diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index b57e3f338e92a..6dca2d9467aa5 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -481,9 +481,20 @@ _GLIBCXX_END_NAMESPACE_VERSION
 // Define if compatibility should be provided for -mlong-double-64.
 #undef _GLIBCXX_LONG_DOUBLE_COMPAT
 
+// Use an alternate macro to test for clang, so as to provide an easy
+// workaround for systems (such as vxworks) whose headers require
+// __clang__ to be defined, even when compiling with GCC.
+#if !defined _GLIBCXX_CLANG && defined __clang__
+# define _GLIBCXX_CLANG __clang__
+// Turn -D_GLIBCXX_CLANG=0 into -U_GLIBCXX_CLANG, so that
+// _GLIBCXX_CLANG can be tested as defined, just like __clang__.
+#elif !_GLIBCXX_CLANG
+# undef _GLIBCXX_CLANG
+#endif
+
 // Define if compatibility should be provided for alternative 128-bit long
 // double formats. Not possible for Clang until __ibm128 is supported.
-#ifndef __clang__
+#ifndef _GLIBCXX_CLANG
 #undef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
 #endif
 
diff --git a/libstdc++-v3/include/bits/locale_facets_nonio.tcc b/libstdc++-v3/include/bits/locale_facets_nonio.tcc
index 8f67be5a6147b..72136f42f0866 100644
--- a/libstdc++-v3/include/bits/locale_facets_nonio.tcc
+++ b/libstdc++-v3/include/bits/locale_facets_nonio.tcc
@@ -1465,7 +1465,7 @@ _GLIBCXX_END_NAMESPACE_LDBL_OR_CXX11
       ctype<_CharT> const& __ctype = use_facet<ctype<_CharT> >(__loc);
       __err = ios_base::goodbit;
       bool __use_state = false;
-#if __GNUC__ >= 5 && !defined(__clang__)
+#if __GNUC__ >= 5 && !defined(_GLIBCXX_CLANG)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wpmf-conversions"
       // Nasty hack.  The C++ standard mandates that get invokes the do_get
diff --git a/libstdc++-v3/include/bits/stl_bvector.h b/libstdc++-v3/include/bits/stl_bvector.h
index d567e26f4e430..52153cadf8f70 100644
--- a/libstdc++-v3/include/bits/stl_bvector.h
+++ b/libstdc++-v3/include/bits/stl_bvector.h
@@ -185,7 +185,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     void
     _M_assume_normalized() const
     {
-#if __has_attribute(__assume__) && !defined(__clang__)
+#if __has_attribute(__assume__) && !defined(_GLIBCXX_CLANG)
       unsigned int __ofst = _M_offset;
       __attribute__ ((__assume__ (__ofst < unsigned(_S_word_bit))));
 #endif
diff --git a/libstdc++-v3/include/c_compatibility/stdatomic.h b/libstdc++-v3/include/c_compatibility/stdatomic.h
index 52daa16c9b1ed..5403b52a036d1 100644
--- a/libstdc++-v3/include/c_compatibility/stdatomic.h
+++ b/libstdc++-v3/include/c_compatibility/stdatomic.h
@@ -125,7 +125,7 @@ using std::atomic_flag_clear_explicit;
 using std::atomic_thread_fence;
 using std::atomic_signal_fence;
 
-#elif defined __clang__
+#elif defined _GLIBCXX_CLANG
 # include_next <stdatomic.h>
 #endif // __cpp_lib_stdatomic_h
 #endif // _GLIBCXX_STDATOMIC_H
diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/include/experimental/bits/simd.h
index 7c52462571902..ea034138fd720 100644
--- a/libstdc++-v3/include/experimental/bits/simd.h
+++ b/libstdc++-v3/include/experimental/bits/simd.h
@@ -606,7 +606,7 @@ template <size_t _Bytes>
     static_assert(_Bytes > 0);
     if constexpr (_Bytes == sizeof(int))
       return int();
-  #ifdef __clang__
+  #ifdef _GLIBCXX_CLANG
     else if constexpr (_Bytes == sizeof(char))
       return char();
   #else
@@ -615,7 +615,7 @@ template <size_t _Bytes>
   #endif
     else if constexpr (_Bytes == sizeof(short))
       return short();
-  #ifndef __clang__
+  #ifndef _GLIBCXX_CLANG
     else if constexpr (_Bytes == sizeof(long))
       return long();
   #endif
@@ -1720,7 +1720,7 @@ template <typename _To, typename _From>
 	  __builtin_memcpy(&__r, &__v, sizeof(_To));
 	  return __r;
 	}
-#if _GLIBCXX_SIMD_X86INTRIN && !defined __clang__
+#if _GLIBCXX_SIMD_X86INTRIN && !defined _GLIBCXX_CLANG
     else if constexpr (__have_avx && sizeof(_From) == 16 && sizeof(_To) == 32)
       return reinterpret_cast<_To>(__builtin_ia32_ps256_ps(
 	reinterpret_cast<__vector_type_t<float, 4>>(__v)));
@@ -1975,7 +1975,7 @@ template <typename _TW>
 
