[Webkit-unassigned] [Bug 170360] New: WebAssembly: downgrade likely unused fast memory modules to slow memory

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=170360

            Bug ID: 170360
           Summary: WebAssembly: downgrade likely unused fast memory
                    modules to slow memory
           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: jfbastien at apple.com
                CC: fpizlo at apple.com, ggaren at apple.com,
                    jfbastien at apple.com, keith_miller at apple.com,
                    mark.lam at apple.com, msaboff at apple.com,
                    sbarati at apple.com
            Blocks: 159775

In setting where we have a limited number of fast memories, it's possible that the GC inadvertently keeps one alive by keeping the WebAssemblyInstance alive. That's a bummer because we're wasting a precious fast memory. We could have a heuristic to figure out whether an Instance executes at all (counters on function entry, or from JS exports, or when the GC scans the stack) and if we think an Instance is dead the downgrade it to a BoundsChecking memory instead. This requires changing its mode, deleting all its code, and making damn sure none of the code is currently on a call stack. Or performing OSR.

This is a lot of work and may not be worth it, but let's keep it in mind.


Referenced Bugs:

https://bugs.webkit.org/show_bug.cgi?id=159775
[Bug 159775] WebAssembly: support the newer spec
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