Add v8::ArrayBuffer::Data

[littledivy]

https://v8.github.io/api/head/classv8_1_1ArrayBuffer.html

More efficient shortcut for GetBackingStore()->Data(). The returned pointer is valid as long as the ArrayBuffer is alive.

[bnoordhuis]

One caveat to be aware of: nodejs/node#44554. Probably doesn't matter for deno though.

[billywhizz]

this looks nice. i'll see if can measure any difference in perf. one experiment i tried in just-js actually worked but one thing i was not sure about is whether that address will change when GC is under memory pressure - i assume V8 GC can move heap allocated ArrayBuffer backing data if heap is fragmented etc.?

https://github.com/just-js/modules/blob/main/memory/memory.cc#L39

I have never seen this happen in practice with the just-js hack and it can also be circumvented by externalising the arraybuffer so we are in control of the backing data and ensuring it doesn't get moved/freed.

another benefit of doing something like this is that passing an integer in as id for a buffer is cheaper than passing an arraybuffer from JS to C++. at least last time i checked it was.

[bnoordhuis]

Good point about arraybuffers moving around after GC. I thought rusty_v8 had that disabled but no.

It's controlled by the v8_typed_array_max_size_in_heap setting. rusty_v8 inherits that from v8/BUILD.gn where it's set to 64. Node explicitly sets it to 0 and so should rusty_v8, most likely.

[billywhizz]

thanks @bnoordhuis. i wonder could that be the source of some of these panics and segfaults we are seeing? 🤔