Weird performance boost when calling QJSValue aliased property

[mjakubowski]

Hello,

I noticed a strange behavior in Qt, checked on 5.5.1 and 5.7.0 beta. Have a look at the code here. It's a simple example, I have a MyObj class with an overloaded method v: one for reading from and the other one for writing into a QVariantMap. An instance of this class is passed to QJSEngine, which then evaluates body like below and calls this via QJSValue::call 5 million times.

(function() { obj.v('test1', obj.v('test1') + 0.05); })

The weird thing is that if I do the following:

QJSValue myObjJS = engine.newQObject(&myObj);
myObjJS.setProperty("z", myObjJS.property("v"));

And take the following function using this new aliased property:

(function() { obj.z('test2', obj.z('test2') + 0.05); })

it is noticeably faster, with both Qt 5.5.1 and 5.7.0, debug and release builds:

~/projects/builds/qjsperftest-5.5.1%> ./qjsperftest
Version =  5.5.1
QVariant(double, 250000)  in  4892  ms
QVariant(double, 250000)  in  2871  ms

~/projects/builds/qjsperftest-5.7.0%> ./qjsperftest
Version =  5.7.0
QVariant(double, 250000)  in  5907  ms
QVariant(double, 250000)  in  3514  ms

~%> gcc --version
gcc (Ubuntu 5.3.1-14ubuntu2) 5.3.1 20160413

Any ideas what could be the reason here? I mean, I'm not going to complain having found a faster way to call these methods, but it seems rather weird.

-- Marcin