[Webkit-unassigned] [Bug 65825] New: DFG JIT does not track speculation decisions for global variables

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=65825

           Summary: DFG JIT does not track speculation decisions for
                    global variables
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: fpizlo at apple.com


The DFG JIT tracks speculation decisions for block-local temporaries (in the backend) and variables than span blocks (during bytecode parsing via local variable tracking).  But it does not do this for global variables.  Hence code that uses a global variable "as if" it were a local variable can experience awkward pathologies.  For example:

x=5
x/=2
x++

Where x is not declared as a 'var' will result in the first line storing the Int32 representation of 5 into x; the second line checking what type x has, realizing that it's an int, and converting it to a double in order to perform a division and then storing the result into x as a double; and the third line speculating that x is an integer and failing speculation.

The DFG JIT should track speculations for global variables, so that such code will at the very least not perform speculations that contravene other speculations performed by the same code.

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