[Webkit-unassigned] [Bug 172115] New: [DFG] Constant Folding Phase should convert MakeRope("", String) => Identity(String)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=172115

            Bug ID: 172115
           Summary: [DFG] Constant Folding Phase should convert
                    MakeRope("", String) => Identity(String)
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: utatane.tea at gmail.com

This optimization is performed in Fixup phase. But in fixup phase, we may not figure out many constant values...
We should later perform this optimization in constant folding phase again.

I investigated ARES-6 Babylon and figured out that Babylon's Array#indexOf repeatedly find roped string.
This is because of readWord1 in Babylon, the implementation is like,

var word = "";
...
...
...
return word + data.slice(...);

Due to the above limitation, this now accidentally produces MakeRope!

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