[webkit-dev] Webkit JavaScript question

Gavin Barraclough barraclough at apple.com
Wed May 27 16:53:40 PDT 2009

Hi Zoltan,

> I don't think so. It is not worth to do it on a desktop pc since
> interpreter is always slower than JIT. However, the story is  
> different for
> embedded systems, when they enter low-memory mode.

Whilst it is certainly true that this would likely be of most benefit  
on embedded platforms, we anticipate mixed-mode to be desirable on the  
desktop too.

In addition to having the potential to providing a more memory  
efficient representation than JIT code (assuming a switch to a more  
compact intermediate code representation – our current bytecode is  
more tuned towards performance then size), this could also be  
beneficial for performance.  As JS application size grows, avoiding  
JIT generation for infrequently executed code (e.g. setup/ 
initialization code can often be run only once) may become more  
important (although right now we not seeing code generation as a  
significant overhead).  Additionally, by running the code through the  
interpreter prior to JIT generation this may give us an opportunity to  
gather information to improve the quality of generated code.

> We made some
> experiments before, and it seems possible to switch between jit and
> interpreter, but I am not sure that the community is interested in  
> such
> features.

Whilst not something we have prioritized yet, it is definitely an area  
of interest.  If you have an implementation, even in a partially  
complete state, it might be worth filing an enhancement request on the  
bug tracker and attaching a patch.


> Zoltan
>>> The last step depends on the architecture (supported or not) and C++
>>> compiler directives. If JIT is enabled (see wtf/Platform.h), it  
>>> always
>>> generates machine code. Otherwise an interpreter executes the byte  
>>> code.
>>> A
>>> mixed environment (both jit and interpreter) is not yet supported.
>> Are there plans for that ? Mozilla's TraceMonkey supports triggering
>> JIT execution at runtine already, iirc.
>> --
>> --Antonio Gomes
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