __format::__char _CharT>
struct formatter<_Tp, _CharT>
{
formatter() = default;
@@ -2607,7 +2610,7 @@ namespace __format
#ifdef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
__ieee128 _M_f128;
__ibm128 _M_ibm128;
-#elif _GLIBCXX_FORMAT_F128
+#elif _GLIBCXX_FORMAT_F128 == 2
__float128_t _M_f128;
#endif
};
@@ -2663,7 +2666,7 @@ namespace __format
else if constexpr (is_same_v<_Tp, unsigned __int128>)
return __u._M_u128;
#endif
-#if _GLIBCXX_FORMAT_F128
+#if _GLIBCXX_FORMAT_F128 == 2
else if constexpr (is_same_v<_Tp, __float128_t>)
return __u._M_f128;
#endif
@@ -2843,13 +2846,15 @@ namespace __format
return type_identity<_Float64>();
# endif
#endif
-#ifdef __FLT128_DIG__
+#if _GLIBCXX_FORMAT_F128
+# if __FLT128_DIG__
else if constexpr (is_same_v<_Td, _Float128>)
return type_identity<__format::__float128_t>();
-#endif
-#if _GLIBCXX_USE_FLOAT128
+# endif
+# if __SIZEOF_FLOAT128__
else if constexpr (is_same_v<_Td, __float128>)
return type_identity<__format::__float128_t>();
+# endif
#endif
else if constexpr (__is_specialization_of<_Td, basic_string_view>)
return type_identity<basic_string_view<_CharT>>();
@@ -2926,7 +2931,7 @@ namespace __format
else if constexpr (is_same_v<_Tp, _Float64>)
return _Arg_f64;
#endif
-#if _GLIBCXX_FORMAT_F128
+#if _GLIBCXX_FORMAT_F128 == 2
else if constexpr (is_same_v<_Tp, __format::__float128_t>)
return _Arg_f128;
#endif
@@ -3015,7 +3020,7 @@ namespace __format
#endif
// TODO _Arg_f16 etc.
-#if _GLIBCXX_FORMAT_F128
+#if _GLIBCXX_FORMAT_F128 == 2
case _Arg_f128:
return std::forward<_Visitor>(__vis)(_M_val._M_f128);
#endif
@@ -5,6 +5,7 @@
#include <string>
#include <limits>
#include <cstdint>
+#include <cstdio>
#include <testsuite_hooks.h>
void
@@ -289,12 +290,27 @@ test_p1652r1() // printf corner cases in std::format
VERIFY( s == "3.31" );
}
+template<typename T>
+bool format_float()
+{
+ auto s = std::format("{:#} != {:<+7.3f}", (T)-0.0, (T)0.5);
+ return s == "-0. != +0.500 ";
+}
+
void
test_float128()
{
#ifdef __SIZEOF_FLOAT128__
- auto s = std::format("{:#} != {:<+7.3f}", (__float128)-0.0, (__float128)0.5);
- VERIFY( s == "-0. != +0.500 " );
+ if constexpr (std::formattable<__float128, char>)
+ VERIFY( format_float<__float128>() );
+ else
+ std::puts("Cannot format __float128 on this target");
+#endif
+#if __FLT128_DIG__
+ if constexpr (std::formattable<_Float128, char>)
+ VERIFY( format_float<_Float128>() );
+ else
+ std::puts("Cannot format _Float128 on this target");
#endif
}