QTypeInfo: use C++11 type traits to deduce if a type is static or complex

All types that can be trivially copied and destructed are by definition
relocatable, and we should apply those semantics when moving them in
memory.

Types that are trivial, are by definition not complex and should be
treated as such.

[ChangeLog][QtCore] Qt Containers and meta type system now use C++11
type traits (std::is_trivial, std::is_trivially_copyable and
std::is_trivially_destructible) to detect the class of a type not
explicitly set by Q_DECLARE_TYPEINFO. (Q_DECLARE_TYPEINFO is still
needed for QList.)

Done-with: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Change-Id: Iebb87ece425ea919e86169d06cd509c54a074282
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 31 insertions, 4 deletions

diff --git a/src/corelib/global/qglobal.cpp b/src/corelib/global/qglobal.cpp
index 8da94c8624..c0d46b73d9 100644
--- a/src/corelib/global/qglobal.cpp
+++ b/src/corelib/global/qglobal.cpp
@@ -4103,6 +4103,13 @@ bool qunsetenv(const char *varName)
     Example of a movable type:
 
     \snippet code/src_corelib_global_qglobal.cpp 39
+
+    Qt will try to detect the class of a type using std::is_trivial or
+    std::is_trivially_copyable. Use this macro to tune the behavior.
+    For instance many types would be candidates for Q_MOVABLE_TYPE despite
+    not being trivially-copyable. For binary compatibility reasons, QList
+    optimizations are only enabled if there is an explicit
+    Q_DECLARE_TYPEINFO even for trivially-copyable types.
 */
 
 /*!
diff --git a/src/corelib/global/qtypeinfo.h b/src/corelib/global/qtypeinfo.h
index 4f79c48c51..567ff5c08e 100644
--- a/src/corelib/global/qtypeinfo.h
+++ b/src/corelib/global/qtypeinfo.h
@@ -49,6 +49,26 @@ QT_BEGIN_NAMESPACE
    QTypeInfo     - type trait functionality
 */
 
+template <typename T>
+static constexpr bool qIsRelocatable()
+{
+#if defined(Q_CC_CLANG) || !defined(Q_CC_GNU) || Q_CC_GNU >= 501
+    return std::is_trivially_copyable<T>::value && std::is_trivially_destructible<T>::value;
+#else
+    return std::is_enum<T>::value || std::is_integral<T>::value;
+#endif
+}
+
+template <typename T>
+static constexpr bool qIsTrivial()
+{
+#if defined(Q_CC_CLANG) || !defined(Q_CC_GNU) || Q_CC_GNU >= 501
+    return std::is_trivial<T>::value;
+#else
+    return std::is_enum<T>::value || std::is_integral<T>::value;
+#endif
+}
+
 /*
   The catch-all template.
 */
@@ -61,9 +81,9 @@ public:
         isSpecialized = std::is_enum<T>::value, // don't require every enum to be marked manually
         isPointer = false,
         isIntegral = std::is_integral<T>::value,
-        isComplex = !isIntegral && !std::is_enum<T>::value,
+        isComplex = !qIsTrivial<T>(),
         isStatic = true,
-        isRelocatable = std::is_enum<T>::value,
+        isRelocatable = qIsRelocatable<T>(),
         isLarge = (sizeof(T)>sizeof(void*)),
         isDummy = false, //### Qt6: remove
         sizeOf = sizeof(T)
@@ -248,9 +268,9 @@ class QTypeInfo<TYPE > \
 public: \
     enum { \
         isSpecialized = true, \
-        isComplex = (((FLAGS) & Q_PRIMITIVE_TYPE) == 0), \
+        isComplex = (((FLAGS) & Q_PRIMITIVE_TYPE) == 0) && !qIsTrivial<TYPE>(), \
         isStatic = (((FLAGS) & (Q_MOVABLE_TYPE | Q_PRIMITIVE_TYPE)) == 0), \
-        isRelocatable = !isStatic || ((FLAGS) & Q_RELOCATABLE_TYPE), \
+        isRelocatable = !isStatic || ((FLAGS) & Q_RELOCATABLE_TYPE) || qIsRelocatable<TYPE>(), \
         isLarge = (sizeof(TYPE)>sizeof(void*)), \
         isPointer = false, \
         isIntegral = std::is_integral< TYPE >::value, \