Optimize text layout.

When laying out text, a significant amount of time was being spent
querying for the font engine. This patch:
* changes the layout to only query for the engine when a new script item
is encountered.
* adds an internal cache of the previous result to
QTextEngine::fontEngine(). This catches the important case of multiline
text with few font engine changes.

With these changes layout costs are now approximately 60% of what they
were previously, as measured by the text layout benchmarks.

Reviewed-by: Eskil Abrahamsen Blomfeldt

1 files changed, 17 insertions, 0 deletions

diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index 5588cbc..7270efb 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -550,6 +550,23 @@ public:
 
     mutable QScriptLineArray lines;
 
+    struct FontEngineCache {
+        FontEngineCache();
+        mutable QFontEngine *prevFontEngine;
+        mutable QFontEngine *prevScaledFontEngine;
+        mutable int prevScript;
+        mutable int prevPosition;
+        mutable int prevLength;
+        inline void reset() {
+            prevFontEngine = 0;
+            prevScaledFontEngine = 0;
+            prevScript = -1;
+            prevPosition = -1;
+            prevLength = -1;
+        }
+    };
+    mutable FontEngineCache feCache;
+
     QString text;
     QFont fnt;
     QTextBlock block;