 // }}}
 // __andnot{{{
-#if _GLIBCXX_SIMD_X86INTRIN && !defined __clang__
+#if _GLIBCXX_SIMD_X86INTRIN && !defined _GLIBCXX_CLANG
 static constexpr struct
 {
   _GLIBCXX_SIMD_INTRINSIC __v4sf
@@ -2035,7 +2035,7 @@ static constexpr struct
   operator()(__v8di __a, __v8di __b) const noexcept
   { return _mm512_andnot_si512(__a, __b); }
 } _S_x86_andnot;
-#endif // _GLIBCXX_SIMD_X86INTRIN && !__clang__
+#endif // _GLIBCXX_SIMD_X86INTRIN && !_GLIBCXX_CLANG
 
 template <typename _TW>
   _GLIBCXX_SIMD_INTRINSIC constexpr _TW
@@ -2046,7 +2046,7 @@ template <typename _TW>
 	using _TVT = conditional_t<__is_simd_wrapper_v<_TW>, _TW,
 				   _VectorTraitsImpl<_TW>>;
 	using _Tp = typename _TVT::value_type;
-#if _GLIBCXX_SIMD_X86INTRIN && !defined __clang__
+#if _GLIBCXX_SIMD_X86INTRIN && !defined _GLIBCXX_CLANG
 	if constexpr (sizeof(_TW) >= 16)
 	  {
 	    const auto __ai = __to_intrin(__a);
@@ -2095,7 +2095,7 @@ template <typename _T0, typename _T1, typename _Fun, size_t... _Is>
     using _Tp = remove_reference_t<decltype(declval<_T0>()[0])>;
     using _RV [[maybe_unused]] = __vector_type_t<_Tp, sizeof...(_Is)>;
 #if __has_builtin(__builtin_shufflevector)
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
     // Clang requires _T0 == _T1
     if constexpr (sizeof(__x) > sizeof(__y) and _N1 == 1)
       return __vec_shuffle(__x, _T0{__y[0]}, __seq, __idx_perm);
diff --git a/libstdc++-v3/include/experimental/bits/simd_builtin.h b/libstdc++-v3/include/experimental/bits/simd_builtin.h
index 505f8083794e7..4abc750cfa1c2 100644
--- a/libstdc++-v3/include/experimental/bits/simd_builtin.h
+++ b/libstdc++-v3/include/experimental/bits/simd_builtin.h
@@ -1177,7 +1177,7 @@ struct _CommonImplBuiltin
       constexpr size_t _Bytes = _ReqBytes == 0 ? sizeof(__x) : _ReqBytes;
       static_assert(sizeof(__x) >= _Bytes);
 
-#if !defined __clang__ && _GLIBCXX_SIMD_WORKAROUND_PR90424
+#if !defined _GLIBCXX_CLANG && _GLIBCXX_SIMD_WORKAROUND_PR90424
       if constexpr (__is_vector_type_v<_TV>)
 	_S_memcpy<_Bytes>(reinterpret_cast<char*>(__addr), reinterpret_cast<const char*>(&__x));
       else
@@ -2022,7 +2022,7 @@ template <typename _Abi, typename>
 	      return __vector_bitcast<float, 2>(_S_plus_minus(__x4, __y));
 	  }
 #endif
-#if !defined __clang__ && __GCC_IEC_559 == 0
+#if !defined _GLIBCXX_CLANG && __GCC_IEC_559 == 0
 	if (__builtin_is_constant_evaluated()
 	      || (__builtin_constant_p(__x) && __builtin_constant_p(__y)))
 	  return (__x + __y) - __y;
diff --git a/libstdc++-v3/include/experimental/bits/simd_detail.h b/libstdc++-v3/include/experimental/bits/simd_detail.h
index 51e130bd36a5e..9ddf912a62b03 100644
--- a/libstdc++-v3/include/experimental/bits/simd_detail.h
+++ b/libstdc++-v3/include/experimental/bits/simd_detail.h
@@ -262,7 +262,7 @@
 #endif
 //}}}
 
