[Development] QML, v8 and freezing the global object

Knoll Lars Lars.Knoll at digia.com
Fri Oct 12 12:08:02 CEST 2012

On Oct 12, 2012, at 10:17 AM, Jedrzej Nowacki <jedrzej.nowacki at digia.com> wrote:

> On Thursday 11. October 2012 16.04.04 Thomas McGuire wrote:
>> Hi,
>> the QML engine "freezes" the global JS object. This is apparently(?) to
>> prevent accidental writes to the global object. For those who don't know,
>> the global object in JS provides objects and properties available in
>> global scope, such as "console" (for console.log) or "qsTr".
>> Now, it turned out that freezing the global object takes a lot of time with
>> v8 on BB10 devices. Even in release mode, just the call to freeze the
>> global object makes the startup time 100ms longer.
>> v8 seems to have an implmentation of freezing that is suboptimal, see for
>> example the benchmark at http://jsperf.com/performance-frozen-object. Note
>> this benchmark is for iterating over properties of frozen objects, not
>> freezing an object, though likely freezing an object would get similar
>> benchmark results.
>> Could we maybe simple get rid of object freezing, and not freeze the global
>> object?
>> What would the consequences of that be, anything bad? I am the opinion that
>> if the user wants to override the "console" object, let him. Maybe there
>> were other reasons for freezing the global object though?
>> Regards,
>> Thomas
> Hi,
>  Freezing global object was a language design decision. The problem was that 
> in QML it would be much easier accidentally add a property to the global 
> object then in JS. Such mistakes would be quite difficult to debug in QML. As a 
> side effect there is a few optimizations in declarative engine that assume 
> frozen global object. As far I remember they speedup property lookup.
>  Algorithm for freezing an object, was never benchmarked. Probably you can 
> optimize it. You my also try to enable v8 snapshots, if it is not enabled on 
> your configuration. According to our test it may increase qml boot time about 
> 5x. I do not know if it is already done but, maybe you can create a snapshot 
> with frozen global object, that could solve your problem.

Yes, enabling snapshots is pretty much a must on ARM devices, and should solve most of the startup time problems you see. We did some work to enable this also when cross compiling (some of Ossi's qmake changes where there to allow for this), but maybe this was never fully finished.


> Good luck :-)
>  Jędrek
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