diff --git a/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp b/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
--- a/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
+++ b/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
@@ -227,18 +227,14 @@
         test_roundtrip_through_nested_T<false>(i);
         test_roundtrip_through_buffer<false>(i);
 
-        // On arm64 on Apple platforms, long double is just double, so we don't
-        // test against int128, but instead against double itself. Otherwise,
-        // we test against int128 if we have those types available.
-#if defined(__aarch64__) && defined(__APPLE__)
-#   define LONG_DOUBLE_IS_DOUBLE
-#endif
-
-#if defined(LONG_DOUBLE_IS_DOUBLE)
+#if defined(__SIZEOF_LONG_DOUBLE__) && defined(__SIZEOF_DOUBLE__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_DOUBLE__
         test_roundtrip_through<double, false>(i);
-#elif !defined(_LIBCPP_HAS_NO_INT128)
+#endif
+#if !defined(_LIBCPP_HAS_NO_INT128)
+#if defined(__SIZEOF_LONG_DOUBLE__) && defined(__SIZEOF_INT128__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_INT128__
         test_roundtrip_through<__int128_t, false>(i);
         test_roundtrip_through<__uint128_t, false>(i);
+#endif
 #endif
     }