[PATCH v2] libstdc++: Remove unnecessary code from reduce implementation

[Jonathan Wakely]

On Wed, 19 Aug 2026 at 09:53, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Tue, Aug 18, 2026 at 10:10 AM Matthias Kretz <MatthiasKretz@gmx.net> wrote:
>>
>> Resending (see patch from 2026-07-02). OK for trunk?

OK for trunk, thanks.

(Is it still needed for gcc-16? Should the drive-by be backported at least?)


>>
>> ------------- 8< -----------
>>
>> From: Matthias Kretz <m.kretz@gsi.de>
>>
>> Previously, this was necessary for efficient reductions. When I tried to
>> turn it into a missed-optimization PR, there was no issue (anymore). The
>> generic _S_static_permute implementation optimizes just fine (or better)
>> at this point.
>>
>> The traits_impl.cc change is a simple drive-by fix.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/bits/simd_vec.h (_M_elements_shifted_to_front): Remove
>>         x86-specific implementation.
>>         * testsuite/std/simd/traits_impl.cc: Fix incorrect macro name.
>>
>> Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
>
> Thanks, this LGTM. Happy to see that optimization is no longer needed.
> Still needs Jonathan ACK.
>>
>> ---
>>  libstdc++-v3/include/bits/simd_vec.h          | 32 +++----------------
>>  .../testsuite/std/simd/traits_impl.cc         |  2 +-
>>  2 files changed, 6 insertions(+), 28 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/bits/simd_vec.h b/libstdc++-v3/include/bits/
>> simd_vec.h
>> index bf6616df227..9473c57b6cd 100644
>> --- a/libstdc++-v3/include/bits/simd_vec.h
>> +++ b/libstdc++-v3/include/bits/simd_vec.h
>> @@ -449,33 +449,11 @@ _M_elements_shifted_to_front() const
>>           static_assert(_Shift < _S_size && -_Shift < _S_size);
>>           if constexpr (_Shift == 0)
>>             return *this;
>> -#ifdef __SSE2__
>> -         else if (!__is_const_known(*this))
>> -           {
>> -             if constexpr (sizeof(_M_data) == 16 && _Shift > 0)
>> -               return reinterpret_cast<_DataType>(
>> -                        __builtin_ia32_psrldqi128(__vec_bit_cast<long
>> long>(_M_data),
>> -                                                  _Shift *
>> sizeof(value_type) * 8));
>> -             else if constexpr (sizeof(_M_data) == 16 && _Shift < 0)
>> -               return reinterpret_cast<_DataType>(
>> -                        __builtin_ia32_pslldqi128(__vec_bit_cast<long
>> long>(_M_data),
>> -                                                  -_Shift *
>> sizeof(value_type) * 8));
>> -             else if constexpr (sizeof(_M_data) < 16)
>> -               {
>> -                 auto __x = reinterpret_cast<__vec_builtin_type_bytes<long
>> long, 16>>(
>> -                              __vec_zero_pad_to_16(_M_data));
>> -                 if constexpr (_Shift > 0)
>> -                   __x = __builtin_ia32_psrldqi128(__x, _Shift *
>> sizeof(value_type) * 8);
>> -                 else
>> -                   __x = __builtin_ia32_pslldqi128(__x, -_Shift *
>> sizeof(value_type) * 8);
>> -                 return
>> _VecOps<_DataType>::_S_extract(__vec_bit_cast<__canon_value_type>(__x));
>> -               }
>> -           }
>> -#endif
>> -         return _S_static_permute(*this, [](int __i) consteval {
>> -                  int __off = __i + _Shift;
>> -                  return __off >= _S_size || __off < 0 ? zero_element :
>> __off;
>> -                });
>> +         else
>> +           return _S_static_permute(*this, [](int __i) consteval {
>> +                    int __off = __i + _Shift;
>> +                    return __off >= _S_size || __off < 0 ? zero_element :
>> __off;
>> +                  });
>>         }
>>
>>        /** @internal
>> diff --git a/libstdc++-v3/testsuite/std/simd/traits_impl.cc b/libstdc++-v3/
>> testsuite/std/simd/traits_impl.cc
>> index dde41c70aca..9f6e9f42b6e 100644
>> --- a/libstdc++-v3/testsuite/std/simd/traits_impl.cc
>> +++ b/libstdc++-v3/testsuite/std/simd/traits_impl.cc
>> @@ -49,7 +49,7 @@ namespace simd
>>        static_assert(__vectorizable<__integer_from<N>>);
>>      }
>>    template for (constexpr int N : {
>> -#ifdef __STDCPP_BFLOAT16_T__
>> +#ifdef __STDCPP_FLOAT16_T__
>>                                   2,
>>  #endif
>>                                   4, 8})
>> --
>> ──────────────────────────────────────────────────────────────────────────
>>  Dr. Matthias Kretz                           https://mattkretz.github.io
>>  GSI Helmholtz Center for Heavy Ion Research               https://gsi.de
>>  std::simd
>> ──────────────────────────────────────────────────────────────────────────
>>
>>
>>
>>