• littledan's avatar
    Implement immutable prototype chains · 0ff7b483
    littledan authored
    This patch implements "immutable prototype exotic objects" from the ECMAScript
    spec, which are objects whose __proto__ cannot be changed, but are not otherwise
    frozen. They are introduced in order to prevent a Proxy from being introduced
    to the prototype chain of the global object.
    
    The API is extended by a SetImmutablePrototype() call in ObjectTemplate, which
    can be used to vend new immutable prototype objects. Additionally, Object.prototype
    is an immutable prototype object.
    
    In the implementation, a new bit is added to Maps to say whether the prototype is
    immutable, which is read by SetPrototype. Map transitions to the immutable prototype
    state are not saved in the transition tree because the main use case is just for
    the prototype chain of the global object, which there will be only one of per
    Context, so no need to take up the extra word for a pointer in each full transition
    tree.
    
    BUG=v8:5149
    
    Review-Url: https://codereview.chromium.org/2108203002
    Cr-Commit-Position: refs/heads/master@{#37482}
    0ff7b483