Optimize OP_LdCustomSpreadIteratorList for arguments object

[kunalspathak]

I was looking into the code of OP_LdCustomSpreadIteratorList and noticed that we optimize it by not creating SpreadArgument if parameter passed to OP_LdCustomSpreadIteratorList does not have user defined iterator. We do this check only for JavascriptArray and TypedArrayBase and we can expand it for ArgumentsObject as well. As per spec, both unmapped and mapped argument objects has default @@iterator set to Array.proto.values.

[pleath]

Is someone actually using the spread operator combined with the arguments object? (Please say no.)

-- Paul

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From: Kunal Pathakmailto:notifications@github.com
Sent: ‎7/‎1/‎2016 9:20 AM
To: Microsoft/ChakraCoremailto:ChakraCore@noreply.github.com
Subject: [Microsoft/ChakraCore] Optimize OP_LdCustomSpreadIteratorList for arguments object (#1230)

I was looking into the codehttps://github.com/Microsoft/ChakraCore/blob/master/lib/Runtime/Language/JavascriptOperators.cpp#L1405 of OP_LdCustomSpreadIteratorList and noticed that we optimize it by not creating SpreadArgument if parameter passed to OP_LdCustomSpreadIteratorList does not have user defined iterator. We do this check only for JavascriptArray and TypedArrayBase and we can expand it for ArgumentsObject as well. As per spec, both unmappedhttps://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2ftc39.github.io%2fecma262%2f%23sec-createunmappedargumentsobject&data=01%7c01%7cpleath%40microsoft.com%7cdc43a74403fd4c97644e08d3a1cb978c%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=1gD16k7vrB6rfqP5ypTeu%2fIlFAxK%2b2ZqjM0qeLXtlso%3d and mappedhttps://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2ftc39.github.io%2fecma262%2f%23sec-createmappedargumentsobject&data=01%7c01%7cpleath%40microsoft.com%7cdc43a74403fd4c97644e08d3a1cb978c%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=t6BOFKQXbEjzMB%2fp6rOSlmkPRtk4cS%2fBGoq48fvrF4Q%3d argument objects has default @@iterator set to Array.proto.values.

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[kunalspathak]

I noticed this while investigating #1228 . Once #1228 is fixed, this will go away, but wanted to have tracking perf improvement we could do. Does spec says that we can't use spread operator with the arguments object?

[dilijev]

@kunalspathak If this will go away after #1228 do we want to advertise this as "Your First PR"?

[kunalspathak]

@dilijev , what I meant was we found the issue while investigating #1228. However there will be other cases like usage of ...arguments which we should optimize in general.

[dilijev]

@kunalspathak got it, makes sense.

[tcare]

@pleath Arguments object has an @@iterator property, in which case we need to have the iterator calls. Do we have plans for general iterator optimization?

See https://tc39.github.io/ecma262/#sec-createunmappedargumentsobject Step 7
and https://tc39.github.io/ecma262/#sec-createmappedargumentsobject Step 23

[agarwal-sandeep]

@pleath do we have work to do for this?