600764 - JM: Make looping to obj.length faster

Status: RESOLVED WORKSFORME

(Core :: JavaScript Engine, defect)

Description

[David Mandelin [:dmandelin]]

This microbenchmark, extracted from A*, loops over an array up to its length:

    function lengthInLoop(a) {
	for (var i = 0; i < a.length; ++i) {
	}
    }

    function makeArray(n) {
	var a = [];
	for (var i = 0; i < 4000; ++i)
	    a.push(0);
	return a;
    }

    var a = makeArray(4000);

    var t0 = new Date;
    for (var i = 0; i < 10000; ++i)
	lengthInLoop(a);
    var t1 = new Date;
    print('lengthInLoop ' + (t1-t0));

The methodjit runs this 2x slower than the tracejit. It seems that it should be able to be about as fast.

If we fix this bug and bug 599214, then the methodjit should be about as fast as the tracer on A*, which will make tuning easier.

Comment 1

[mmc]

The following line in makeArray():
  for (var i = 0; i < 4000; ++i)

should read:
  for (var i = 0; i < n; ++i)

Comment 2

[Ryan VanderMeulen [:RyanVM]]

Interp: 2040
TM: 2053
JM: 409
JM+TI: 37
d8: 57

Looks like TI made this fast.