1046197 - Make ArrayIteratorNext and StringIteratorNext easier to optimized with Scalar Replacement.

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Nicolas B. Pierron [:nbp]]

The current implementation of scalar replacement does not accept to have a Phi which is used to merge 4 MNewObject, even if the 4 of them are non-escaped objects in practice.  The following micro benchmark is hitting this case with the inlining of ArrayIteratorNext.

    function f(arr) {
        var t = new Date;
        var sum = 0;
        //for (var i=0; i<100000; i++) {
            var iter = arr["@@iterator"]();
            while (true) {
                var obj = iter.next();
                if (obj.done) {
                    obj = undefined;
                    break;
                }
                sum += obj.value;
                obj = undefined;
            }
        //}
        print(new Date - t);
        print(sum);
    }
     
    var a = [];
    for (var i=0; i<10000000; i++)
        a.push(i % 3);
    f(a);

To handle this properly in the Scalar replacement phase we would have to check that all objects following into the Phi are non-escaped and have the same shape as well as checking that the Phi is not escaped either.  This is what an escape analysis should do, but our current conservative one is too trivial to detect such cases and it is easier to tweak our builtins to fit the optimizable pattern.

Comment 1

[Nicolas B. Pierron [:nbp]]

Attachment: Make Array & String IteratorNext easier to optimized with Scalar Replacement.

Comment 2

[Nicolas B. Pierron [:nbp]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/48f6eb406821

Comment 3

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/48f6eb406821