[GTK] Tree optimization: speedup TreeItem constructor by using known

iterator eclipse-platform#882

This change eliminates a single hotspot. The overall complexity of
tree building and element reveal is still quadratic.

Performance improvement is proven by running
org.eclipse.swt.tests.junit.performance.Test_org_eclipse_swt_widgets_Tree.jfaceReveal()

In test jfaceReveal[Shape: STAR, virtual: true]:
10_000 elements: 620_682_717ns -> 501_378_662ns
100_000 elements: 62_804_214_489ns -> 52_781_899_817ns (-16%)

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Tree.java

@@ -191,19 +191,22 @@ TreeItem _getItem (long iter) {
 		parentIter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
 		GTK.gtk_tree_model_get_iter (modelHandle, parentIter, path);
 	}
-	items [id] = new TreeItem (this, parentIter, SWT.NONE, indices [indices.length -1], false);
+	items [id] = new TreeItem (this, parentIter, SWT.NONE, indices [indices.length -1], iter);
 	GTK.gtk_tree_path_free (path);
 	if (parentIter != 0) OS.g_free (parentIter);
 	return items [id];
 }
 
 TreeItem _getItem (long parentIter, int index) {
 	long iter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
-	GTK.gtk_tree_model_iter_nth_child(modelHandle, iter, parentIter, index);
-	int id = getId (iter, true);
-	OS.g_free (iter);
-	if (items [id] != null) return items [id];
-	return items [id] = new TreeItem (this, parentIter, SWT.NONE, index, false);
+	try {
+		GTK.gtk_tree_model_iter_nth_child (modelHandle, iter, parentIter, index);
+		int id = getId (iter, true);
+		if (items [id] != null) return items [id];
+		return items [id] = new TreeItem (this, parentIter, SWT.NONE, index, iter);
+	} finally {
+		OS.g_free (iter);
+	}
 }
 
 void reallocateIds(int newSize) {
@@ -3532,7 +3535,7 @@ void setItemCount (long parentIter, int count) {
 		OS.g_free (iters);
 	} else {
 		for (int i=itemCount; i<count; i++) {
-			new TreeItem (this, parentIter, SWT.NONE, itemCount, true);
+			new TreeItem (this, parentIter, SWT.NONE, itemCount, 0);
 		}
 	}
 	if (!isVirtual) setRedraw (true);

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/TreeItem.java

@@ -70,7 +70,7 @@ public class TreeItem extends Item {
  * @see Widget#getStyle
  */
 public TreeItem (Tree parent, int style) {
-	this (checkNull (parent), 0, style, -1, true);
+	this (checkNull (parent), 0, style, -1, 0);
 }
 
 /**
@@ -103,7 +103,7 @@ public TreeItem (Tree parent, int style) {
  * @see Tree#setRedraw
  */
 public TreeItem (Tree parent, int style, int index) {
-	this (checkNull (parent), 0, style, checkIndex (index), true);
+	this (checkNull (parent), 0, style, checkIndex (index), 0);
 }
 
 /**
@@ -129,7 +129,7 @@ public TreeItem (Tree parent, int style, int index) {
  * @see Widget#getStyle
  */
 public TreeItem (TreeItem parentItem, int style) {
-	this (checkNull (parentItem).parent, parentItem.handle, style, -1, true);
+	this (checkNull (parentItem).parent, parentItem.handle, style, -1, 0);
 }
 
 /**
@@ -158,17 +158,19 @@ public TreeItem (TreeItem parentItem, int style) {
  * @see Tree#setRedraw
  */
 public TreeItem (TreeItem parentItem, int style, int index) {
-	this (checkNull (parentItem).parent, parentItem.handle, style, checkIndex (index), true);
+	this (checkNull (parentItem).parent, parentItem.handle, style, checkIndex (index), 0);
 }
 
-TreeItem (Tree parent, long parentIter, int style, int index, boolean create) {
+TreeItem (Tree parent, long parentIter, int style, int index, long iter) {
 	super (parent, style);
 	this.parent = parent;
-	if (create) {
+	if (iter == 0) {
 		parent.createItem (this, parentIter, index);
 	} else {
+		assert handle == 0;
 		handle = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
-		GTK.gtk_tree_model_iter_nth_child (parent.modelHandle, handle, parentIter, index);
+		if (handle == 0) error(SWT.ERROR_NO_HANDLES);
+		C.memmove(handle, iter, GTK.GtkTreeIter_sizeof ());
 	}
 }