Bug 186724 – [DFG] DFG fixup checkArray typically considers only one Structure

• Status: NEW
• Component: JavaScriptCore

[Yusuke Suzuki]

The simple example is below.

function test()
{
    var array = [0, 1, 2, 3];
    for (var i = 0; i < 4; ++i) {
        array[0] = array[0] + 1;
    }
    return array;
}
noInline(test);

for (var i = 0; i < 1e6; ++i)
    test();


Our ArrayProfile takes only one array. So typically, `array[0]` op_put_by_val says "Yeah, the array seems ArrayWithInt32".
But actually, this should see both `CopyOnWriteArrayWithInt32` and `ArrayWithInt32`.
The above example first emit CheckStructure for ArrayWithInt32. Then it fails repeatedly, and the op_put_by_val emits

PutByVal(Check:Untyped:@26, Check:Untyped:@29, Check:Untyped:Kill:@47, MustGen|VarArgs, Generic+OriginalNonArray+OutOfBounds+AsIs+Write, R:World, W:Heap, Exits, ClobbersExit, bc#37, ExitValid)

It is bad. Ideally, we should emit ArrayifyToStructure for ArrayWithInt32 first.

[Yusuke Suzuki]

I think this is the reason why Kraken crypto-aes and crypto-ccm cause regression right now.

[Yusuke Suzuki]

Keith, Saam do you have any idea to fix this?
One way fixing this is https://bugs.webkit.org/show_bug.cgi?id=186193.

[Keith Miller]

I think the simplest solution to this have ArrayProfile track if it has seen a CoW array. Then, if fixup sees original and maybe copy on write can can do an arrayify to structure. Thoughts?