Bug 167797 – Line-heavy SVG example spends a lot of time in SVGPointListValues::valueAsString()

• Status: NEW
• Component: SVG

[Simon Fraser (smfr)]

Load https://bl.ocks.org/monfera/fb2ba8d3b6c3fbeb5f1f54b7dc4d58ea and sample.

Lots of time under:

      2100 vmEntryToJavaScript  (in JavaScriptCore) + 299  [0x1106c4ceb]
        2093 ???  (in <unknown binary>)  [0x2026beba1496]
        | 2092 WebCore::setJSSVGPointX(JSC::ExecState*, long long, long long)  (in WebCore) + 380  [0x1125e194c]
        | + 2077 WebCore::SVGAttributeToPropertyMap::synchronizeProperty(WebCore::SVGElement&, WebCore::QualifiedName const&) const  (in WebCore) + 121  [0x112aee7b9]
        | + ! 1943 WebCore::SVGPolyElement::synchronizePoints(WebCore::SVGElement*)  (in WebCore) + 136  [0x112b4d3e8]
        | + ! : 952 WebCore::SVGPointListValues::valueAsString() const  (in WebCore) + 350  [0x112b4d2be]
        | + ! : | 812 WTF::StringBuilder::appendNumber(double, unsigned int, WTF::TrailingZerosTruncatingPolicy)  (in JavaScriptCore) + 53  [0x1108defe5]
        | + ! : | + 725 WTF::numberToFixedPrecisionString(double, unsigned int, char*, bool)  (in JavaScriptCore) + 70  [0x10fed06e6]
        | + ! : | + ! 476 WTF::double_conversion::DoubleToStringConverter::ToPrecision(double, int, WTF::double_conversion::StringBuilder*) const  (in JavaScriptCore) + 290  [0x10fed08a2]

Ironically examples like this are cited as being slow because they don't use hardware acceleration, but in this case most of the time is in number-to-string code. Why so much?

[Simon Fraser (smfr)]

Every time JS sets the x value of a point, we synchronously update the 'points' attribute of the polyline. It seems like we could do this lazily.

[Simon Fraser (smfr)]

That happens via this line in SVGAnimatedProperty::commitChange():

    // Needed to synchronize with CSSOM for presentation attributes with SVG DOM.
    m_contextElement->synchronizeAnimatedSVGAttribute(m_attributeName);

which was added in r171341. This patch has some explanation:
https://bug-135046-attachments.webkit.org/attachment.cgi?id=235119

[Radar WebKit Bug Importer]

<rdar://problem/35909900>