Repository: facebook/react
Pull request: [compiler][rewrite] PropagateScopeDeps hir rewrite

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at bottom):
* __->__ #30894
* #30949
* #30948

Resubmission of #30079 -- core logic unchanged, but needed to rebase past #30573

### Quick background
#### Temporaries

The compiler currently treats temporaries and named variables (e.g. `x`) differently in this pass.
- named variables may be reassigned (in fact, since we're running after LeaveSSA, a single named identifier's IdentifierId may map to multiple `Identifier` instances -- each with its own scope and mutable range)
- temporaries are replaced with their represented expressions during codegen. This is correct (mostly correct, see #29878) as we're careful to always lower the correct evaluation semantics. However, since we rewrite reactive scopes entirely (to if/else blocks), we need to track temporaries that a scope produces in `ReactiveScope.declarations` and later promote them to named variables.
In the same example, $4, $5, and $6 need to be promoted: $2 ->`t0`,  $5 ->`t1`, and $6 ->`t2`.
```js
[1] $2 = LoadGlobal(global) foo
[2] $3 = LoadLocal bar$1
scope 0:
  [3] $4 = Call $2(<unknown> $3)
scope 1:
  [4] $5 = Object {  }
scope 2:
  [5] $6 = Object { a: $4, b: $5 }
[6] $8 = StoreLocal Const x$7 = $6
```

#### Dependencies
`ReactiveScope.dependencies` records the set of (read-only) values that a reactive scope is dependent on. This is currently limited to just variables (named variables from source and promoted temporaries) and property-loads.
All dependencies we record need to be hoistable -- i.e. reordered to just before the ReactiveScope begins. Not all PropertyLoads are hoistable.

In this example, we should not evaluate `obj.a.b` without before creating x and checking `objIsNull`.
```js
// reduce-reactive-deps/no-uncond.js
function useFoo({ obj, objIsNull }) {
  const x = [];
  if (isFalse(objIsNull)) {
    x.push(obj.a.b);
  }
  return x;
}
```

While other memoization strategies with different constraints exist, the current compiler requires that `ReactiveScope.dependencies` be re-orderable to the beginning of the reactive scope. But.. `PropertyLoad`s from null values will throw `TypeError`. This means that evaluating hoisted dependencies should throw if and only if the source program throws. (It is also a bug if source throws and compiler output does not throw. See https://github.com/facebook/react-forget/pull/2709)

---
### Rough high level overview
1. Pass 1
Walk over instructions to gather every temporary used outside of its defining scope (same as ReactiveFunction version). These determine the sidemaps we produce, as temporaries used outside of their declaring scopes get promoted to named variables later (and are not considered hoistable rvals).
2. Pass 2 (collectTemporariesSidemap)
Walk over instructions to generate a sidemap of temporary identifier -> named variable and property path (e.g. `$3 -> {obj: props, path: ["a", "b"]}`)
2. Pass 2 (collectHoistablePropertyLoads)
  a. Build a sidemap of block -> accessed variables and properties (e.g. `bb0 -> [ {obj: props, path: ["a", "b"]} ]`)
  b. Propagate "non-nullness" i.e. variables and properties for which we can safely evaluate `PropertyLoad`.
  A basic block can unconditionally read from identifier X if any of the following applies:
    - the block itself reads from identifier X
    - all predecessors of the block read from identifier X
    - all successors of the block read from identifier X
4. Pass 3: (collectDependencies)
Walks over instructions again to record dependencies and declarations, using the previously produced sidemaps. We do not record any control-flow here
5. Merge every scope's recorded dependencies with the set of hoistable PropertyLoads

Tested by syncing internally and (1) checking compilation output differences ([internal link](https://www.internalfb.com/intern/everpaste/?handle=GPCfUBt_HCoy_S4EAJDVFJyJJMR0bsIXAAAB)), running internally e2e tests ([internal link](https://fburl.com/sandcastle/cs5mlkxq))

---
### Followups:
1. Rewrite function expression deps
This change produces much more optimal output as the compiler now uses the function CFG to understand which variables / paths are assumed to be non-null. However, it may exacerbate [this function-expr hoisting bug](https://github.com/facebook/react/blob/main/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/bug-invalid-hoisting-functionexpr.tsx). A short term fix here is to simply call some form of `collectNonNullObjects` on every function expression to find hoistable variable / paths. In the longer term, we should refactor out `FunctionExpression.deps`.

2. Enable optional paths
(a) don't count optional load temporaries as dependencies (e.g. `collectOptionalLoadRValues(...)`).
(b) record optional paths in both collectHoistablePropertyLoads and dependency collection

Changed files:
filename: compiler/packages/babel-plugin-react-compiler/src/HIR/CollectHoistablePropertyLoads.ts
patch: @@ -67,7 +67,7 @@ export function collectHoistablePropertyLoads(
   temporaries: ReadonlyMap<IdentifierId, ReactiveScopeDependency>,
 ): ReadonlyMap<ScopeId, BlockInfo> {
   const nodes = collectPropertyLoadsInBlocks(fn, temporaries);
-  deriveNonNull(fn, nodes);
+  propagateNonNull(fn, nodes);
 
   const nodesKeyedByScopeId = new Map<ScopeId, BlockInfo>();
   for (const [_, block] of fn.body.blocks) {
@@ -295,7 +295,7 @@ function collectPropertyLoadsInBlocks(
   return nodes;
 }
 
-function deriveNonNull(
+function propagateNonNull(
   fn: HIRFunction,
   nodes: ReadonlyMap<BlockId, BlockInfo>,
 ): void {
@@ -316,7 +316,7 @@ function deriveNonNull(
    * can assume are non-null can be calculated from the following:
    * X = Union(Intersect(X_neighbors), X)
    */
-  function recursivelyDeriveNonNull(
+  function recursivelyPropagateNonNull(
     nodeId: BlockId,
     direction: 'forward' | 'backward',
     traversalState: Map<BlockId, 'active' | 'done'>,
@@ -347,7 +347,7 @@ function deriveNonNull(
     let changed = false;
     for (const pred of neighbors) {
       if (!traversalState.has(pred)) {
-        const neighborChanged = recursivelyDeriveNonNull(
+        const neighborChanged = recursivelyPropagateNonNull(
           pred,
           direction,
           traversalState,
@@ -407,7 +407,7 @@ function deriveNonNull(
     i++;
     changed = false;
     for (const [blockId] of fn.body.blocks) {
-      const forwardChanged = recursivelyDeriveNonNull(
+      const forwardChanged = recursivelyPropagateNonNull(
         blockId,
         'forward',
         traversalState,
@@ -417,7 +417,7 @@ function deriveNonNull(
     }
     traversalState.clear();
     for (const [blockId] of reversedBlocks) {
-      const backwardChanged = recursivelyDeriveNonNull(
+      const backwardChanged = recursivelyPropagateNonNull(
         blockId,
         'backward',
         traversalState,