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

[Matthias Kretz]

Resending (see patch from 2026-07-02). OK for trunk?

------------- 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>
---
 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
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