[Webkit-unassigned] [Bug 119058] New: Shrink RuleData by 16 bytes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=119058

           Summary: Shrink RuleData by 16 bytes
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: BlinkMergeCandidate
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rniwa at webkit.org
                CC: koivisto at iki.fi, akling at apple.com


Consider merging https://chromium.googlesource.com/chromium/blink/+/7e234c2b0cfa9704d33eb6d6b5bc11f5f0e0b3d2

This CL shrunks RuleData by 16 bytes by avoiding materializing the array of
decendant selector hashes. Instead of storing the hashes in each RuleData, we
find the hashes stored in the StringImpl objects referenced by the underlying
CSSSelector.

There are 2202 RuleData objects in Mobile Gmail, which means this CL saves at
least 35 kB. When measuring the savings using VmRSS, this CL appears to save 92
kB. The truth is probably somewhere in between.

This CL is risky in the sense that it makes the fast-reject path more
complicated. Rather than iterating over an array, this code path now needs to
walk the selector list. Experiments with html5-full-render indiciate that the
performance difference is in the noise, but we'll watch the main perf bots
carefully to see if this CL affects performance.

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