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
 #define _GLIBCXX_SIMD_NORMAL_MATH
 #define _GLIBCXX_SIMD_ALWAYS_INLINE_LAMBDA
 #else
@@ -277,7 +277,7 @@
 #define _GLIBCXX_SIMD_IS_UNLIKELY(__x) __builtin_expect(__x, 0)
 #define _GLIBCXX_SIMD_IS_LIKELY(__x) __builtin_expect(__x, 1)
 
-#if _GLIBCXX_SIMD_HAVE_SVE || __STRICT_ANSI__ || defined __clang__
+#if _GLIBCXX_SIMD_HAVE_SVE || __STRICT_ANSI__ || defined _GLIBCXX_CLANG
 #define _GLIBCXX_SIMD_CONSTEXPR
 #define _GLIBCXX_SIMD_USE_CONSTEXPR_API const
 #else
@@ -285,7 +285,7 @@
 #define _GLIBCXX_SIMD_USE_CONSTEXPR_API constexpr
 #endif
 
-#if defined __clang__
+#if defined _GLIBCXX_CLANG
 #define _GLIBCXX_SIMD_USE_CONSTEXPR const
 #else
 #define _GLIBCXX_SIMD_USE_CONSTEXPR constexpr
@@ -330,7 +330,7 @@
 #endif
 
 // integer division not optimized
-#ifndef __clang__
+#ifndef _GLIBCXX_CLANG
 #define _GLIBCXX_SIMD_WORKAROUND_PR90993 1
 #endif
 
diff --git a/libstdc++-v3/include/experimental/bits/simd_x86.h b/libstdc++-v3/include/experimental/bits/simd_x86.h
index 517c4b4a5bec7..4ab933b573c61 100644
--- a/libstdc++-v3/include/experimental/bits/simd_x86.h
+++ b/libstdc++-v3/include/experimental/bits/simd_x86.h
@@ -363,7 +363,7 @@ template <typename _Tp>
 
 // }}}
 
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
 template <size_t _Np, typename _Tp, typename _Kp>
   _GLIBCXX_SIMD_INTRINSIC constexpr auto
   __movm(_Kp __k) noexcept
@@ -408,7 +408,7 @@ template <size_t _Np, typename _Tp, typename _Kp>
     else
       __assert_unreachable<_Tp>();
   }
-#endif // __clang__
+#endif // _GLIBCXX_CLANG
 
 #ifdef _GLIBCXX_SIMD_WORKAROUND_PR85048
 #include "simd_x86_conversions.h"
@@ -674,7 +674,7 @@ struct _CommonImplX86 : _CommonImplBuiltin
       using _Tp = typename _VectorTraits<_TV>::value_type;
       static_assert(sizeof(_TV) >= 16);
       static_assert(sizeof(_Tp) <= 8);
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
       return __movm<_VectorTraits<_TV>::_S_full_size, _Tp>(__k) ? __b : __a;
 #else
       using _IntT
@@ -3505,9 +3505,9 @@ template <typename _Abi, typename>
 	    // optimize masked unary increment and decrement as masked sub +/-1
 	    constexpr int __pm_one
 	      = is_same_v<_Op<void>, __increment<void>> ? -1 : 1;
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
 	    return __movm<_Np, _Tp>(__k._M_data) ? __v._M_data - __pm_one : __v._M_data;
-#else // __clang__
+#else // _GLIBCXX_CLANG
 	    using _TV = __vector_type_t<_Tp, _Np>;
 	    constexpr size_t __bytes = sizeof(__v) < 16 ? 16 : sizeof(__v);
 	    constexpr size_t __width = __bytes / sizeof(_Tp);
@@ -3561,7 +3561,7 @@ template <typename _Abi, typename>
 #undef _GLIBCXX_SIMD_MASK_SUB_512
 #undef _GLIBCXX_SIMD_MASK_SUB
 	      }
-#endif // __clang__
+#endif // _GLIBCXX_CLANG
 	  }
 	else
 	  return _Base::template _S_masked_unary<_Op>(__k, __v);
diff --git a/libstdc++-v3/include/experimental/simd b/libstdc++-v3/include/experimental/simd
index f45ad4ed2e6cc..d2d081e0d1a2a 100644
--- a/libstdc++-v3/include/experimental/simd
+++ b/libstdc++-v3/include/experimental/simd
@@ -59,7 +59,7 @@
 #pragma GCC diagnostic push
 // Many [[gnu::vector_size(N)]] types might lead to a -Wpsabi warning which is
 // irrelevant as those functions never appear on ABI borders
-#ifndef __clang__
+#ifndef _GLIBCXX_CLANG
 #pragma GCC diagnostic ignored "-Wpsabi"
 #endif
 
diff --git a/libstdc++-v3/include/pstl/pstl_config.h b/libstdc++-v3/include/pstl/pstl_config.h
index ccb9dd32838d3..7157a8a492ed9 100644
--- a/libstdc++-v3/include/pstl/pstl_config.h
+++ b/libstdc++-v3/include/pstl/pstl_config.h
@@ -53,7 +53,7 @@
 // the actual GCC version on the system.
 #define _PSTL_GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
 
