1782677 - Optimize away `MakeIteratorWrapper` and/or function created inside it

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement, P3)

Description

[Tooru Fujisawa [:arai]]

Bug 1781061 will fix the situation around callContentFunction, iterator, and JSOp::DebugCheckSelfHosted.
After that, MakeIteratorWrapper function or the function created inside it could be optimized away.

Comment 1

[Tooru Fujisawa [:arai]]

The situation is the following:

• "Get" operation on [Symbol.iterator] property is performed at step 4
• We use for-of syntax to implement the step 5.e loop
• for-of performs [Symbol.iterator] property access, but we don't want to perform it again
• We use a wrapper that skips the [Symbol.iterator] access, and also call [Symbol.iterator] with correct this value

That's the reason why we have the wrapper object and function.
We can skip them by either:

• (a) add self-hosted special syntax to skip [Symbol.iterator] access in for-of, but receive it
• (b) implement the loop with while loop, instead of for-of

https://searchfox.org/mozilla-central/rev/14781effaa15c12b1652beb75f021489567bad8f/js/src/builtin/Array.js#801-803,816-817,822-823,879,885,888-891

    // Step 4.
    // Inlined: GetMethod, steps 1-2.
    var usingIterator = items[GetBuiltinSymbol("iterator")];
...
        // Step 5.c.
        var iterator = MakeIteratorWrapper(items, usingIterator);
...
        // Step 5.e
        for (var nextValue of allowContentIter(iterator)) {
...
function MakeIteratorWrapper(items, method) {
...
    return {
...
        [GetBuiltinSymbol("iterator")]: function IteratorMethod() {
            return callContentFunction(method, items);
        },
    };

Comment 2

[Tooru Fujisawa [:arai]]

Attachment: Bug 1782677 - Optimize away extra object and function for already-retrieved iterator method in for-of in self-hosted JS. r?jandem!

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/07d6704acf75
Optimize away extra object and function for already-retrieved iterator method in for-of in self-hosted JS. r=jandem

Comment 4

[Andreea Pavel [:apavel]]

https://hg.mozilla.org/mozilla-central/rev/07d6704acf75