[Webkit-unassigned] [Bug 117781] New: Consider using String rope for XMLHttpRequest.responseText

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=117781

           Summary: Consider using String rope for
                    XMLHttpRequest.responseText
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: BlinkMergeCandidate
          Severity: Normal
          Priority: P2
         Component: HTML DOM
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rniwa at webkit.org
                CC: ggaren at apple.com, barraclough at apple.com


Consider merging https://chromium.googlesource.com/chromium/blink/+/3d7d1c4f000b6a955ca23a17aa1bee48e8c281b3

MLHttpRequest#responseText should use a rope

Instead of building a flat WTF::String from XMLHttpRequest response data, we
should build a v8::String rope. That way when web sites query the responseText
field repeatedly, every query will share the same underlying data.

This CL causes StringImpl::hashSlowCase to drop way down the CPU profile of
reloading Mobile Gmail:

Before: 1.08%  content_shell  content_shell          [.] WTF::StringImpl::hashSlowCase() const
After:  0.37%  content_shell  content_shell          [.] WTF::StringImpl::hashSlowCase() const

There are likely other savings as well, but the effect on StringImpl::hashSlowCase
is the most dramatic change to the profile.

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