WASM isn't machine code: if it were, it wouldn't be universal which is a fundamental requirement of the web. (Well, okay, it's compiled code—it is, after all, compiled from C/C++/etc., but in the same way JS is compiled to byte code in VMs today.) As such, there's a further compilation step needed from WASM to machine code to get near-native performance out of it.
The existing WASM implementations do a mixture of JITing code at load-time and JITing code at first-call, depending on the size of the WASM blob, AFAIK. The gain performance wise over asm.js is primarily in parse-time, as far as I'm aware.
WASM VM itself is perfectly possible to be memory safe: WASM code cannot read outside of the memory allocated to it (and malloc/free are implemented on top of that memory allocated to the VM, hence there's memory safety at the VM level).
The big problems come when you need to guarantee your JIT code doesn't violate memory safety, and that's something Rust's type system cannot (currently) solve. You need guarantees that the generated code will never have any memory access errors, and will never race for memory reads/writes, because it's running with the privilege of the VM, not the limited powers of the WASM code running within it.
The existing WASM implementations do a mixture of JITing code at load-time and JITing code at first-call, depending on the size of the WASM blob, AFAIK. The gain performance wise over asm.js is primarily in parse-time, as far as I'm aware.
WASM VM itself is perfectly possible to be memory safe: WASM code cannot read outside of the memory allocated to it (and malloc/free are implemented on top of that memory allocated to the VM, hence there's memory safety at the VM level).
The big problems come when you need to guarantee your JIT code doesn't violate memory safety, and that's something Rust's type system cannot (currently) solve. You need guarantees that the generated code will never have any memory access errors, and will never race for memory reads/writes, because it's running with the privilege of the VM, not the limited powers of the WASM code running within it.