[Webkit-unassigned] [Bug 78515] New: DFG should be able to emit code on control flow edges

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=78515

           Summary: DFG should be able to emit code on control flow edges
           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


Currently the DFG can emit code for the basic blocks that appeared in the program.  But it is often necessary or useful to emit code on synthetic basic blocks created on control flow edges.  For example we may have B1, B2, B3 such that B1 branches to B2 or some other block, and B3 jumps to B2.  We may want some code to execute only when B1 branches to B2 but not when it branches elsewhere; and for a different piece of code to execute when B3 jumps to B2.

Currently the code that emits branches makes this difficult, since if it directly emits branches, and there is no way to say that some code should be inserted just on one of the edges of the branch.

To make this work, we should add the notion of control flow edge code, and abstract this behind an API for emitting branches so that most of the code generator doesn't have to worry about it.  Since currently we do not yet use this feature, we should have a verification mode that emits dummy code in control flow edges to test that this feature is working correctly.

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