Optimize QTextLayout/QTextEngine usage outside of QTextDocument.

When QTextLayout is used in a QTextDocument, many code paths use
special caches and thus greatly outperform the raw QTextLayout version
that operates directly on a QString.

This patch brings some of these optimizations also to the raw version.
We now also use a QFormatCollection in such cases and enable the
functionality of QTextEngine::indexAdditionalFormats() and
QTextEngine::resolveAdditionalFormats(). Thanks to that, we can greatly
speed up QTextEngine::format(), which now uses an amort O(1) hash table
lookup instead of a O(N) linear search.

The added benchmark shows a gain in the order of one magnitude:

./tst_bench_QText formattedLayout:long-many

before applying the patch:
378.19 msecs per iteration (total: 37,820, iterations: 100)
after applying the patch:
25.80 msecs per iteration (total: 2,580, iterations: 100)

Note: This change is source-incompatible for applications using the private
QTextEngine API.

Task-number: QTBUG-8389
Change-Id: Ifcf7a8902a394428979ea06a6d955f886ee739c7
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index d8ab222475..c2362e6dc5 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -541,7 +541,12 @@ public:
 #ifdef QT_BUILD_COMPAT_LIB
         return 0; // Compat should never reference this symbol
 #else
-        return block.docHandle()->formatCollection();
+        if (block.docHandle())
+            return block.docHandle()->formatCollection();
+        else if (specialData)
+            return specialData->formats.data();
+
+        return 0;
 #endif
     }
     QTextCharFormat format(const QScriptItem *si) const;
@@ -619,6 +624,8 @@ public:
         QList<QTextLayout::FormatRange> addFormats;
         QVector<int> addFormatIndices;
         QVector<int> resolvedFormatIndices;
+        // only used when no docHandle is available
+        QScopedPointer<QTextFormatCollection> formats;
     };
     SpecialData *specialData;