Fallback implementation of Q_ALIGNOF

For all practical purposes, the fallback introduced here returns the
desired value. Having a fallback enables unconditional use of Q_ALIGNOF.

For compilers that provide native support for it, Q_ALIGNOF is otherwise
 #defined in qcompilerdetection.h.

Change-Id: Ie148ca8936cbbf8b80fe87771a14797c39a9d30c
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

5 files changed, 50 insertions, 17 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index d0d6e851ad..ee577a7563 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -248,6 +248,56 @@ typedef quint64 qulonglong;
 
 #if defined(__cplusplus)
 
+namespace QtPrivate {
+    template <class T>
+    struct AlignOfHelper
+    {
+        char c;
+        T type;
+
+        AlignOfHelper();
+        ~AlignOfHelper();
+    };
+
+    template <class T>
+    struct AlignOf_Default
+    {
+        enum { Value = sizeof(AlignOfHelper<T>) - sizeof(T) };
+    };
+
+    template <class T> struct AlignOf : AlignOf_Default<T> { };
+    template <class T> struct AlignOf<T &> : AlignOf<T> {};
+    template <size_t N, class T> struct AlignOf<T[N]> : AlignOf<T> {};
+
+#ifdef Q_COMPILER_RVALUE_REFS
+    template <class T> struct AlignOf<T &&> : AlignOf<T> {};
+#endif
+
+#if defined(Q_PROCESSOR_X86_32) && !defined(Q_OS_WIN)
+    template <class T> struct AlignOf_WorkaroundForI386Abi { enum { Value = sizeof(T) }; };
+
+    // x86 ABI weirdness
+    // Alignment of naked type is 8, but inside struct has alignment 4.
+    template <> struct AlignOf<double>  : AlignOf_WorkaroundForI386Abi<double> {};
+    template <> struct AlignOf<qint64>  : AlignOf_WorkaroundForI386Abi<qint64> {};
+    template <> struct AlignOf<quint64> : AlignOf_WorkaroundForI386Abi<quint64> {};
+#ifdef Q_CC_CLANG
+    // GCC and Clang seem to disagree wrt to alignment of arrays
+    template <size_t N> struct AlignOf<double[N]>   : AlignOf_Default<double> {};
+    template <size_t N> struct AlignOf<qint64[N]>   : AlignOf_Default<qint64> {};
+    template <size_t N> struct AlignOf<quint64[N]>  : AlignOf_Default<quint64> {};
+#endif
+#endif
+} // namespace QtPrivate
+
+#define QT_EMULATED_ALIGNOF(T) \
+    (size_t(QT_PREPEND_NAMESPACE(QtPrivate)::AlignOf<T>::Value))
+
+#ifndef Q_ALIGNOF
+#define Q_ALIGNOF(T) QT_EMULATED_ALIGNOF(T)
+#endif
+
+
 /*
   quintptr and qptrdiff is guaranteed to be the same size as a pointer, i.e.
 
diff --git a/src/corelib/kernel/qmetaobjectbuilder.cpp b/src/corelib/kernel/qmetaobjectbuilder.cpp
index 59740960c9..8d6d7cbe91 100644
--- a/src/corelib/kernel/qmetaobjectbuilder.cpp
+++ b/src/corelib/kernel/qmetaobjectbuilder.cpp
@@ -1095,11 +1095,7 @@ int QMetaStringTable::enter(const QByteArray &value)
 
 int QMetaStringTable::preferredAlignment()
 {
-#ifdef Q_ALIGNOF
     return Q_ALIGNOF(QByteArrayData);
-#else
-    return sizeof(void *);
-#endif
 }
 
 // Returns the size (in bytes) required for serializing this string table.
diff --git a/src/corelib/tools/qcontiguouscache.h b/src/corelib/tools/qcontiguouscache.h
index 4dc763f35d..4e01d7b586 100644
--- a/src/corelib/tools/qcontiguouscache.h
+++ b/src/corelib/tools/qcontiguouscache.h
@@ -170,11 +170,7 @@ private:
     }
     int alignOfTypedData() const
     {
-#ifdef Q_ALIGNOF
         return qMax<int>(sizeof(void*), Q_ALIGNOF(Data));
-#else
-        return 0;
-#endif
     }
 };
 
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 73162b6cf1..ef003c8a71 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -257,13 +257,8 @@ class QHash
         return reinterpret_cast<Node *>(node);
     }
 
-#ifdef Q_ALIGNOF
     static inline int alignOfNode() { return qMax<int>(sizeof(void*), Q_ALIGNOF(Node)); }
     static inline int alignOfDummyNode() { return qMax<int>(sizeof(void*), Q_ALIGNOF(DummyNode)); }
-#else
-    static inline int alignOfNode() { return 0; }
-    static inline int alignOfDummyNode() { return 0; }
-#endif
 
 public:
     inline QHash() : d(const_cast<QHashData *>(&QHashData::shared_null)) { }
diff --git a/src/corelib/tools/qvector.h b/src/corelib/tools/qvector.h
index 226d0bb258..04ab9e6e80 100644
--- a/src/corelib/tools/qvector.h
+++ b/src/corelib/tools/qvector.h
@@ -331,11 +331,7 @@ private:
     }
     static Q_DECL_CONSTEXPR int alignOfTypedData()
     {
-#ifdef Q_ALIGNOF
         return Q_ALIGNOF(AlignmentDummy);
-#else
-        return sizeof(void *);
-#endif
     }
 };