Bug 39181 – Reduce the size of MutableStyleProperties

• Status: ASSIGNED
• Component: CSS

[Simon Fraser (smfr)]

After the short membuster test, we have 15673 live CSSMutableStyleDeclarations at 128 bytes each for a total of 2,006,144 bytes. It would be good to shrink that.

[Simon Fraser (smfr)]

CSSMutableStyleDeclaration has this vector with inline size:
    Vector<CSSProperty, 4> m_properties;

In the short membuster test, here is the frequency distribution of m_properties.size():


0	47
1	424
2	92
3	45
4	27
5	15
6	11
7	4
8	15
9	23
10	14
11	16
12	15
13	17
14	14
15	7
16	8
17	4
18	10
19+	42

so the majority of CSSMutableStyleDeclarations have just one property. Maybe we should reduce that inline size.

[Simon Fraser (smfr)]

Results from 29-page membuster of reducing line vector capacity to 2:

    COUNT     BYTES       AVG   CLASS_NAME                                       TYPE    BINARY
    =====     =====       ===   ==========                                       ====    ======
before:
    15673   2006144     128.0   WebCore::CSSMutableStyleDeclaration              C++     WebCore           
after:
    15556   1493376      96.0   WebCore::CSSMutableStyleDeclaration              C++     WebCore

[Simon Fraser (smfr)]

Here's the patch:

diff --git a/WebCore/css/CSSMutableStyleDeclaration.cpp b/WebCore/css/CSSMutableStyleDeclaration.cpp
index 44e0b17..7e8cb91 100644
--- a/WebCore/css/CSSMutableStyleDeclaration.cpp
+++ b/WebCore/css/CSSMutableStyleDeclaration.cpp
@@ -788,7 +788,7 @@ void CSSMutableStyleDeclaration::removePropertiesInSet(const int* set, unsigned
     for (unsigned i = 0; i < length; ++i)
         toRemove.add(set[i]);
     
-    Vector<CSSProperty, 4> newProperties;
+    Vector<CSSProperty, 2> newProperties;
     newProperties.reserveInitialCapacity(m_properties.size());
     
     unsigned size = m_properties.size();
diff --git a/WebCore/css/CSSMutableStyleDeclaration.h b/WebCore/css/CSSMutableStyleDeclaration.h
index f7759f4..9c64c0a 100644
--- a/WebCore/css/CSSMutableStyleDeclaration.h
+++ b/WebCore/css/CSSMutableStyleDeclaration.h
@@ -159,7 +159,7 @@ private:
     Vector<CSSProperty>::const_iterator findPropertyWithId(int propertyId) const;
     Vector<CSSProperty>::iterator findPropertyWithId(int propertyId);
 
-    Vector<CSSProperty, 4> m_properties;
+    Vector<CSSProperty, 2> m_properties;
 
     Node* m_node;
     unsigned m_variableDependentValueCount : 24;

[Sam Weinig]

One thing Anders and I have been doing is also looking at the distributions for things like opening all of Safari's default bookmarks (eg, the News folder and Popular folder) as a sanity check.  Membuster can be a little odd sometimes.

[Simon Fraser (smfr)]

The inline capacity was originally added in
http://trac.webkit.org/changeset/40554

[Simon Fraser (smfr)]

MutableStyleProperties is still 96 bytes:

 15$ $ ./Tools/Scripts/dump-class-layout -c Release WebCore MutableStyleProperties
  +0 < 96> MutableStyleProperties
  +0 <  8>     WebCore::StyleProperties WebCore::StyleProperties
  +0 <  8>         WebCore::StylePropertiesBase WebCore::StylePropertiesBase
  +0 <  4>             WTF::RefCounted<WebCore::StylePropertiesBase> WTF::RefCounted<WebCore::StylePropertiesBase>
  +0 <  4>                 WTF::RefCountedBase WTF::RefCountedBase
  +0 <  4>                   unsigned int m_refCount
  +4 < :3>           unsigned int m_cssParserMode : 3
  +4 < :2>           unsigned int m_type : 2
  +4 < :27>           unsigned int m_arraySize : 27
  +8 < 80>     WTF::Vector<WebCore::CSSProperty, 4, WTF::CrashOnOverflow, 16> m_propertyVector
  +8 < 80>         WTF::VectorBuffer<WebCore::CSSProperty, 4> WTF::VectorBuffer<WebCore::CSSProperty, 4>
  +8 < 16>             WTF::VectorBufferBase<WebCore::CSSProperty> WTF::VectorBufferBase<WebCore::CSSProperty>
  +8 <  8>               WebCore::CSSProperty * m_buffer
 +16 <  4>               unsigned int m_capacity
 +20 <  4>               unsigned int m_size
 +24 < 64>           std::__1::aligned_storage<16, 8>::type [4] m_inlineBuffer
 +88 <  8>     std::__1::unique_ptr<WebCore::PropertySetCSSStyleDeclaration, std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration> > m_cssomWrapper
 +88 <  8>         std::__1::__compressed_pair<WebCore::PropertySetCSSStyleDeclaration *, std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration> > __ptr_
 +88 <  8>             std::__1::__compressed_pair_elem<WebCore::PropertySetCSSStyleDeclaration *, 0, false> std::__1::__compressed_pair_elem<WebCore::PropertySetCSSStyleDeclaration *, 0, false>
 +88 <  8>               WebCore::PropertySetCSSStyleDeclaration * __value_
 +88 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration>, 1, true>
 +88 <  1>                 std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration> std::__1::default_delete<WebCore::PropertySetCSSStyleDeclaration>
Total byte size: 96
Total pad bytes: 0