[Webkit-unassigned] [Bug 155153] New: How we load new.target in arrow functions is broken

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=155153

            Bug ID: 155153
           Summary: How we load new.target in arrow functions is broken
    Classification: Unclassified
           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: sbarati at apple.com
                CC: gskachkov at gmail.com

function foo() {
    let arr = () => {
        if (false) {
            print(new.target);
        } else {
            print(new.target);
        }
    };
    arr();
}
new foo();

This prints undefined. It shouldn't.
The reason is that we make a classic mistake
where we emit code to load new.target once.
Even if that code is behind a branch in byte code,
we assume all uses can use that already loaded value.
That's wrong. We either need to always emit the load from
the scope for each new.target, or we need to emit the load
at the function prologue before any uses of new.target.

I think loading it from the scope is probably cleaner because
the alternative punishes programs that never use new.target.
That said, always loading it from the scope will punish programs
that do the load in a loop.

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