[gcc r16-9575] libstdc++: Remove unnecessary code from reduce implementation

[Matthias Kretz]

https://gcc.gnu.org/g:1356b29f6cdf4a9cae63afb98d30880b5397bbff

commit r16-9575-g1356b29f6cdf4a9cae63afb98d30880b5397bbff
Author: Matthias Kretz <m.kretz@gsi.de>
Date:   Thu Jul 2 13:37:57 2026 +0200

    libstdc++: Remove unnecessary code from reduce implementation
    
    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.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/simd_vec.h (_M_elements_shifted_to_front): Remove
            x86-specific implementation.
    
    Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
    (cherry picked from commit f09bafa778caaeac1faaa16eacf4d0ad7adb6c92)

Diff:
---
 libstdc++-v3/include/bits/simd_vec.h | 32 +++++---------------------------
 1 file changed, 5 insertions(+), 27 deletions(-)

diff --git a/libstdc++-v3/include/bits/simd_vec.h b/libstdc++-v3/include/bits/simd_vec.h
index 5624ec781426..7fe4290cc4ce 100644
--- a/libstdc++-v3/include/bits/simd_vec.h
+++ b/libstdc++-v3/include/bits/simd_vec.h
@@ -443,33 +443,11 @@ namespace simd
 	  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