Reduce use of downcast<>() in StyleProperties

https://bugs.webkit.org/show_bug.cgi?id=270758

Reviewed by Anne van Kesteren.

Reduce use of downcast<>() in StyleProperties, for performance.

* Source/WebCore/css/StyleProperties.cpp:
(WebCore::StyleProperties::immutableCopyIfNeeded const):
* Source/WebCore/css/StylePropertiesInlines.h:
(WebCore::StyleProperties::propertyAt const):
(WebCore::StyleProperties::propertyCount const):
(WebCore::StyleProperties::findPropertyIndex const):
(WebCore::StyleProperties::findCustomPropertyIndex const):

Canonical link: https://commits.webkit.org/275896@main

Source/WebCore/css/StyleProperties.cpp

@@ -52,9 +52,9 @@ constexpr unsigned maxShorthandsForLonghand = 4; // FIXME: Generate this from CS
 
 Ref<ImmutableStyleProperties> StyleProperties::immutableCopyIfNeeded() const
 {
-    if (auto* immutableProperties = dynamicDowncast<ImmutableStyleProperties>(*this))
-        return const_cast<ImmutableStyleProperties&>(*immutableProperties);
-    return downcast<MutableStyleProperties>(*this).immutableDeduplicatedCopy();
+    if (m_isMutable)
+        return uncheckedDowncast<MutableStyleProperties>(*this).immutableDeduplicatedCopy();
+    return const_cast<ImmutableStyleProperties&>(uncheckedDowncast<ImmutableStyleProperties>(*this));
 }
 
 String serializeLonghandValue(CSSPropertyID property, const CSSValue& value)

Source/WebCore/css/StylePropertiesInlines.h

@@ -45,16 +45,16 @@ inline StyleProperties::StyleProperties(CSSParserMode mode, unsigned immutableAr
 
 inline StyleProperties::PropertyReference StyleProperties::propertyAt(unsigned index) const
 {
-    if (auto* mutableProperties = dynamicDowncast<MutableStyleProperties>(*this))
-        return mutableProperties->propertyAt(index);
-    return downcast<ImmutableStyleProperties>(*this).propertyAt(index);
+    if (m_isMutable)
+        return uncheckedDowncast<MutableStyleProperties>(*this).propertyAt(index);
+    return uncheckedDowncast<ImmutableStyleProperties>(*this).propertyAt(index);
 }
 
 inline unsigned StyleProperties::propertyCount() const
 {
-    if (auto* mutableProperties = dynamicDowncast<MutableStyleProperties>(*this))
-        return mutableProperties->propertyCount();
-    return downcast<ImmutableStyleProperties>(*this).propertyCount();
+    if (m_isMutable)
+        return uncheckedDowncast<MutableStyleProperties>(*this).propertyCount();
+    return uncheckedDowncast<ImmutableStyleProperties>(*this).propertyCount();
 }
 
 inline void StyleProperties::deref() const
@@ -72,16 +72,16 @@ inline void StyleProperties::deref() const
 
 inline int StyleProperties::findPropertyIndex(CSSPropertyID propertyID) const
 {
-    if (auto* mutableProperties = dynamicDowncast<MutableStyleProperties>(*this))
-        return mutableProperties->findPropertyIndex(propertyID);
-    return downcast<ImmutableStyleProperties>(*this).findPropertyIndex(propertyID);
+    if (m_isMutable)
+        return uncheckedDowncast<MutableStyleProperties>(*this).findPropertyIndex(propertyID);
+    return uncheckedDowncast<ImmutableStyleProperties>(*this).findPropertyIndex(propertyID);
 }
 
 inline int StyleProperties::findCustomPropertyIndex(StringView propertyName) const
 {
-    if (auto* mutableProperties = dynamicDowncast<MutableStyleProperties>(*this))
-        return mutableProperties->findCustomPropertyIndex(propertyName);
-    return downcast<ImmutableStyleProperties>(*this).findCustomPropertyIndex(propertyName);
+    if (m_isMutable)
+        return uncheckedDowncast<MutableStyleProperties>(*this).findCustomPropertyIndex(propertyName);
+    return uncheckedDowncast<ImmutableStyleProperties>(*this).findCustomPropertyIndex(propertyName);
 }
 
 inline bool StyleProperties::isEmpty() const