Bug 155153 – How we load new.target in arrow functions is broken

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Saam Barati]

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.

[GSkachkov]

Created attachment 273320 [details]
Patch

Patch

[GSkachkov]

Created attachment 273321 [details]
Patch

Fix merge issue

[Build Bot]

Comment on attachment 273321 [details]
Patch

Attachment 273321 [details] did not pass mac-ews (mac):
Output: http://webkit-queues.webkit.org/results/943397

Number of test failures exceeded the failure limit.

[Build Bot]

Created attachment 273333 [details]
Archive of layout-test-results from ews103 for mac-yosemite

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: ews103  Port: mac-yosemite  Platform: Mac OS X 10.10.5

[Build Bot]

Comment on attachment 273321 [details]
Patch

Attachment 273321 [details] did not pass mac-wk2-ews (mac-wk2):
Output: http://webkit-queues.webkit.org/results/943414

Number of test failures exceeded the failure limit.

[Build Bot]

Created attachment 273337 [details]
Archive of layout-test-results from ews107 for mac-yosemite-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: ews107  Port: mac-yosemite-wk2  Platform: Mac OS X 10.10.5

[Build Bot]

Comment on attachment 273321 [details]
Patch

Attachment 273321 [details] did not pass mac-debug-ews (mac):
Output: http://webkit-queues.webkit.org/results/943409

Number of test failures exceeded the failure limit.

[Build Bot]

Created attachment 273338 [details]
Archive of layout-test-results from ews114 for mac-yosemite

The attached test failures were seen while running run-webkit-tests on the mac-debug-ews.
Bot: ews114  Port: mac-yosemite  Platform: Mac OS X 10.10.5

[GSkachkov]

Created attachment 273400 [details]
Patch

Fix tests

[Saam Barati]

Comment on attachment 273400 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=273400&action=review

> Source/JavaScriptCore/ChangeLog:8
> +        Fixed not correct approach of caching new target. In current path was added code feature

I think you mean "patch" instead of path

> Source/JavaScriptCore/ChangeLog:9
> +        flag that shows that current function is using new.target, so in initing of arrow function

"so in initing of arrow function" => "when generating byte code for an arrow function"

[GSkachkov]

Committed 197928: <http://trac.webkit.org/changeset/197928>

[GSkachkov]

All reviewed patches have been landed.  Closing bug.