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}
For a prototype chain foo -> global_proxy -> global_object, we used to
register a dependency from foo -> global_object. This is incorrect when
the global_proxy/global_object pairing is modified, e.g. when navigating
in iframes. With this patch, we properly register foo -> global_proxy and
global_proxy -> global_object dependencies.
Additionally, when a prototype's prototype changes from null to something
else, this new usage relation must be registered if there are other users
further down on the prototype chain that might expect a complete chain of
registrations to exist (which was the case before, and must be preserved).
BUG=chromium:571517
LOG=n
R=verwaest@chromium.org
Review URL: https://codereview.chromium.org/1559323002
Cr-Commit-Position: refs/heads/master@{#33119}
