[Webkit-unassigned] [Bug 210694] New: VM should have a bottomCellForSetLocal that is understood to be a true bottom value by the DFG

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=210694

            Bug ID: 210694
           Summary: VM should have a bottomCellForSetLocal that is
                    understood to be a true bottom value by the DFG
           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: keith_miller at apple.com

Right now we handle bottom values semantically needed for a SetLocal by vending a JSValue that roughly matches the predicted value for the non-bottom path. This comes up for op_iterator_open/next. Unfortunately, this still causes AI to be unhappy. If we had a constant cell that was understood by the DFG to be a bottom value (and presumably crashes whenever accessed), the DFG would generate better code for op_iterator_open/next.

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