[Webkit-unassigned] [Bug 144186] New: Rationalize DFG DCE handling of nodes that perform checks that propagate through AI

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=144186

            Bug ID: 144186
           Summary: Rationalize DFG DCE handling of nodes that perform
                    checks that propagate through AI
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fpizlo at apple.com

If I do ArithAdd(Int32Use, Int32Use, CheckOverflow) then AI will prove that this returns Int32. We may later perform code simplifications based on the proof that this is Int32, and we may kill all DFG users of this ArithAdd. Then we may prove that there is no exit site at which the ArithAdd is live. This is sufficient to then kill the ArithAdd - except that we still need the overflow check!

Currently we mishandle this:

- In places where we want the overflow check we need to use MustGenerate(@ArithAdd) as a hack to keep it alive. That's dirty and it's just indicative of a deeper issue.

- Our MovHint removal doesn't do Phantom canonicalization which essentially makes it powerless. This was sort of hiding the bug.

- Nodes that have checks that AI leverages should always be NodeMustGenerate. You can't kill something that you are relying on for subsequent simplifications.

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