[Webkit-unassigned] [Bug 193938] New: We should add code to validate expected GC activity modelled by doesGC() against what the runtime encounters.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=193938

            Bug ID: 193938
           Summary: We should add code to validate expected GC activity
                    modelled by doesGC() against what the runtime
                    encounters.
           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: mark.lam at apple.com

For example, in a debug build (or maybe a ASAN build, when we expect everything to be slow), we should try to do the following:

1. JIT compile time verification

   Have a list of DFG operations that are expected to not have GC activity.
   At compile time, maybe in DFG's SpeculativeJIT or FTL LowerTOB3, assert that the expected GC activity modeled by doesGC() matches the operations we're going to call.  This is 

   The reliability of this verification relies on us declaring correctly if a DFG operation is expected to not GC.

2. Runtime verification

   For any operations that are not expected to GC:
   a. Before the operation call, set a flag in the VM.
   b. Modify the GC to clear that flag.
   c. After the operation call, check the flag to make sure that it is still set.
      If the flag is not set, report the issue and crash.
      Else, clear the flag.

   The reliability of this verification relies on us having sufficient code coverage in our test suite to be able to trigger GCs if they are possible in the relevant code paths we are asserting.

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