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Joseph Savona
beb6e1431c
This implements an alternative approach to reference kind inference in the new
architecture based on feedback. Here, we track an environment that maps
top-level identifiers (IdentifierId) to the "kind" of value stored: immutable,
mutable, frozen, or maybe-frozen. We then do a forward data flow analysis
updating this environment based on the semantics of each instruction combined
with the types of values present. For example a reference of a value in a
"mutable" position is inferred as readonly if the value is known to be frozen or
immutable. Similarly, a usage of a reference in a "freeze" position is inferred
as a freeze if the value is not yet definitively frozen, and inferred as
readonly if the value is already frozen.
When multiple control paths converge we merge the previous and new incoming
environments, and only reprocess the block if the environment changed relative
to the previous value. This has some noticeable benefits over the previous
version:
* We now infer precisely where `makeReadOnly()` calls need to be inserted, aka
points where a value needs to be frozen may not yet be frozen.
* We track immutable values and can infer their usage as readonly rather than
mutable.
* The system handles aliasing by representing values as distinct from variables,
so that we can handle situations such as:
```javascript
const a = []; // env: {a: value0; value0: mutable}
const b = a; // env: {a: value0, b: value0; value0: mutable}
freeze(a); // env: {a: value0, b: value0; value0: frozen}
mayMutate(b); // ordinarily inferred as a mutable reference, but we know its
readonly
```