libstdc++: Fix simd<char> conversion for -fno-signed-char for Clang

The special case for Clang in the trait producing a signed integer type
lead to the trait returning 'char' where it should have been 'signed
char'. This workaround was introduced because on Clang the return type
of vector compares was not convertible to '_SimdWrapper<
__int_for_sizeof_t<...' unless '__int_for_sizeof_t<char>' was an alias
for 'char'. In order to not rewrite the complete mask type code (there
is code scattered around the implementation assuming signed integers),
this needs to be 'signed char'; so the special case for Clang needs to
be removed.
The conversion issue is now solved in _SimdWrapper, which now
additionally allows conversion from vector types with compatible
integral type.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	PR libstdc++/115308
	* include/experimental/bits/simd.h (__int_for_sizeof): Remove
	special cases for __clang__.
	(_SimdWrapper): Change constructor overload set to allow
	conversion from vector types with integral conversions via bit
	reinterpretation.

(cherry picked from commit 8e36cf4c5c9140915d0019999db132a900b48037)

1 files changed, 27 insertions, 18 deletions

diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/include/experimental/bits/simd.h
index 2cc2807..365f515 100644
--- a/libstdc++-v3/include/experimental/bits/simd.h
+++ b/libstdc++-v3/include/experimental/bits/simd.h
@@ -584,19 +584,12 @@ template <size_t _Bytes>
     static_assert(_Bytes > 0);
     if constexpr (_Bytes == sizeof(int))
       return int();
-  #ifdef __clang__
-    else if constexpr (_Bytes == sizeof(char))
-      return char();
-  #else
     else if constexpr (_Bytes == sizeof(_SChar))
       return _SChar();
-  #endif
     else if constexpr (_Bytes == sizeof(short))
       return short();
-  #ifndef __clang__
     else if constexpr (_Bytes == sizeof(long))
       return long();
-  #endif
     else if constexpr (_Bytes == sizeof(_LLong))
       return _LLong();
   #ifdef __SIZEOF_INT128__
@@ -2712,6 +2705,8 @@ template <typename _BuiltinType>
 
 // }}}
 // _SimdWrapper{{{
+struct _DisabledSimdWrapper;
+
 template <typename _Tp, size_t _Width>
   struct _SimdWrapper<
     _Tp, _Width,
@@ -2721,16 +2716,17 @@ template <typename _Tp, size_t _Width>
 			      == sizeof(__vector_type_t<_Tp, _Width>),
 		       __vector_type_t<_Tp, _Width>>
   {
-    using _Base
-      = _SimdWrapperBase<__has_iec559_behavior<__signaling_NaN, _Tp>::value
-			   && sizeof(_Tp) * _Width
-				== sizeof(__vector_type_t<_Tp, _Width>),
-			 __vector_type_t<_Tp, _Width>>;
+    static constexpr bool _S_need_default_init
+      = __has_iec559_behavior<__signaling_NaN, _Tp>::value
+	  and sizeof(_Tp) * _Width == sizeof(__vector_type_t<_Tp, _Width>);
+
+    using _BuiltinType = __vector_type_t<_Tp, _Width>;
+
+    using _Base = _SimdWrapperBase<_S_need_default_init, _BuiltinType>;
 
     static_assert(__is_vectorizable_v<_Tp>);
     static_assert(_Width >= 2); // 1 doesn't make sense, use _Tp directly then
 
-    using _BuiltinType = __vector_type_t<_Tp, _Width>;
     using value_type = _Tp;
 
     static inline constexpr size_t _S_full_size
@@ -2766,13 +2762,26 @@ template <typename _Tp, size_t _Width>
     _GLIBCXX_SIMD_INTRINSIC constexpr _SimdWrapper&
     operator=(_SimdWrapper&&) = default;
 
-    template <typename _V, typename = enable_if_t<disjunction_v<
-			     is_same<_V, __vector_type_t<_Tp, _Width>>,
-			     is_same<_V, __intrinsic_type_t<_Tp, _Width>>>>>
+    // Convert from exactly matching __vector_type_t
+    using _SimdWrapperBase<_S_need_default_init, _BuiltinType>::_SimdWrapperBase;
+
+    // Convert from __intrinsic_type_t if __intrinsic_type_t and __vector_type_t differ, otherwise
+    // this ctor should not exist. Making the argument type unusable is our next best solution.
+    _GLIBCXX_SIMD_INTRINSIC constexpr
+    _SimdWrapper(conditional_t<is_same_v<_BuiltinType, __intrinsic_type_t<_Tp, _Width>>,
+			       _DisabledSimdWrapper, __intrinsic_type_t<_Tp, _Width>> __x)
+    : _Base(__vector_bitcast<_Tp, _Width>(__x)) {}
+
+    // Convert from different __vector_type_t, but only if bit reinterpretation is a correct
+    // conversion of the value_type
+    template <typename _V, typename _TVT = _VectorTraits<_V>,
+	      typename = enable_if_t<sizeof(typename _TVT::value_type) == sizeof(_Tp)
+				       and sizeof(_V) == sizeof(_BuiltinType)
+				       and is_integral_v<_Tp>
+				       and is_integral_v<typename _TVT::value_type>>>
       _GLIBCXX_SIMD_INTRINSIC constexpr
       _SimdWrapper(_V __x)
-      // __vector_bitcast can convert e.g. __m128 to __vector(2) float
-      : _Base(__vector_bitcast<_Tp, _Width>(__x)) {}
+      : _Base(reinterpret_cast<_BuiltinType>(__x)) {}
 
     template <typename... _As,
 	      typename = enable_if_t<((is_same_v<simd_abi::scalar, _As> && ...)