wasmtime_environ/
ref_bits.rs

1//! Definitions for bits in the in-memory / in-table representation of references.
2
3/// An "initialized bit" in a funcref table.
4///
5/// We lazily initialize tables of funcrefs, and this mechanism
6/// requires us to interpret zero as "uninitialized", triggering a
7/// slowpath on table read to possibly initialize the element. (This
8/// has to be *zero* because that is the only value we can cheaply
9/// initialize, e.g. with newly mmap'd memory.)
10///
11/// However, the user can also store a null reference into a table. We
12/// have to interpret this as "actually null", and not "lazily
13/// initialize to the original funcref that this slot had".
14///
15/// To do so, we rewrite nulls into the "initialized null" value. Note
16/// that this should *only exist inside the table*: whenever we load a
17/// value out of a table, we immediately mask off the low bit that
18/// contains the initialized-null flag. Conversely, when we store into
19/// a table, we have to translate a true null into an "initialized
20/// null".
21///
22/// We can generalize a bit in order to simply the table-set logic: we
23/// can set the LSB of *all* explicitly stored values to 1 in order to
24/// note that they are indeed explicitly stored. We then mask off this
25/// bit every time we load.
26///
27/// Note that we take care to set this bit and mask it off when
28/// accessing tables directly in fastpaths in generated code as well.
29pub const FUNCREF_INIT_BIT: usize = 1;
30
31/// The mask we apply to all refs loaded from funcref tables.
32///
33/// This allows us to use the LSB as an "initialized flag" (see below)
34/// to distinguish from an uninitialized element in a
35/// lazily-initialized funcref table.
36pub const FUNCREF_MASK: usize = !FUNCREF_INIT_BIT;
wasmtime_environ/ref_bits.rs

wasmtime_environ/
ref_bits.rs
