Cocoa: optimize backingstore flush on 10-bit displays

Qt draws the backing store to the window using CoreGrahics,
which will trigger a slow RGB32 -> RGB64 conversion
when the output display is a deep color display.

Disable NSWindow dynamicDepthLimit and force the depthLimit
to WindowDepthTwentyforBitRGB for the common case of
8-bit-per-component raster surfaces.

This was benchmarked by resizing a simple QRasterWindow
test case which fills the window area using QPainter::fillRect().

Before:

67.1%  rgba64_image_mark_rgb32
10.8%  __vImageCopyBuffer_block_invoke
6.0%   madvise
5.0%   _kernelrpc_mach_vm_deallocate_trap
4.1%   qt_memfill32(unsigned int*, unsigned int, int)

After:

30.7%  __vImageCopyBuffer_block_invoke
20.3%  madvise
12.3%  __vOverwriteChannelsWithScalar_ARGB8888_block_invoke
12.2%  qt_memfill32(unsigned int*, unsigned int, int)
4.6%   _kernelrpc_mach_vm_deallocate_trap

The test program now spends significantly more of its
time allocating/deallocating the backing store (madvise),
and running the Qt paint event (qt_memfill32).

Task-number: QTBUG-47660
Change-Id: I878be7a0e6eee4ad798f7a53f7f9f79b7950af26
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoawindow.mm b/src/plugins/platforms/cocoa/qcocoawindow.mm
index c6fa6795c1..5cd4beb4f0 100644
--- a/src/plugins/platforms/cocoa/qcocoawindow.mm
+++ b/src/plugins/platforms/cocoa/qcocoawindow.mm
@@ -1887,6 +1887,21 @@ QCocoaNSWindow *QCocoaWindow::createNSWindow(bool shouldBeChildNSWindow, bool sh
 
     applyContentBorderThickness(window);
 
+    // Prevent CoreGraphics RGB32 -> RGB64 backing store conversions on deep color
+    // displays by forcing 8-bit components, unless a deep color format has been
+    // requested. This conversion uses significant CPU time.
+    QSurface::SurfaceType surfaceType = QPlatformWindow::window()->surfaceType();
+    bool usesCoreGraphics = surfaceType == QSurface::RasterSurface || surfaceType == QSurface::RasterGLSurface;
+    QSurfaceFormat surfaceFormat = QPlatformWindow::window()->format();
+    bool usesDeepColor = surfaceFormat.redBufferSize() > 8 ||
+                         surfaceFormat.greenBufferSize() > 8 ||
+                         surfaceFormat.blueBufferSize() > 8;
+    bool usesLayer = view().layer;
+    if (usesCoreGraphics && !usesDeepColor && !usesLayer) {
+        [window setDynamicDepthLimit:NO];
+        [window setDepthLimit:NSWindowDepthTwentyfourBitRGB];
+    }
+
     return window;
 }