Currently, when a WebAssembly function calls another function (JS or WASM), it will type check the return value and coerce it into the expected type. Similarly, when a WebAssembly function is called by another function (JS or WASM), it will type check the arguments and coerce them into the expected types. We can avoid this type checking and type coercion if we know that the caller or the callee is a WebAssembly function, because WebAssembly is statically typed.
We can probably take it further by avoiding boxing and unboxing of values completely when a WebAssembly function calls another WebAssembly function.
See also: Geoff's comments https://bugs.webkit.org/show_bug.cgi?id=149033#c6