Darwin: Use direct runtime interface to manage autorelease pools

The Objective-C runtime supports autorelease pools via a language
specific ABI supplement, akin to the “Itanium” generic ABI for C++.

 https://clang.llvm.org/docs/AutomaticReferenceCounting.html#runtime-support

These interfaces are used by NSAutoreleasePool internally, as well as
inserted by the compiler when using the @autoreleasepool syntax in
Objective-C code.

We have our own wrapper, QMacAutoReleasePool, which allows us to
set up pools in C++ code as well. We now use these lower level
interfaces in the implementation, instead of NSAutoreleasePool,
as this reduces overhead due to not needing to allocate and destroy
a NSAutoreleasePool.

This also opens up the possibility of using Automatic Reference Counting
(ARC) in Qt down the road, as explicit NSAutoreleasePool usage is forbidden
in that mode (while @autoreleasepool is not, and uses the runtime ABI
internally as before).

Change-Id: I06fdb4a24ae4972820f866e0a129a1b355bc8a6b
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

1 files changed, 12 insertions, 5 deletions

diff --git a/src/corelib/kernel/qcore_mac.mm b/src/corelib/kernel/qcore_mac.mm
index d3bb2e7c4f..2ad835b754 100644
--- a/src/corelib/kernel/qcore_mac.mm
+++ b/src/corelib/kernel/qcore_mac.mm
@@ -224,10 +224,20 @@ QT_USE_NAMESPACE
 QT_NAMESPACE_ALIAS_OBJC_CLASS(QMacAutoReleasePoolTracker);
 #endif // QT_DEBUG
 
+// Use the direct runtime interface to manage autorelease pools, as it
+// has less overhead then allocating NSAutoreleasePools, and allows for
+// a future where we use ARC (where NSAutoreleasePool is not allowed).
+// https://clang.llvm.org/docs/AutomaticReferenceCounting.html#runtime-support
+
+extern "C" {
+void *objc_autoreleasePoolPush(void);
+void objc_autoreleasePoolPop(void *pool);
+}
+
 QT_BEGIN_NAMESPACE
 
 QMacAutoReleasePool::QMacAutoReleasePool()
-    : pool([[NSAutoreleasePool alloc] init])
+    : pool(objc_autoreleasePoolPush())
 {
 #ifdef QT_DEBUG
     static const bool debugAutoReleasePools = qEnvironmentVariableIsSet("QT_DARWIN_DEBUG_AUTORELEASEPOOLS");
@@ -272,10 +282,7 @@ QMacAutoReleasePool::QMacAutoReleasePool()
 
 QMacAutoReleasePool::~QMacAutoReleasePool()
 {
-    // Drain behaves the same as release, with the advantage that
-    // if we're ever used in a garbage-collected environment, the
-    // drain acts as a hint to the garbage collector to collect.
-    [static_cast<NSAutoreleasePool*>(pool) drain];
+    objc_autoreleasePoolPop(pool);
 }
 
 #ifndef QT_NO_DEBUG_STREAM