Exploit sharing when comparing and copying groups in ComputedStyle.

third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl

Index: third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
diff --git a/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl b/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
index 9ded9eaa63683bd3fcf314347bd275ad3e251695..ff20cf6ed7180fb05620e3236b94254183c67165 100644
--- a/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
+++ b/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
@@ -53,3 +53,36 @@ unsigned {{field.name}} : {{field.size}}; // {{field.type_name}}
 {{field.type_name}} {{field.name}};
 {%- endif %}
 {% endmacro %}
+
+{# Given a group and a list of fields to compare, this generates a set of
+   equality comparisons on those fields. The generated comparisons take
+   advantage of group sharing. #}
+{% macro fieldwise_compare(group, fields_to_compare) %}
+{% for subgroup in group.subgroups %}
+  {# If every field in this subgroup is to be compared, we can compare the
+     group pointer instead. #}
+  {% if subgroup.all_fields|reject("in", fields_to_compare)|list|length == 0 -%}
+    {{subgroup.member_name}} == o.{{subgroup.member_name}} &&
+  {# Otherwise, we would have to recursively generate comparison operations
+     on fields in the subgroup. #}
+  {% elif subgroup.fields|select("in", fields_to_compare)|list|length > 0 -%}
+    {{fieldwise_compare(subgroup, fields_to_compare)}}
+  {% endif %}
+{% endfor %}
+{% for field in group.fields|select("in", fields_to_compare) -%}
+  {{getter_expression(field)}} == o.{{getter_expression(field)}} &&
+{% endfor %}
+{% endmacro %}
+
+{% macro fieldwise_copy(group, fields_to_copy) %}
+{% for subgroup in group.subgroups %}
+  {% if subgroup.all_fields|reject("in", fields_to_copy)|list|length == 0 -%}
+    {{subgroup.member_name}} = other.{{subgroup.member_name}};
+  {% elif subgroup.fields|select("in", fields_to_copy)|list|length > 0 -%}
+    {{fieldwise_copy(subgroup, fields_to_copy)}}
+  {% endif %}
+{% endfor %}
+{% for field in group.fields|select("in", fields_to_copy) -%}
+  {{setter_expression(field)}} = other.{{getter_expression(field)}};
+{% endfor %}
+{% endmacro %}