Closed Bug 1339535 Opened 7 years ago Closed 7 years ago

JSObject::setHadElementsAccess happens too early


(Core :: JavaScript Engine, defect, P1)




Tracking Status
firefox54 --- fixed


(Reporter: evilpie, Assigned: jandem)


(Blocks 2 open bugs)



(1 file)

Calling JSObject::setHadElementsAccess in SetObjectElementOperation always forces a new shape. This commonly prevents us from attaching CacheIR addSlot stubs. The flag HAD_ELEMENTS_ACCESS however is only really considered for objects with more than 120 properties, so setting this flag really early is useless. I suggest we only try to set this flag when we have at least a few dozen elements, not with zero or one.
Summary: JSObject::setHadElementsAccess happens to early → JSObject::setHadElementsAccess happens too early
And we shouldn't do this for symbol properties, as object[symbol] is the only way to add symbol props so we can't assume it's a hashamp (and a hashmap with only symbol keys would be pretty weird). This flag is just a hack for sunspider unpack-code and the like.
Blocks: 1245279
Attached patch PatchSplinter Review
This uses MAX_HEIGHT_WITH_ELEMENTS_ACCESS / 3 (= 42). It fixes the regression in bug 1341067 comment 6. We can tweak this more later but this will avoid setting the flag in a lot of cases.
Assignee: nobody → jdemooij
Attachment #8841487 - Flags: review?(evilpies)
Attachment #8841487 - Flags: review?(evilpies) → review+
Pushed by
Set the hadElementsAccess flag less eagerly to avoid unnecessary Shape changes. r=evilpie
Closed: 7 years ago
Resolution: --- → FIXED
Target Milestone: --- → mozilla54
Jan, would you have expected this to help with Speedometer? (see bug 1339557 comment 3)
(In reply to :Ehsan Akhgari from comment #5)
> Jan, would you have expected this to help with Speedometer? (see bug 1339557
> comment 3)

It helped Speedometer but I think most of that was fixing the regression from bug 1341067. This bug was mostly an issue for Baseline and after bug 1341067 it also affected Ion.

35% improvement is unlikely though, I'm not seeing that locally or on AWFY.
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