SVGTextMetricsBuilder::measureTextRenderer exhibits O(n^2) behavior

https://bugs.webkit.org/show_bug.cgi?id=264076

Reviewed by Chris Dumez.

Prior to this PR, RenderSVGText::willLayout() called SVGTextLayoutAttributesBuilder's
rebuildMetricsForTextRenderer and therefore SVGTextMetricsBuilder's measureTextRenderer
on each descendant RenderObject of RenderSVGText. Since rebuildMetricsForTextRenderer
does a tree traversal from the ancestor RenderSVGText to the specified node, this
exhibited O(1+2+3+ ... +n) = O(n^2) behavior.

This PR rectifies this situation by combining all rebuildMetricsForTextRenderer calls
for descendants as a single call to SVGTextLayoutAttributesBuilder's now renamed
rebuildMetricsForSubtree.

This PR also eliminates O(n^2) behavior in updateFontInAllDescendants in RenderSVGText.cpp
by combining all calls to rebuildMetricsForTextRenderer.

* Source/WebCore/rendering/svg/RenderSVGText.cpp:
(WebCore::RenderSVGText::willLayout):
(WebCore::updateFontInAllDescendants):
(WebCore::RenderSVGText::layout):
* Source/WebCore/rendering/svg/SVGTextLayoutAttributesBuilder.cpp:
(WebCore::SVGTextLayoutAttributesBuilder::rebuildMetricsForSubtree):
(WebCore::SVGTextLayoutAttributesBuilder::rebuildMetricsForTextRenderer): Deleted.
* Source/WebCore/rendering/svg/SVGTextLayoutAttributesBuilder.h:
* Source/WebCore/rendering/svg/SVGTextMetricsBuilder.cpp:
(WebCore::SVGTextMetricsBuilder::measureTextRenderer):
* Source/WebCore/rendering/svg/SVGTextMetricsBuilder.h:

Canonical link: https://commits.webkit.org/270110@main

Source/WebCore/rendering/svg/RenderSVGText.cpp

@@ -264,10 +264,7 @@ void RenderSVGText::willLayout()
 
     // Only update the metrics cache, but not the text positioning element cache
     // nor the layout attributes cached in the leaf #text renderers.
-    for (RenderObject* descendant = firstChild(); descendant; descendant = descendant->nextInPreOrder(this)) {
-        if (is<RenderSVGInlineText>(*descendant))
-            m_layoutAttributesBuilder.rebuildMetricsForTextRenderer(downcast<RenderSVGInlineText>(*descendant));
-    }
+    m_layoutAttributesBuilder.rebuildMetricsForSubtree(*this);
 }
 
 void RenderSVGText::subtreeTextDidChange(RenderSVGInlineText* text)
@@ -296,16 +293,16 @@ void RenderSVGText::subtreeTextDidChange(RenderSVGInlineText* text)
     }
 }
 
-static inline void updateFontInAllDescendants(RenderObject* start, SVGTextLayoutAttributesBuilder* builder = nullptr)
+static inline void updateFontInAllDescendants(RenderSVGText& text, SVGTextLayoutAttributesBuilder* builder = nullptr)
 {
-    for (RenderObject* descendant = start; descendant; descendant = descendant->nextInPreOrder(start)) {
+    for (RenderObject* descendant = &text; descendant; descendant = descendant->nextInPreOrder(&text)) {
         if (!is<RenderSVGInlineText>(*descendant))
             continue;
         auto& text = downcast<RenderSVGInlineText>(*descendant);
         text.updateScaledFont();
-        if (builder)
-            builder->rebuildMetricsForTextRenderer(text);
     }
+    if (builder)
+        builder->rebuildMetricsForSubtree(text);
 }
 
 void RenderSVGText::layout()
@@ -343,7 +340,7 @@ void RenderSVGText::layout()
         // and propogate resulting SVGLayoutAttributes to all RenderSVGInlineText children in the subtree.
         ASSERT(m_layoutAttributes.isEmpty());
         collectLayoutAttributes(this, m_layoutAttributes);
-        updateFontInAllDescendants(this);
+        updateFontInAllDescendants(*this);
         m_layoutAttributesBuilder.buildLayoutAttributesForForSubtree(*this);
 
         m_needsReordering = true;
@@ -354,7 +351,7 @@ void RenderSVGText::layout()
         // When the x/y/dx/dy/rotate lists change, recompute the layout attributes, and eventually
         // update the on-screen font objects as well in all descendants.
         if (m_needsTextMetricsUpdate) {
-            updateFontInAllDescendants(this);
+            updateFontInAllDescendants(*this);
             m_needsTextMetricsUpdate = false;
         }
 
