Bug 224589

Summary: Avoid indirect load in ContainerNode::hasOneChild()
Product: WebKit Reporter: Ryosuke Niwa <rniwa>
Component: DOMAssignee: Ryosuke Niwa <rniwa>
Status: RESOLVED FIXED    
Severity: Normal CC: cdumez, darin, esprehn+autocc, ews-watchlist, kangil.han, webkit-bug-importer, ysuzuki
Priority: P2 Keywords: InRadar
Version: WebKit Nightly Build   
Hardware: Unspecified   
OS: Unspecified   
Attachments:
Description Flags
Patch none

Ryosuke Niwa
Reported 2021-04-14 19:16:46 PDT
Getting nextSibling() will involve loading the first child into the register. Avoid that and just check m_firstChild and m_lastChild instead to avoid this extra memory load.
Attachments
Patch (1.62 KB, patch)
2021-04-14 19:43 PDT, Ryosuke Niwa 		no flags
DetailsFormatted DiffDiff
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Ryosuke Niwa
Comment 1 2021-04-14 19:43:00 PDT
Created attachment 426070 [details] Patch
Chris Dumez
Comment 2 2021-04-14 19:47:33 PDT
Comment on attachment 426070 [details] Patch If you say this is more efficient, I believe you. R=me. The new code definitely still is correct.
Ryosuke Niwa
Comment 3 2021-04-14 19:52:44 PDT
(In reply to Chris Dumez from comment #2) > Comment on attachment 426070 [details] > Patch > > If you say this is more efficient, I believe you. R=me. The new code > definitely still is correct. Yeah because m_firstChild and m_lastChild are both in the same object whereas m_firstChild isn't. CPU needs to load the content of m_firstChild and then retrieve m_nextSibling, which is an indirect load that CPU will have a hard time predicting so whilst prefetched would detect this and will fetch it, it can still result in either TLB hit miss and/or L2/L3 cache misses, which would be horrifyingly expensive.
Ryosuke Niwa
Comment 4 2021-04-14 20:34:17 PDT
Comment on attachment 426070 [details] Patch Clearing flags on attachment: 426070 Committed r275997 (236549@main): <https://commits.webkit.org/236549@main>
Ryosuke Niwa
Comment 5 2021-04-14 20:34:19 PDT
All reviewed patches have been landed. Closing bug.
Radar WebKit Bug Importer
Comment 6 2021-04-14 20:35:15 PDT
<rdar://problem/76682121>
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