REG: Fix unwanted cache flush in Freetype engine

The Freetype cache was almost completely disabled by
134c6db8587a8ce156d4fa31ffa62605821851b2 because after that
change, the lockedAlphaMapForGlyph() function would no longer
cut off early for empty glyphs like spaces, but rather go
through all alpha map functions before it realized that there
was nothing to render. This would in turn invalidate the cache
for every empty glyph, causing all glyphs to be rerendered for
every isolated word.

This change adds back a cut off. This is only needed in the
lockedAlphaMapForGlyph() function, since the superclass implementation
of the other alpha map functions already contains a cut off for
width/height == 0.

[ChangeLog][Qt Gui][Text] Fixed a performance regression in Freetype
engine that was introduced in Qt 5.5.

Change-Id: I381285939909e99cc5fb5f3497fecf9fa871f29a
Task-number: QTBUG-49452
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/gui/text/qfontengine_ft.cpp b/src/gui/text/qfontengine_ft.cpp
index 4de41dfa99..7c878da27f 100644
--- a/src/gui/text/qfontengine_ft.cpp
+++ b/src/gui/text/qfontengine_ft.cpp
@@ -1716,7 +1716,7 @@ glyph_metrics_t QFontEngineFT::alphaMapBoundingBox(glyph_t glyph, QFixed subPixe
 
 static inline QImage alphaMapFromGlyphData(QFontEngineFT::Glyph *glyph, QFontEngine::GlyphFormat glyphFormat)
 {
-    if (glyph == Q_NULLPTR)
+    if (glyph == Q_NULLPTR || glyph->height == 0 || glyph->width == 0)
         return QImage();
 
     QImage::Format format = QImage::Format_Invalid;
@@ -1764,11 +1764,15 @@ QImage *QFontEngineFT::lockedAlphaMapForGlyph(glyph_t glyphIndex, QFixed subPixe
 
     currentlyLockedAlphaMap = alphaMapFromGlyphData(glyph, neededFormat);
 
+    const bool glyphHasGeometry = glyph != Q_NULLPTR && glyph->height != 0 && glyph->width != 0;
     if (!cacheEnabled && glyph != &emptyGlyph) {
         currentlyLockedAlphaMap = currentlyLockedAlphaMap.copy();
         delete glyph;
     }
 
+    if (!glyphHasGeometry)
+        return Q_NULLPTR;
+
     if (currentlyLockedAlphaMap.isNull())
         return QFontEngine::lockedAlphaMapForGlyph(glyphIndex, subPixelPosition, neededFormat, t, offset);