[Webkit-unassigned] [Bug 145204] New: Node::origin.forExit should only be set for those nodes that are allowed to exit

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=145204

            Bug ID: 145204
           Summary: Node::origin.forExit should only be set for those
                    nodes that are allowed to exit
    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

We often have sequences of operations like this:

a: GetStuff(...)
b: Check(Thing:@a)
c: Call(@a, ...)
d: MovHint(@c, loc42)

Notice that GetStuff, Check, and Call can all exit. But after Call, you can't exit anymore. In particular, MovHint cannot exit.

But, the fact that MovHint cannot exit is not communicated in the IR.  We should change that: a Node's forExit origin is set only when that node can exit.

This means that we can for example immediately see that we cannot hoist an exiting node just above the MovHint. But, we would also see that we could hoist an exiting node above the Call, or above @a or @b.

This would harden the IR for fancier CFG transformations, and make it easier to split blocks and insert control flow.

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