Reduce CSSProperty's StylePropertyMetadata memory footprint by half when used inside a ImmutableStylePropertySet.
https://bugs.webkit.org/show_bug.cgi?id=117715
Reviewed by Andreas Kling.
Today CSSProperty holds its metadata in the following way :
--------------------------------------------------------------------------------------------
| m_propertyID : 14 We use 14 bits because CSSPropertyIDs start at 1001. |
| m_shorthandID : 14 id of the shorthand this property was set, 0 if not part of a shorthand|
| m_important : 1 |
| m_implicit : 1 |
| m_inherited : 1 |
--------------------------------------------------------------------------------------------
The proposal to decrease the memory footprint on CSSProperty's metadata
only stand when stored inside ImmutableStylePropertySet which uses a custom
way to allocate and lay out the StylePropertyMetadata and the CSSValues in
memory because the idea behind is that the content will not change.
The MutableStylePropertySet uses a regular vector to retrieve, remove
and modify the CSSProperties. ImmutableStylePropertySet is used by default
when parsing up until someone start to access the CSSOM like
div.style which will convert the immutable to a mutable set. It is also good
to note that a CSSProperty is created for every single statement inside a block
in a stylesheet so we do have quite a bunch around. Another consideration is
that the only client to the m_shorthandID is the inspector which uses it
to group the longhands into a shorthand drop down list.
The new proposal is the following one :
- Reduce m_propertyID to 10 bits by not starting the CSSPropertyIDs from
1001 but rather 0 (or 3 as two are hardcoded CSSPropertyInvalid and CSSPropertyVariable).
- Use the fact that we statically know which longhand belong to which shorthand. So
we create a static mapping between longhands and shorthands.
Here is the new layout :
------------------------------------------------------------------------------
| m_propertyID : 10 (up to 1024 properties), we have less than 400 today |
| m_isSetFromShorthand : 1 and then use the mapping in StylePropertyShorthand |
| m_indexInShorthandsVector : 2 |
| m_important : 1 (unchanged) |
| m_implicit : 1 (unchanged) |
| m_inherited : 1 (unchanged) |
------------------------------------------------------------------------------
it was set from using the new code in StylePropertyShorthand.
- m_indexInShorthandsVector : 2 bits, unfortunately there are few longhands which belong to multiple
shorthands so we need to store which was this longhand was part at parsing time. Notice
that it does not store the CSSPropertyID of the matching shorthand but rather its position
in the vector of matching shorthands. CSSProperty::m_shorthandID() method make it transparent
for call sites and return the actual CSSPropertyID of the shorthand. So far 2 bits seems
enough as there is only few longhands with ambiguity and they belong to 3 shorthands.
Profiling the benchmark with Intel Vtune to find out the performance regression
showed that copying uint16_t, so StylePropertyMetadata, is not
a fast operation in term of assembly code and has a big penalty on Windows
MSVC over unsigned for example. The latter produces a much faster code
when using unsigned over uint16_t (45% difference in the benchmark).
The patch avoid the copies of StylePropertyMetadata when applicable (by using const ref).
The second part of the fix is avoiding the conversion from an int (enum) to
an uint16_t in a tight loop such as StylePropertySet::findPropertyIndex
(which is the hotspot of the benchmark).
On my Windows 7 64 bits Core i5 machine CSSPropertySetterGetter results are :
- avg : 2714 runs/s with the patch
- avg : 2696 runs/s without the patch
According to Andreas Kling this patch save up ~1.8Mb on membuster.
No new tests : refactor, old ones should cover.
* css/CSSComputedStyleDeclaration.cpp:
(WebCore::ComputedStyleExtractor::getBackgroundShorthandValue):
* css/CSSParser.cpp:
(WebCore::CSSParser::addPropertyWithPrefixingVariant):
(WebCore::CSSParser::addProperty):
* css/CSSProperty.cpp:
(WebCore::StylePropertyMetadata::shorthandID):
(WebCore::borderDirections):
* css/CSSProperty.h:
(WebCore::StylePropertyMetadata::StylePropertyMetadata): Use uint16_t type for bitfields so sizeof() returns 2 bytes.
(WebCore::CSSProperty::CSSProperty):
(WebCore::CSSProperty::isSetFromShorthand):
(WebCore::CSSProperty::shorthandID):
* css/StylePropertySet.cpp:
(WebCore::ImmutableStylePropertySet::ImmutableStylePropertySet):
(WebCore::getIndexInShorthandVectorForPrefixingVariant):
(WebCore::MutableStylePropertySet::appendPrefixingVariantProperty):
(WebCore::MutableStylePropertySet::setPrefixingVariantProperty):
* css/StylePropertySet.h:
(WebCore::StylePropertySet::PropertyReference::shorthandID):
* css/StylePropertyShorthand.cpp:
(WebCore::backgroundShorthand):
(WebCore::backgroundPositionShorthand):
(WebCore::backgroundRepeatShorthand):
(WebCore::borderShorthand):
(WebCore::borderAbridgedShorthand):
(WebCore::borderBottomShorthand):
(WebCore::borderColorShorthand):
(WebCore::borderImageShorthand):
(WebCore::borderLeftShorthand):
(WebCore::borderRadiusShorthand):
(WebCore::webkitBorderRadiusShorthand):
(WebCore::borderRightShorthand):
(WebCore::borderSpacingShorthand):
(WebCore::borderStyleShorthand):
(WebCore::borderTopShorthand):
(WebCore::borderWidthShorthand):
(WebCore::listStyleShorthand):
(WebCore::fontShorthand):
(WebCore::marginShorthand):
(WebCore::markerShorthand):
(WebCore::outlineShorthand):
(WebCore::overflowShorthand):
(WebCore::paddingShorthand):
(WebCore::transitionShorthand):
(WebCore::webkitAnimationShorthand):
(WebCore::webkitAnimationShorthandForParsing):
(WebCore::webkitBorderAfterShorthand):
(WebCore::webkitBorderBeforeShorthand):
(WebCore::webkitBorderEndShorthand):
(WebCore::webkitBorderStartShorthand):
(WebCore::webkitColumnsShorthand):
(WebCore::webkitColumnRuleShorthand):
(WebCore::webkitFlexFlowShorthand):
(WebCore::webkitFlexShorthand):
(WebCore::webkitMarginCollapseShorthand):
(WebCore::webkitGridColumnShorthand):
(WebCore::webkitGridRowShorthand):
(WebCore::webkitMarqueeShorthand):
(WebCore::webkitMaskShorthand):
(WebCore::webkitMaskPositionShorthand):
(WebCore::webkitMaskRepeatShorthand):
(WebCore::webkitTextEmphasisShorthand):
(WebCore::webkitTextStrokeShorthand):
(WebCore::webkitTransitionShorthand):
(WebCore::webkitTransformOriginShorthand):
(WebCore::widthShorthand):
(WebCore::heightShorthand):
(WebCore::matchingShorthandsForLonghand):
(WebCore::indexOfShorthandForLonghand):
* css/StylePropertyShorthand.h:
(WebCore::StylePropertyShorthand::StylePropertyShorthand):
(WebCore::StylePropertyShorthand::id):
* css/makeprop.pl:
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153581 268f45cc-cd09-0410-ab3c-d52691b4dbfc
10 files changed
