This is a temporary fix for the issue we discovered on our first integration,
where destructuring of a function return value is emitting the function call
multiple times:
```javascript
// Input
const [x, setX] = useState(null);
// Output
const x = useState(null)[0];
const setX = useState(null)[1];
```
The reason this happens is that we lower `useState(null)` to a temporary, and
then generate a ComputedLoad for each of x and setX. Codegen doesn't emit
temporaries eagerly - it assumes they are going to be used exactly once and it
re-emits the value each time the temporary is used. Hence why the
`useState(null)` part gets duplicated in the output.
Right now destructuring is the only place i'm aware of where we reuse
temporaries this way. And we do want to change codegen to preserve destructuring
in the output to correctly handle array patterns. However, that's a more
involved change. For now, this PR is a stopgap. During the pass where we promote
temporaries used in scopes to named variables, we now check to see if those
temporaries are used multiple times and promote them.
The above example would then generate something like
```javascript
const t0 = useState(null);
const x = t0[0];
const setX = t0[1];
```
This is still incorrect (it assumes t0 is an array), but it's more likely to
work in practice. I'll revert this change once we correctly handle
destructuring.
Configures typescript-eslint for the project with an initial configuration that
starts with their recommended rules, and adds/disables a few (generally either
disabling warnings or promoting them to errors). The new `yarn lint` command is
not hooked up to CI yet, so for now this is something we can opt-in to running
locally. If you have some free time, help get us down to zero errors!
My general philosophy for linting, which I propose we follow, is that lints
should be very high-signal:
* Error, don't warn. If it's worth mentioning it's worth fixing.
* Enable rules that consistently identify real problems. If we frequently would
have to disable the rule due to false positives, it isn't high-signal.
* Enable rules that help improve consistent style (to avoid code review about
style rather than substance).
I realized we hadn't updated InferReferenceEffects to match our latest thinking
on hooks. Specifically, we will default to assuming that hooks can mutate their
arguments and return mutable values — this works with our model since we don't
treat hooks specially for reactive scope construction. Ie, first we figure out
what variables construct together, then we create scopes, then we prune scopes
that contain hooks. So changing the reference effects for hooks "just works".
Note that it is helpful for our unit tests to have an example hook that we know
_does_ freeze its input and return a frozen value, so i've temporarily added
`useFreeze()` to the list of defined hooks. That is meant as a stopgap: the
right solution is to allow some way to tell the compiler about specific custom
hooks and their semantics.
Adds a subclass of ReactiveFunctionVisitor, ReactiveFunctionTransform, which
makes it easier to write passes that change the shape of a ReactiveFunction. The
two use-cases converted so far are both flattening away certain categories of
reactive scopes — this will make it easier to add a similar pass to prune scopes
that contain hook calls.
Effect.Capture is very similar to Effect.Read, but the only difference is that
this reference is stored somewhere via a Effect.Store.
Previously, any operand associated with a Effect.Store in the same instruction
would get aliased -- so there was no need to explicitly differentiate between a
"normal read" and "read that gets stored".
This difference is now explicit with FunctionExpression where every dependency
is "read" but only a few are "captured" for store (and mutation). In a follow up
PR, the mutating deps will have an Effect.Capture to differentiate from the
other non mutating deps (Effect.Read).
This commit adds a new Program visitor to our Babel plugin which then calls our
FunctionDeclaration visitor. Babel does some "smart" merging of plugin passes so
so even if plugin A is inserted prior to plugin B, if A does not have a Program
visitor and B does, B will run first.
Note that we also can't use Forget inside of a Babel preset as plugins run
_before_ presets (https://babeljs.io/docs/en/plugins/#plugin-ordering).
When this flag is enabled, Forget will only compile function declarations opted
in via the `'use forget'` directive. By default this is false.
Tested internally, see related diffs
Improves DCE, using fixpoint iteration to detect values that are updated across
loops but otherwise never read. There is still some further optimization we can
do (the dce-loop case could optimize out `y`), but this seems like plenty for
now.
Actually, we probably have to add some return statements to our fixtures before
landing this, because otherwise most of the code goes away.
This is a first pass at DCE without having read any literate on the subject, so
lemme know if there's a better approach. That said the algorithm is:
* Keep a `Set<Identifier>` of identifiers that are used (and whose constructing
logic cannot be removed).
* Do a first RPO iteration of all block's phis. Any phi operand that
participates in a loop is preemptively marked as "used" even if it isn't
strictly used somewhere. This step is necessary bc these operands may otherwise
not be used.
* Do a second post-order iteration of all blocks, including iterating first
their terminals, then reverse iteration of instructions, then their phis. Mark
the operands of each as used as we encounter them, and prune instructions whose
lvalue is never used.
For now I was conservative about which types of instructions can be pruned. For
example, call instructions are never pruned, even if the result of the call is
never used.
However one catch is that we currently prune instructions that cause values to
become frozen. We had planned to add runtime calls (in dev) to freeze values for
runtime enforcement, and if we want to do that we can always add these
instructions back (or replace them with explicit freeze calls).
There are a few potential next steps but we should discuss whether they're worth
it:
* Use fixpoint iteration to find exactly which operands are actually used. This
would allow us to to prune cases such as `let x = 0; while (...) { x += 1 }` eg
where there's a phi but the result is never used. Such cases should be rare in
practice though.
* Eliminate more types of instructions, eg eliminate function calls that don't
have any mutable arguments.
There's a bug in the HIR->ReactiveFunction conversion for certain categories of
compound value blocks where we replace operands (which must be a Place) with a
ReactiveValue. This approach worked in practice for lots of cases so I thought a
type coercion was safe, but then I found a case where this assumption breaks
(see new test).
The updated logic fixes the bug and is simpler. When a value block gets split up
(because there was a nested value block), instead of replacing the earlier value
in the later instructions, we append the instructions together. This can result
in some extra nesting (which if we wanted we could flatten away) but ensures
that we maintain type-safety.
A SequenceExpression currently doesn't store the InstructionId that produced its
final `.value`. This PR adds that instruction id, which is then used in the next
PR as we compose SequenceExpressions.
Just small things I noticed when looking at InferTypes. I was thinking about how
we'd adjust this pass to account for hooks, i'll probably pause that for now but
putting this up in case you like the changes. If not no big deal!
We currently lower switch case test values within the wrong scope: the test
value really should be a value block rather than a `Place`. Until then, this PR
adds a bailout for complex test values: we allow primitives and identifiers
which should cover most real-world use-cases.
No need for InferAliasForStores to know about the semantics of each instruction
anymore. It's just a simple pass that iterates over every operand and lvalue.
The FunctionExpression is special cased because it's slightly different but I
have a follow up that removes this special casing.
This doesn't change codegen as the lvalue is unused but it lets us make this
pass be semantically the same across all instructions -- "alias lvalue and
operands of an instr".
Adds limited support for UpdateExpressions (`x++`). We now support the postfix
form (`x++` ok, `++x` is a todo) and only when the argument is an identifier. We
can relax these restrictions with more work, but this PR should be sufficient
for the examples we've seen so far.
Sathya Gunasekaran
Joe Savona
Lauren Tan
Mofei Zhang
mofeiZ