Before this change, we would enumerate all the keys with
[[OwnPropertyKeys]], and then do [[GetOwnPropertyDescriptor]] twice for
each key as we went through them.
We now only do one [[GetOwnPropertyDescriptor]] per key, which
drastically reduces the number of proxy traps when those are involved.
The new trap sequence matches what you get with V8, so I don't think
anyone will be unpleasantly surprised here.
Instead of creating a new iterator result Object for every step of
for..in iteration, we can create a single object up front and reuse it
for every step. This avoids generating a bunch of garbage that isn't
observable by author code anyway.
We can also reuse the existing premade shape for these objects.
Instead of pruning as-we-go, which means a ton of hash lookups,
we now only do a single pass to prune all non-enumerable keys when
setting up for for..in iteration.
This is no longer done. One of the comments is also innacurate for a
second reason - the call stack is never empty in that case, and is
verified as such only a few lines above.
From what I understand, the suspension steps are not required now,
or in the future for our implementation, or any other. The intent
is already implemented in the spec pushing on another execution
context to the stack and leaving the running execution context as-is.
The resume steps are a slightly different story as there is some subtle
behavior which the spec is trying to convey where some custom logic may
need to be done when one execution context changes from one to another.
It may be worth implementing those steps at a later point in time so
that this behavior is a bit easier to follow in those cases.
To make the situation more confusing - from what I can gather from the
spec, not all cases that the spec mentions resume actually means
anything normative. Resume is only _actually_ needed in a limited set
of locations.
For now, let's just remove the unneeded FIXMEs that indicate that there
is something to be done for the suspension steps, as there is not, and
leave the resume steps as is.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
When the cached value was not an accessor, it was simply ignored.
This is the value we really want, so we can just return it.
Shows up to 5x improvements on some benchmarks,
and 1.4x in general js-benchmarks.
