QList: prepare for tag dispatching based on memory layout

Add one tag class for each of QList's three different memory layouts
to QListData, and inherit QList<T>::MemoryLayout from exactly one of
them.

To simplify overloading, added tag classes that express the negation
of the two extreme poles of memory layout (C-compatible and heap),
too. The "missing" one could be added when needed, too.

Change-Id: I45ea603731499fd3fdfb37d60a0a98fb22ac15ec
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 51 insertions, 0 deletions

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 023dafb223..255816d0ba 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -66,6 +66,14 @@ template <typename T> struct QListSpecialMethods { };
 template <> struct QListSpecialMethods<QByteArray>;
 
 struct Q_CORE_EXPORT QListData {
+    // tags for tag-dispatching of QList implementations,
+    // based on QList's three different memory layouts:
+    struct NotArrayCompatibleLayout {};
+    struct NotIndirectLayout {};
+    struct ArrayCompatibleLayout   : NotIndirectLayout {};                           // data laid out like a C array
+    struct InlineWithPaddingLayout : NotArrayCompatibleLayout, NotIndirectLayout {}; // data laid out like a C array with padding
+    struct IndirectLayout          : NotArrayCompatibleLayout {};                    // data allocated on the heap
+
     struct Data {
         QtPrivate::RefCount ref;
         int alloc, begin, end;
@@ -99,6 +107,17 @@ struct Q_CORE_EXPORT QListData {
 template <typename T>
 class QList : public QListSpecialMethods<T>
 {
+public:
+    struct MemoryLayout
+        : QtPrivate::if_<
+            QTypeInfo<T>::isStatic || QTypeInfo<T>::isLarge,
+            QListData::IndirectLayout,
+            typename QtPrivate::if_<
+                sizeof(T) == sizeof(void*),
+                QListData::ArrayCompatibleLayout,
+                QListData::InlineWithPaddingLayout
+             >::type>::type {};
+private:
     struct Node { void *v;
 #if defined(Q_CC_BOR)
         Q_INLINE_TEMPLATE T &t();
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index 95744051e4..1207986dde 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -35,6 +35,17 @@
 #include <QtTest/QtTest>
 #include <QList>
 
+template <typename T, class MemoryLayout>
+class is_qlist_array_memory_layout {
+    struct No { char c; };
+    struct Yes { No n[2]; };
+    Q_STATIC_ASSERT(sizeof(No) != sizeof(Yes));
+    static No check(...);
+    static Yes check(MemoryLayout);
+public:
+    enum { value = sizeof(check(typename QList<T>::MemoryLayout())) == sizeof(Yes) };
+};
+
 struct Movable {
     Movable(char input = 'j')
         : i(input)
@@ -235,6 +246,27 @@ Q_STATIC_ASSERT(!QTypeInfo<Optimal>::isStatic);
 Q_STATIC_ASSERT(QTypeInfo<Optimal>::isComplex);
 Q_STATIC_ASSERT(QTypeInfo<Complex>::isStatic);
 Q_STATIC_ASSERT(QTypeInfo<Complex>::isComplex);
+// iow:
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<int, QListData::NotIndirectLayout>       ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<int, QListData::IndirectLayout>          ::value));
+
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Optimal, QListData::InlineWithPaddingLayout> ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Optimal, QListData::NotArrayCompatibleLayout>::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Optimal, QListData::NotIndirectLayout>       ::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Optimal, QListData::ArrayCompatibleLayout>   ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Optimal, QListData::IndirectLayout>          ::value));
+
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Movable, QListData::InlineWithPaddingLayout> ::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Movable, QListData::NotArrayCompatibleLayout>::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Movable, QListData::NotIndirectLayout>       ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Movable, QListData::ArrayCompatibleLayout>   ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Movable, QListData::IndirectLayout>          ::value));
+
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Complex, QListData::InlineWithPaddingLayout> ::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Complex, QListData::NotArrayCompatibleLayout>::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Complex, QListData::NotIndirectLayout>       ::value));
+Q_STATIC_ASSERT((!is_qlist_array_memory_layout<Complex, QListData::ArrayCompatibleLayout>   ::value));
+Q_STATIC_ASSERT(( is_qlist_array_memory_layout<Complex, QListData::IndirectLayout>          ::value));
 
 class tst_QList : public QObject
 {