[Webkit-unassigned] [Bug 193764] New: Template Literal GC'ed Against Specs

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=193764

            Bug ID: 193764
           Summary: Template Literal GC'ed Against Specs
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: andrea.giammarchi at gmail.com

Created attachment 360000

  --> https://bugs.webkit.org/attachment.cgi?id=360000&action=review

the alert shown on Safari Technology Preview

Accordingly with Note 2 of ECMAScript Language Specification:
http://www.ecma-international.org/ecma-262/#sec-gettemplateobject

Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.9.6).
The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated.
Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

However, as you can easily test on this code pen https://codepen.io/WebReflection/pen/OdVrqN?editors=0010 , you will see an alert, at some point, demonstrating that the template literal has been collected and recreated, hence it's both not unique and also observable in terms of GC operation.

This happens in both latest Safari on macOS Mojave, and Safari Technology Preview, and it makes code relying on uniqueness of template literals incapable of performing well in Safari ('cause templates needs to be transformed per their value instead of their unique reference).

Please provide an ETA so I can also evaluate if I should penalize safari performance in various libraries based on that part of the specifications.

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