Optimise the deletion of a QSharedPointer with no custom deleter

When QSharedPointer is created with no user-specified custom deleter,
instead of storing a pointer in ExternalRefCount::destroyer to a
static function which, in turn, calls normalDeleter<T> indirectly (via
another function pointer), specialise the CustomDeleter class and make
it not store the pointer, but instead do the deleting directly.

The benefits are:
 - the QSharedPointer's private data is smaller
 - there is no double-indirection via indirect jumps to the actual
 deleter

Change-Id: Ice5653c144912efb1226e432267a047b9799aaca
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp
index 7dc1caf57a..167771027e 100644
--- a/src/corelib/tools/qsharedpointer.cpp
+++ b/src/corelib/tools/qsharedpointer.cpp
@@ -230,6 +230,13 @@
     structure will be 12+8+4 = 24 bytes on 32-bit architectures, 16+16+8 = 40
     bytes on 64-bit ones.
 
+    If the deleter was not specified when creating the QSharedPointer object
+    (i.e., if a standard \tt delete call is expected), then there's an
+    optimization that avoids the need to store another function pointer in
+    ExternalRefCountWithCustomDeleter. Instead, a template specialization makes
+    a direct delete call. The size of the structure, in this case, is 12+4 = 16
+    bytes on 32-bit architectures, 16+8 = 24 bytes on 64-bit ones.
+
     \section3 QtSharedPointer::ExternalRefCountWithContiguousData
 
     This class also derives from ExternalRefCountData and it is