WebAssembly/Reference types

The WebAssembly reference types extension adds support for two opaque types - externref and funcref. They are managed by the host and cannot be stored in linear memory. They can be used as function parameters (they can only be passed by value), local variables, function results, and as WebAssembly globals.

Free Pascal support

Partial support for reference types was merged into the main FPC branch on Jun 26 2023.

externref

There is a new type in the System unit, called WasmExternRef. It represents the externref WebAssembly type.

funcref

Procedural types can now be declared as WasmFuncRef. For example:

pascal

type
  TMyWasmFuncRef = function(a: longint; b: int64): longint; WasmFuncRef;

These types represent the funcref WebAssembly type.

Restrictions

The WebAssembly reference types don't have an in-memory representation. Therefore, the following is not allowed:

Taking their address
Taking their size, using sizeof() or bitsizeof()
Using them as a field in a record, object or class
Passing them as var, constref or out parameters
Typecasting them to pointer, or integer, or any other type that is not a WebAssembly reference type
Comparing them by value (they can only be compared to nil)

Allowed operations

The WebAssembly reference types can be used in the following ways:

as procedure and function parameters, passed by value
as function results
as local variables
as WebAssembly globals
they can be assigned the value nil
they can be compared to nil, or used as a parameter to assigned()
they can be copied (a := b)

Future features (Not yet implemented)

The following seem to be possible to implement in the future, but is not yet implemented:

putting reference types inside tables, which can be modeled in Pascal as dynamic arrays, declared as WebAssembly globals
calling funcrefs
converting a normal Pascal function to a funcref
with the multivalue extension, it is possible to allow them as out or var parameters. In this case, they would need to be converted as extra function results.

An additional WebAssembly proposal might allow equality comparison between reference types (currently, only comparison to nil is allowed).