64-bit WASM support

[tidoust]

Raised in:

• Bringing Desktop-Class Creative Expression to the Web

[lars-t-hansen]

Firefox has a working prototype for 64-bit wasm; it is off by default but I expect it to be enabled by default in Firefox Nightly later this month. Our implementation has not yet received all desirable optimizations, those will arrive over the next few months. The main tracking bug is here. Once the feature is standardized, we expect to ship it.

While wasm64 will enable wasm applications to use more than 4GB RAM on suitable systems, run-time performance will likely not match that of wasm32 since 64-bit heap accesses will need to use more expensive bounds checks. Reducing this cost as much as possible is the focus of ongoing work.