@@ -374,7 +371,7 @@ void RenderSVGText::layout()
         if (m_needsTextMetricsUpdate || isLayoutSizeChanged) {
             // If the root layout size changed (eg. window size changes) or the transform to the root
             // context has changed then recompute the on-screen font size.
-            updateFontInAllDescendants(this, &m_layoutAttributesBuilder);
+            updateFontInAllDescendants(*this, &m_layoutAttributesBuilder);
 
             ASSERT(!m_needsReordering);
             ASSERT(!m_needsPositioningValuesUpdate);

Source/WebCore/rendering/svg/SVGTextLayoutAttributesBuilder.cpp

@@ -73,9 +73,9 @@ bool SVGTextLayoutAttributesBuilder::buildLayoutAttributesForForSubtree(RenderSV
     return true;
 }
 
-void SVGTextLayoutAttributesBuilder::rebuildMetricsForTextRenderer(RenderSVGInlineText& text)
+void SVGTextLayoutAttributesBuilder::rebuildMetricsForSubtree(RenderSVGText& text)
 {
-    m_metricsBuilder.measureTextRenderer(text);
+    m_metricsBuilder.measureTextRenderer(text, nullptr);
 }
 
 static inline void processRenderSVGInlineText(const RenderSVGInlineText& text, unsigned& atCharacter, bool& lastCharacterWasSpace)

Source/WebCore/rendering/svg/SVGTextLayoutAttributesBuilder.h

@@ -44,7 +44,7 @@ class SVGTextLayoutAttributesBuilder {
     bool buildLayoutAttributesForForSubtree(RenderSVGText&);
     void buildLayoutAttributesForTextRenderer(RenderSVGInlineText&);
 
-    void rebuildMetricsForTextRenderer(RenderSVGInlineText&);
+    void rebuildMetricsForSubtree(RenderSVGText&);
 
     // Invoked whenever the underlying DOM tree changes, so that m_textPositions is rebuild.
     void clearTextPositioningElements() { m_textPositions.clear(); }

Source/WebCore/rendering/svg/SVGTextMetricsBuilder.cpp

@@ -197,14 +197,10 @@ void SVGTextMetricsBuilder::walkTree(RenderElement& start, RenderSVGInlineText*
     }
 }
 
-void SVGTextMetricsBuilder::measureTextRenderer(RenderSVGInlineText& text)
+void SVGTextMetricsBuilder::measureTextRenderer(RenderSVGText& textRoot, RenderSVGInlineText* stopAtLeaf)
 {
-    auto* textRoot = RenderSVGText::locateRenderSVGTextAncestor(text);
-    if (!textRoot)
-        return;
-
     MeasureTextData data(nullptr);
-    walkTree(*textRoot, &text, &data);
+    walkTree(textRoot, stopAtLeaf, &data);
 }
 
 void SVGTextMetricsBuilder::buildMetricsAndLayoutAttributes(RenderSVGText& textRoot, RenderSVGInlineText* stopAtLeaf, SVGCharacterDataMap& allCharactersMap)

Source/WebCore/rendering/svg/SVGTextMetricsBuilder.h

@@ -34,7 +34,7 @@ class SVGTextMetricsBuilder {
     WTF_MAKE_NONCOPYABLE(SVGTextMetricsBuilder);
 public:
     SVGTextMetricsBuilder();
-    void measureTextRenderer(RenderSVGInlineText&);
+    void measureTextRenderer(RenderSVGText&, RenderSVGInlineText* stopAtLeaf);
     void buildMetricsAndLayoutAttributes(RenderSVGText&, RenderSVGInlineText* stopAtLeaf, SVGCharacterDataMap& allCharactersMap);
 
 private: