Constant wasmtime_environ::FUNCREF_INIT_BIT

pub const FUNCREF_INIT_BIT: usize = 1;

An “initialized bit” in a funcref table.

We lazily initialize tables of funcrefs, and this mechanism requires us to interpret zero as “uninitialized”, triggering a slowpath on table read to possibly initialize the element. (This has to be zero because that is the only value we can cheaply initialize, e.g. with newly mmap’d memory.)

However, the user can also store a null reference into a table. We have to interpret this as “actually null”, and not “lazily initialize to the original funcref that this slot had”.

To do so, we rewrite nulls into the “initialized null” value. Note that this should only exist inside the table: whenever we load a value out of a table, we immediately mask off the low bit that contains the initialized-null flag. Conversely, when we store into a table, we have to translate a true null into an “initialized null”.

We can generalize a bit in order to simply the table-set logic: we can set the LSB of all explicitly stored values to 1 in order to note that they are indeed explicitly stored. We then mask off this bit every time we load.

Note that we take care to set this bit and mask it off when accessing tables direclty in fastpaths in generated code as well.