Check before setting initial border image styles

Border-image and its friend mask-box-image are shorthands that
expand out into 4 css properties[1]. When setting the initial values
for these we would blindly create a new NinePieceBorderImage which
is large and contains non-trivial destructors. This patch first
checks whether the current value is already equal to the initial value
before setting.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/border-image

Review URL: https://codereview.chromium.org/340703004

git-svn-id: svn://svn.chromium.org/blink/trunk@176492 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/build/scripts/templates/StyleBuilderFunctions.cpp.tmpl

@@ -156,27 +156,75 @@ namespace WebCore {
 {{apply_auto('CSSPropertyWidows')}}
 {{apply_auto('CSSPropertyZIndex')}}
 
+static bool lengthTypeAndValueMatch(const Length& length, LengthType type, float value)
+{
+    return length.type() == type && length.value() == value;
+}
+
+static bool lengthTypeAndValueMatch(const LengthBox& lengthBox, LengthType type, float value)
+{
+    return (lengthTypeAndValueMatch(lengthBox.left(), type, value)
+        && lengthTypeAndValueMatch(lengthBox.right(), type, value)
+        && lengthTypeAndValueMatch(lengthBox.top(), type, value)
+        && lengthTypeAndValueMatch(lengthBox.bottom(), type, value));
+}
+
+static bool lengthTypeAndValueMatch(const BorderImageLength& borderImageLength, LengthType type, float value)
+{
+    return borderImageLength.isLength() && lengthTypeAndValueMatch(borderImageLength.length(), type, value);
+}
+
+static bool lengthTypeAndValueMatch(const BorderImageLengthBox& borderImageLengthBox, LengthType type, float value)
+{
+    return (lengthTypeAndValueMatch(borderImageLengthBox.left(), type, value)
+        && lengthTypeAndValueMatch(borderImageLengthBox.right(), type, value)
+        && lengthTypeAndValueMatch(borderImageLengthBox.top(), type, value)
+        && lengthTypeAndValueMatch(borderImageLengthBox.bottom(), type, value));
+}
+
 {% macro apply_border_image_modifier(property_id, modifier_type) %}
 {% set is_mask_box = 'MaskBox' in property_id %}
 {% set getter = 'maskBoxImage' if is_mask_box else 'borderImage' %}
 {% set setter = 'setMaskBoxImage' if is_mask_box else 'setBorderImage' %}
 {{ declare_initial_function(property_id) }}
 {
-    NinePieceImage image(state.style()->{{getter}}());
+    const NinePieceImage& currentImage = state.style()->{{getter}}();
+    {# Check for equality in case we can bail out before creating a new NinePieceImage. #}
+    {% if modifier_type == 'Outset' %}
+    if (lengthTypeAndValueMatch(currentImage.outset(), Fixed, 0))
+        return;
+    {% elif modifier_type == 'Repeat' %}
+    if (currentImage.horizontalRule() == StretchImageRule && currentImage.verticalRule() == StretchImageRule)
+        return;
+    {% elif modifier_type == 'Slice' and is_mask_box %}
+    // Masks have a different initial value for slices. Preserve the value of 0 for backwards compatibility.
+    if (currentImage.fill() == true && lengthTypeAndValueMatch(currentImage.imageSlices(), Fixed, 0))
+        return;
+    {% elif modifier_type == 'Slice' and not is_mask_box %}
+    if (currentImage.fill() == false && lengthTypeAndValueMatch(currentImage.imageSlices(), Percent, 100))
+        return;
+    {% elif modifier_type == 'Width' and is_mask_box %}
+    // Masks have a different initial value for widths. Preserve the value of 'auto' for backwards compatibility.
+    if (lengthTypeAndValueMatch(currentImage.borderSlices(), Auto, 0))
+        return;
+    {% elif modifier_type == 'Width' and not is_mask_box %}
+    if (lengthTypeAndValueMatch(currentImage.borderSlices(), Fixed, 1))
+        return;
+    {% endif %}
+
+    NinePieceImage image(currentImage);
     {% if modifier_type == 'Outset' %}
     image.setOutset(Length(0, Fixed));
     {% elif modifier_type == 'Repeat' %}
     image.setHorizontalRule(StretchImageRule);
     image.setVerticalRule(StretchImageRule);
     {% elif modifier_type == 'Slice' and is_mask_box %}
-    // Masks have a different initial value for slices. Preserve the value of 0 for backwards compatibility.
     image.setImageSlices(LengthBox({{ (['Length(0, Fixed)']*4) | join(', ') }}));
     image.setFill(true);
     {% elif modifier_type == 'Slice' and not is_mask_box %}
     image.setImageSlices(LengthBox({{ (['Length(100, Percent)']*4) | join(', ') }}));
     image.setFill(false);
     {% elif modifier_type == 'Width' %}
-    // Masks have a different initial value for widths. Preserve the value of 'auto' for backwards compatibility.
     image.setBorderSlices({{ 'Length(Auto)' if is_mask_box else '1.0' }});
     {% endif %}
     state.style()->{{setter}}(image);