Now that Declaration implements nsIStyleRule, we don't need the memory
overhead of storing a StyleRule object for style attributes.
We also need to change this prior to patch 5, because the changes in
patch 5 that will allow rules to change (but declarations not) would
otherwise break due to style attribute object merging done by
nsAttrValue::ParseStyleAttribute.
The bulk of this commit was generated by running:
run-clang-tidy.py \
-checks='-*,llvm-namespace-comment' \
-header-filter=^/.../mozilla-central/.* \
-fix
I confirmed that we're actually using this codepath by manually testing
<input type=color>: it works with the patch, but if I comment out the
call to nsHTMLCSSStyleSheet::PseudoElementRulesMatching from
nsStyleSet::RuleNodeWithReplacement, then the color swatch breaks, which
proves that we're depending on the code.
I think I included this in the queue because it is needed for patch 22,
although I've forgotten the full reasoning.
This allows nsStyleSet::RuleNodeWithReplacement to call it without
constructing an entire (and unnecessary) ElementRuleProcessorData, which
will happen in patch 3.
Note that this removes the Reset method and the mURL and mDocument
members (and arguments to set them) from nsHTMLCSSStyleSheet.
On the other hand, from nsHTMLStyleSheet it only removes mURL (and
equivalent arguments), and nsHTMLStyleSheet keeps the SetOwningDocument
method that was previously part of nsIStyleSheet (but no longer virtual).
This patch enables sharing of an nsAttrValue's MiscContainer between nodes for style rules. MiscContainers of type eCSSStyleRule are now refcounted (with some clever struct packing to ensure that the amount of memory allocated for MiscContainer remains unchanged on 32 and 64 bit). This infrastructure can be used to share most MiscContainer types in the future if we find advantages to sharing other types than just eCSSStyleRuley. A cache mapping strings to MiscContainers has been added to nsHTMLCSSStyleSheet. MiscContainers can be shared between nsAttrValues when one nsAttrValue is SetTo another nsAttrValue or when there is a cache hit in this cache. This patch also adds the ability to tell a style rule that it belongs to an nsHTMLCSSStyleSheet, with appropriate accessor functions to separate that from the existing case of belonging to an nsCSSStyleSheet.
The primary use case is to reduce memory use for pages that have lots of inline style attributes with the same value. This can happen easily with large pages that are automatically generated. An (admittedly pathological) testcase in Bug 686975 sees over 250 MB of memory savings with this change. Reusing the same MiscContainer for multiple nodes saves the overhead of maintaining separate copies of the string containing the serialized value of the style attribute and of creating separate style rules for each node. Eliminating duplicate style rules enables further savings in layout through style context sharing. The testcase sees the amount of memory used by style contexts go from over 250 MB to 10 KB.
Because the cache is based on the text value of the style attribute, it will not handle attributes that have different text values but are parsed into identical style rules. We also do not attempt to share MiscContainers when the node's base URI differs from the document URI. The effect of these limitations is expected to be low.
