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
(cherry picked from commit 0c56bef89b6e3fe4c9fb32eb8b51a6ea316a89fa)

3 files changed, 71 insertions, 25 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index 8374c62..03717dc 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1392,6 +1392,7 @@ void QTextEngine::invalidate()
     maxWidth = 0;
     if (specialData)
         specialData->resolvedFormatIndices.clear();
+    feCache.reset();
 }
 
 void QTextEngine::clearLineData()
@@ -1783,6 +1784,13 @@ QFont QTextEngine::font(const QScriptItem &si) const
     return font;
 }
 
+QTextEngine::FontEngineCache::FontEngineCache()
+{
+    reset();
+}
+
+//we cache the previous results of this function, as calling it numerous times with the same effective
+//input is common (and hard to cache at a higher level)
 QFontEngine *QTextEngine::fontEngine(const QScriptItem &si, QFixed *ascent, QFixed *descent, QFixed *leading) const
 {
     QFontEngine *engine = 0;
@@ -1791,28 +1799,47 @@ QFontEngine *QTextEngine::fontEngine(const QScriptItem &si, QFixed *ascent, QFix
 
     QFont font = fnt;
     if (hasFormats()) {
-        QTextCharFormat f = format(&si);
-        font = f.font();
-
-        if (block.docHandle() && block.docHandle()->layout()) {
-            // Make sure we get the right dpi on printers
-            QPaintDevice *pdev = block.docHandle()->layout()->paintDevice();
-            if (pdev)
-                font = QFont(font, pdev);
+        if (feCache.prevFontEngine && feCache.prevPosition == si.position && feCache.prevLength == length(&si) && feCache.prevScript == script) {
+            engine = feCache.prevFontEngine;
+            scaledEngine = feCache.prevScaledFontEngine;
         } else {
-            font = font.resolve(fnt);
-        }
-        engine = font.d->engineForScript(script);
-        QTextCharFormat::VerticalAlignment valign = f.verticalAlignment();
-        if (valign == QTextCharFormat::AlignSuperScript || valign == QTextCharFormat::AlignSubScript) {
-            if (font.pointSize() != -1)
-                font.setPointSize((font.pointSize() * 2) / 3);
-            else
-                font.setPixelSize((font.pixelSize() * 2) / 3);
-            scaledEngine = font.d->engineForScript(script);
+            QTextCharFormat f = format(&si);
+            font = f.font();
+
+            if (block.docHandle() && block.docHandle()->layout()) {
+                // Make sure we get the right dpi on printers
+                QPaintDevice *pdev = block.docHandle()->layout()->paintDevice();
+                if (pdev)
+                    font = QFont(font, pdev);
+            } else {
+                font = font.resolve(fnt);
+            }
+            engine = font.d->engineForScript(script);
+            QTextCharFormat::VerticalAlignment valign = f.verticalAlignment();
+            if (valign == QTextCharFormat::AlignSuperScript || valign == QTextCharFormat::AlignSubScript) {
+                if (font.pointSize() != -1)
+                    font.setPointSize((font.pointSize() * 2) / 3);
+                else
+                    font.setPixelSize((font.pixelSize() * 2) / 3);
+                scaledEngine = font.d->engineForScript(script);
+            }
+            feCache.prevFontEngine = engine;
+            feCache.prevScaledFontEngine = scaledEngine;
+            feCache.prevScript = script;
+            feCache.prevPosition = si.position;
+            feCache.prevLength = length(&si);
         }
     } else {
-        engine = font.d->engineForScript(script);
+        if (feCache.prevFontEngine && feCache.prevScript == script && feCache.prevPosition == -1)
+            engine = feCache.prevFontEngine;
+        else {
+            engine = font.d->engineForScript(script);
+            feCache.prevFontEngine = engine;
+            feCache.prevScript = script;
+            feCache.prevPosition = -1;
+            feCache.prevLength = -1;
+            feCache.prevScaledFontEngine = 0;
+        }
     }
 
     if (si.analysis.flags == QScriptAnalysis::SmallCaps) {
diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index 86551a4..c920c7b 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -558,6 +558,23 @@ 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;
diff --git a/src/gui/text/qtextlayout.cpp b/src/gui/text/qtextlayout.cpp
index 86c6f48..183bcea 100644
--- a/src/gui/text/qtextlayout.cpp
+++ b/src/gui/text/qtextlayout.cpp
@@ -386,6 +386,7 @@ QTextLayout::~QTextLayout()
 void QTextLayout::setFont(const QFont &font)
 {
     d->fnt = font;
+    d->feCache.reset();
 }
 
 /*!
@@ -518,6 +519,7 @@ void QTextLayout::setAdditionalFormats(const QList<FormatRange> &formatList)
     }
     if (d->block.docHandle())
         d->block.docHandle()->documentChange(d->block.position(), d->block.length());
+    d->feCache.reset();
 }
 
 /*!
@@ -1867,14 +1869,14 @@ void QTextLine::layout_helper(int maxGlyphs)
             lbh.currentPosition = qMax(line.from, current.position);
             end = current.position + eng->length(item);
             lbh.glyphs = eng->shapedGlyphs(&current);
+            QFontEngine *fontEngine = eng->fontEngine(current);
+            if (lbh.fontEngine != fontEngine) {
+                lbh.fontEngine = fontEngine;
+                lbh.minimumRightBearing = qMin(QFixed(),
+                                               QFixed::fromReal(fontEngine->minRightBearing()));
+            }
         }
         const QScriptItem &current = eng->layoutData->items[item];
-        QFontEngine *fontEngine = eng->fontEngine(current);
-        if (lbh.fontEngine != fontEngine) {
-            lbh.fontEngine = fontEngine;
-            lbh.minimumRightBearing = qMin(QFixed(),
-                                           QFixed::fromReal(fontEngine->minRightBearing()));
-        }
 
         lbh.tmpData.leading = qMax(lbh.tmpData.leading + lbh.tmpData.ascent,
                                    current.leading + current.ascent) - qMax(lbh.tmpData.ascent,