[Webkit-unassigned] [Bug 67343] New: DFG graph has no way of distinguishing or reconciling between static and dynamic predictions

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=67343

           Summary: DFG graph has no way of distinguishing or reconciling
                    between static and dynamic predictions
           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 graph has the ability to store predictions about program statements and variables.  It can even merge multiple different predictions; for example if one data point indicates that something is likely integer and another data point indicates that it is likely double, then the graph is smart enough to mark the prediction as "numeric", which indicates that can be either int or double but probably not anything else.  But what the graph cannot do is deal with a combination of dynamic and static predictions.  A dynamic prediction should always take precedence over a static one.  But if for some reason no dynamic prediction can be made (for example because we have not yet seen the relevant statement(s) execute) then a static prediction is probably better than no prediction.

The DFG graph should have a mechanism for tracking where a prediction came from (static guess or dynamic profiling) and be able to merge static predictions with dynamic ones, so that a dynamic prediction always takes precedence.

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