[Interest] QJSEngine vs. (deprecated) QScriptEngine performance - old thing winning!

Ulf Hermann ulf.hermann at qt.io
Wed Oct 12 13:30:55 CEST 2016

> The usage scenario fits my needs, that is the reason to evaluate exactly this way.
> I mean - should the QScriptEngine be deprecated, the QQmlEngine/QJSEngine should also have ways to utilize some form of JIT/preparse. Even in the first pass where JIT cannot be effective (or when I parse just the text expression) QScriptEngine beats newer and more shiny QJSEngine a lot. That is confusing to me and my point - why is QJSEngine offered as replacement of QScriptEngine when it is so much slower?

QJSEngine does not cache the compilation results if you just pass in plain strings. I also don't think QScriptEngine can do this here (but I haven't checked). QScriptEngine might be clever enough to automatically use the interpreter rather than JIT-compiling such small expressions. You can set the QV4_FORCE_INTERPRETER environment variable to force QJSEngine to use the interpreter rather than the JIT.

In any case it's a waste to parse the expressions from strings over and over and to not use the JIT. If you care about performance, you should try to parse the expressions only once.


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