diff --git a/cmake/QtPlatformTargetHelpers.cmake b/cmake/QtPlatformTargetHelpers.cmake
index 0b2025666de..90f159c06c6 100644
--- a/cmake/QtPlatformTargetHelpers.cmake
+++ b/cmake/QtPlatformTargetHelpers.cmake
@@ -47,15 +47,6 @@ function(qt_internal_setup_public_platform_target)
         target_compile_options(Platform INTERFACE "$<$<CXX_COMPILER_ID:Clang>:-fno-direct-access-external-data>")
     endif()
 
-    # Qt checks if a given platform supports 128 bit integers
-    # by checking if __SIZEOF_128__ is defined
-    # VXWORKS doesn't support 128 bit integers
-    # but it uses clang which defines __SIZEOF_128__
-    # which breaks the detection mechanism
-    if(VXWORKS)
-        target_compile_definitions(Platform INTERFACE "-DQT_NO_INT128")
-    endif()
-
     qt_set_msvc_cplusplus_options(Platform INTERFACE)
 
     # Propagate minimum C++ 17 via Platform to Qt consumers (apps), after the global features
diff --git a/src/corelib/global/qsystemdetection.h b/src/corelib/global/qsystemdetection.h
index b29f2e94964..0cbcef27407 100644
--- a/src/corelib/global/qsystemdetection.h
+++ b/src/corelib/global/qsystemdetection.h
@@ -239,4 +239,12 @@
 #  define Q_OF_MACH_O
 #endif
 
+#if defined(__SIZEOF_INT128__)
+// Compiler used in VxWorks SDK declares __SIZEOF_INT128__ but VxWorks doesn't support this type,
+// so we can't rely solely on compiler here.
+#if !defined(Q_OS_VXWORKS)
+#  define QT_COMPILER_SUPPORTS_INT128 __SIZEOF_INT128__
+#endif
+#endif // defined(__SIZEOF_INT128__)
+
 #endif // QSYSTEMDETECTION_H
diff --git a/src/corelib/global/qtypes.cpp b/src/corelib/global/qtypes.cpp
index 9a4616b9d89..bd01498ce2d 100644
--- a/src/corelib/global/qtypes.cpp
+++ b/src/corelib/global/qtypes.cpp
@@ -503,7 +503,7 @@ static_assert(sizeof(size_t) == sizeof(qsizetype)); // implied by the definition
 static_assert((std::is_same<qsizetype, qptrdiff>::value));
 static_assert(std::is_same_v<std::size_t, size_t>);
 
-#ifdef __SIZEOF_INT128__
+#ifdef QT_COMPILER_SUPPORTS_INT128
 # ifndef QT_SUPPORTS_INT128
 #  error Qt needs to be compiled in a mode that enables INT128 \
     if the compiler supports it in principle.
diff --git a/src/corelib/global/qtypes.h b/src/corelib/global/qtypes.h
index d5dffe60477..65bf01fac14 100644
--- a/src/corelib/global/qtypes.h
+++ b/src/corelib/global/qtypes.h
@@ -6,6 +6,7 @@
 #define QTYPES_H
 
 #include <QtCore/qprocessordetection.h>
+#include <QtCore/qsystemdetection.h>
 #include <QtCore/qtconfigmacros.h>
 #include <QtCore/qassert.h>
 
@@ -66,8 +67,8 @@ typedef quint64 qulonglong;
 
 #ifdef Q_QDOC // QDoc always needs to see the typedefs
 #  define QT_SUPPORTS_INT128 16
-#elif defined(__SIZEOF_INT128__) && !defined(QT_NO_INT128)
-#  define QT_SUPPORTS_INT128 __SIZEOF_INT128__
+#elif defined(QT_COMPILER_SUPPORTS_INT128) && !defined(QT_NO_INT128)
+#  define QT_SUPPORTS_INT128 QT_COMPILER_SUPPORTS_INT128
 #  if defined(__GLIBCXX__) && defined(__STRICT_ANSI__) // -ansi/-std=c++NN instead of gnu++NN
 #    undef QT_SUPPORTS_INT128                          // breaks <type_traits> on libstdc++
 #  endif
diff --git a/src/corelib/plugin/quuid.h b/src/corelib/plugin/quuid.h
index 49f2b6749cd..435b7bb572d 100644
--- a/src/corelib/plugin/quuid.h
+++ b/src/corelib/plugin/quuid.h
@@ -7,6 +7,7 @@
 #include <QtCore/qcompare.h>
 #include <QtCore/qendian.h>
 #include <QtCore/qstring.h>
+#include <QtCore/qsystemdetection.h>
 
 #if defined(Q_OS_WIN) || defined(Q_QDOC)
 #ifndef GUID_DEFINED
@@ -61,7 +62,7 @@ public:
         quint16 data16[8];
         quint32 data32[4];
         quint64 data64[2];
-#if defined(__SIZEOF_INT128__)
+#if defined(QT_COMPILER_SUPPORTS_INT128)
 QT_WARNING_PUSH
 QT_WARNING_DISABLE_GCC("-Wpedantic")    // ISO C++ does not support ‘__int128’ for ‘data128’
         unsigned __int128 data128[1];