As the summary says: make .style faster. Measurements suggest the upcoming patch makes it 3.5x faster. The change to nsXULElement::GetStyle overlaps with a similar change in bug 569719; hence the dependency. Zack should land his stuff first.

Two notes: 1) This doesn't handle SVG elements. That's a different interface, but would be easy to do if we have a lot of free bits in the typemapping. Do we? 2) This is still slower than webkit. Remaining time spent is 55% under xpc_qsXPCOMObjectToJsval (self time, time in NativeInterface2JSObject, time in FindInJSObjectScope, QI on the decl). 22% is spent in the GetStyle quickstub (a lot of this setting up the ToJsval function call with its many arguments). 5% is spent calling GetStyle on the element. 9% is spent in trace-generated code, and 6% in GetPropertyWithNativeGetter.

Comment on attachment 462694 [details] [diff] [review] Fix >+nsIDOMCSSStyleDeclaration* >+nsStyledElement::GetStyle(nsresult* retval) > { >+ nsXULElement* xulElement = nsXULElement::FromContent(this); >+ if (xulElement) { >+ nsresult rv = xulElement->EnsureLocalStyle(); >+ if (NS_FAILED(rv)) { It would be nice if EnsureLocalStyle returned void (all it throws is OUT_OF_MEMORY?). Oh well. > nsGenericElement::nsDOMSlots *slots = GetDOMSlots(); >- NS_ENSURE_TRUE(slots, NS_ERROR_OUT_OF_MEMORY); >+ if (!slots) { I don't think this can happen anymore? >diff --git a/content/html/content/src/nsGenericHTMLElement.cpp b/content/html/content/src/nsGenericHTMLElement.cpp >+ NS_IMETHOD GetStyle(nsIDOMCSSStyleDeclaration** aStyle) { Brace on its own line. >diff --git a/content/xul/content/src/nsXULElement.cpp b/content/xul/content/src/nsXULElement.cpp >+ NS_IMETHOD GetStyle(nsIDOMCSSStyleDeclaration** aStyle) { Same here. The bitmap has 32 bits. Adding nsSVGStylableElement looks fine to me.

> all it throws is OUT_OF_MEMORY? I think so, but I don't think it's following infallible-only codepaths yet. Once it is, I'd hope static analysis would win for us. > I don't think this can happen anymore? Indeed. Took that null-check out. > Brace on its own line. Done, both places. I added nsSVGStylableElement.

Pushed http://hg.mozilla.org/mozilla-central/rev/e7b3d37b2bbd

Some combination of this and bug 584287 gave us a 5% win on Dromaeo (DOM) on tinderbox on Linux and Mac.