[PATCH] libstdc++: Fix Cylindrical Bessel functions for infinity [PR119143]

[Jonathan Wakely]

On Fri, 10 Jul 2026 at 12:47, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Fri, Jul 10, 2026 at 1:30 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> The result should be zero at infinity.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         PR libstdc++/119143
>>         * include/tr1/bessel_function.tcc (__cyl_bessel_jn_asymp):
>>         Return zero for infinite x.
>>         * testsuite/special_functions/08_cyl_bessel_j/check_inf.cc: New
>>         test.
>>         * testsuite/special_functions/10_cyl_neumann/check_inf.cc: New
>>         test.
>>         * testsuite/tr1/5_numerical_facilities/special_functions/11_cyl_neumann/check_inf.cc:
>>         New test.
>> ---
>>
>> Tested x86_64-linux.
>>
>>  libstdc++-v3/include/tr1/bessel_function.tcc  |  6 +++
>>  .../08_cyl_bessel_j/check_inf.cc              | 38 +++++++++++++++++++
>>  .../10_cyl_neumann/check_inf.cc               | 38 +++++++++++++++++++
>>  .../11_cyl_neumann/check_inf.cc               | 37 ++++++++++++++++++
>>  4 files changed, 119 insertions(+)
>>  create mode 100644 libstdc++-v3/testsuite/special_functions/08_cyl_bessel_j/check_inf.cc
>>  create mode 100644 libstdc++-v3/testsuite/special_functions/10_cyl_neumann/check_inf.cc
>>  create mode 100644 libstdc++-v3/testsuite/tr1/5_numerical_facilities/special_functions/11_cyl_neumann/check_inf.cc
>>
>> diff --git a/libstdc++-v3/include/tr1/bessel_function.tcc b/libstdc++-v3/include/tr1/bessel_function.tcc
>> index 7e06d98ce3c6..bba792bd89d3 100644
>> --- a/libstdc++-v3/include/tr1/bessel_function.tcc
>> +++ b/libstdc++-v3/include/tr1/bessel_function.tcc
>> @@ -362,6 +362,12 @@ namespace tr1
>>      void
>>      __cyl_bessel_jn_asymp(_Tp __nu, _Tp __x, _Tp & __Jnu, _Tp & __Nnu)
>>      {
>> +      if (__builtin_expect(__builtin_isinf(__x), 0))
>> +       {
>> +         __Jnu = __Nnu = _Tp(0);
>> +         return;
>> +       }
>> +
>>        const _Tp __mu = _Tp(4) * __nu * __nu;
>>        const _Tp __8x = _Tp(8) * __x;
>>
>> diff --git a/libstdc++-v3/testsuite/special_functions/08_cyl_bessel_j/check_inf.cc b/libstdc++-v3/testsuite/special_functions/08_cyl_bessel_j/check_inf.cc
>> new file mode 100644
>> index 000000000000..50c1f3098297
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/special_functions/08_cyl_bessel_j/check_inf.cc
>> @@ -0,0 +1,38 @@
>> +// { dg-do run { target c++11 } }
>> +// { dg-require-c-std "" }
>> +// { dg-options "-D__STDCPP_WANT_MATH_SPEC_FUNCS__" }
>> +// { dg-add-options ieee }
>> +
>> +// ISO/IEC IS 29124 8.1.8 cyl_bessel_j
>> +// J_nu(+inf) == 0 for all finite nu >= 0
>> +
>> +#include <cmath>
>> +#include <limits>
>> +#include <testsuite_hooks.h>
>> +
>> +void
>> +test01()
>
> I would template this on RealType and instantiate it for float and double.
> long double instead, but this is also OK.

I did consider that, but the new tests match the form of the existing
check_nan.cc tests.

I realized that I forgot to add  09_cyl_bessel_j/check_inf.cc to the
TR1 tests, so the committed patch includes a fourth new test file.