-#if defined(__clang__)
+#if defined(_GLIBCXX_CLANG)
 // according to clang documentation, version can be vendor specific
 #    define _PSTL_CLANG_VERSION (__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__)
 #endif
@@ -62,7 +62,7 @@
 #if (defined(_OPENMP) && _OPENMP >= 201307) || \
     (defined(__INTEL_COMPILER) && __INTEL_COMPILER >= 1600) || \
     (!defined(__INTEL_COMPILER) && _PSTL_GCC_VERSION >= 40900) || \
-    defined(__clang__)
+    defined(_GLIBCXX_CLANG)
 #    define _PSTL_PRAGMA_SIMD _PSTL_PRAGMA(omp simd)
 #    define _PSTL_PRAGMA_DECLARE_SIMD _PSTL_PRAGMA(omp declare simd)
 #    define _PSTL_PRAGMA_SIMD_REDUCTION(PRM) _PSTL_PRAGMA(omp simd reduction(PRM))
diff --git a/libstdc++-v3/include/std/complex b/libstdc++-v3/include/std/complex
index 415c735665c3e..be810b6d05d25 100644
--- a/libstdc++-v3/include/std/complex
+++ b/libstdc++-v3/include/std/complex
@@ -47,7 +47,7 @@
 // Get rid of a macro possibly defined in <complex.h>
 #undef complex
 
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wc99-extensions"
 #endif
@@ -2646,7 +2646,7 @@ _GLIBCXX_END_NAMESPACE_VERSION
 
 #endif  // C++11
 
-#ifdef __clang__
+#ifdef _GLIBCXX_CLANG
 #pragma clang diagnostic pop
 #endif
 
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index b1e827c9a7241..3f335b95a0864 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -4687,7 +4687,7 @@ namespace views::__adaptor
   class zip_view<_Vs...>::_Iterator
     : public __detail::__zip_view_iter_cat<_Const, _Vs...>
   {
-#ifdef __clang__ // LLVM-61763 workaround
+#ifdef _GLIBCXX_CLANG // LLVM-61763 workaround
   public:
 #endif
     tuple<iterator_t<__detail::__maybe_const_t<_Const, _Vs>>...> _M_current;
@@ -4710,7 +4710,7 @@ namespace views::__adaptor
 	return input_iterator_tag{};
     }
 
-#ifndef __clang__ // LLVM-61763 workaround
+#ifndef _GLIBCXX_CLANG // LLVM-61763 workaround
     template<move_constructible _Fp, input_range... _Ws>
       requires (view<_Ws> && ...) && (sizeof...(_Ws) > 0) && is_object_v<_Fp>
 	&& regular_invocable<_Fp&, range_reference_t<_Ws>...>
@@ -5387,7 +5387,7 @@ namespace views::__adaptor
   template<bool _Const>
   class adjacent_view<_Vp, _Nm>::_Iterator
   {
-#ifdef __clang__ // LLVM-61763 workaround
+#ifdef _GLIBCXX_CLANG // LLVM-61763 workaround
   public:
 #endif
     using _Base = __detail::__maybe_const_t<_Const, _Vp>;
@@ -5430,7 +5430,7 @@ namespace views::__adaptor
 
     friend class adjacent_view;
 
-#ifndef __clang__ // LLVM-61763 workaround
+#ifndef _GLIBCXX_CLANG // LLVM-61763 workaround
     template<forward_range _Wp, move_constructible _Fp, size_t _Mm>
       requires view<_Wp> && (_Mm > 0) && is_object_v<_Fp>
         && regular_invocable<__detail::__unarize<_Fp&, _Mm>, range_reference_t<_Wp>>
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 371cbb90f54a6..51aaa62085170 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1758,7 +1758,7 @@ namespace __detail::__variant
 	  }, __rhs);
       }
 
-#if defined(__clang__) && __clang_major__ <= 7
+#if defined(_GLIBCXX_CLANG) && __clang_major__ <= 7
     public:
       using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
 #endif
