Bug 177734 – Prototype structure transition should be a deferred transition

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

[Saam Barati]

I think OPC set depends on the structure being changed before the watchpoint fires. If true, let's make it use the deferred transition mechanism.

[Saam Barati]

So I was right that this was wrong, however, we catch ourselves from bugs because this check:

```bool PropertyCondition::isWatchableWhenValid(
    Structure* structure, WatchabilityEffort effort) const
{
    if (structure->transitionWatchpointSetHasBeenInvalidated()) {
        return false;
    }
    ...
}````

We will always return false here because the Structure in question just transitioned by changing its prototype, therefore, it will always have an invalidated transition watchpoint set. The consequence here is if we always set our prototype to be the same object, we'll end up invalidating these watchpoints repeatedly.

[Saam Barati]

(In reply to Saam Barati from comment #1)
> So I was right that this was wrong, however, we catch ourselves from bugs
> because this check:
> 
> ```bool PropertyCondition::isWatchableWhenValid(
>     Structure* structure, WatchabilityEffort effort) const
> {
>     if (structure->transitionWatchpointSetHasBeenInvalidated()) {
>         return false;
>     }
>     ...
> }````
> 
> We will always return false here because the Structure in question just
> transitioned by changing its prototype, therefore, it will always have an
> invalidated transition watchpoint set. The consequence here is if we always
> set our prototype to be the same object, we'll end up invalidating these
> watchpoints repeatedly.

We actually skip transitioning if we set the prototype to the same value. This makes sense. Anyways, I'm going to make this change for sanity reasons. It seems like we've been relying on the wrong code path all along.

[Saam Barati]

Created attachment 323358 [details]
patch

[Keith Miller]

Comment on attachment 323358 [details]
patch

r=me.

[WebKit Commit Bot]

Comment on attachment 323358 [details]
patch

Clearing flags on attachment: 323358

Committed r223161: <http://trac.webkit.org/changeset/223161>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Radar WebKit Bug Importer]

<rdar://problem/34924463>