| 1 | # pruneUnusedScopes |
| 2 | |
| 3 | ## File |
| 4 | `src/ReactiveScopes/PruneUnusedScopes.ts` |
| 5 | |
| 6 | ## Purpose |
| 7 | This pass converts reactive scopes that have no meaningful outputs into "pruned scopes". A pruned scope is no longer memoized - its instructions are executed unconditionally on every render. This optimization removes unnecessary memoization overhead for scopes that don't produce values that need to be cached. |
| 8 | |
| 9 | ## Input Invariants |
| 10 | - The input is a `ReactiveFunction` that has already been transformed into reactive scope form |
| 11 | - Scopes have been created and have `declarations`, `reassignments`, and potentially `earlyReturnValue` populated |
| 12 | - The pass is called after: |
| 13 | - `pruneUnusedLabels` - cleans up unnecessary labels |
| 14 | - `pruneNonEscapingScopes` - removes scopes whose outputs don't escape |
| 15 | - `pruneNonReactiveDependencies` - removes non-reactive dependencies from scopes |
| 16 | - Scopes may already be marked as pruned by earlier passes |
| 17 | |
| 18 | ## Output Guarantees |
| 19 | Scopes that meet ALL of the following criteria are converted to `pruned-scope`: |
| 20 | - No return statement within the scope |
| 21 | - No reassignments (`scope.reassignments.size === 0`) |
| 22 | - Either no declarations (`scope.declarations.size === 0`), OR all declarations "bubbled up" from inner scopes |
| 23 | |
| 24 | Pruned scopes: |
| 25 | - Keep their original scope metadata (for debugging/tracking) |
| 26 | - Keep their instructions intact |
| 27 | - Will be executed unconditionally during codegen (no memoization check) |
| 28 | |
| 29 | ## Algorithm |
| 30 | |
| 31 | The pass uses the visitor pattern with `ReactiveFunctionTransform`: |
| 32 | |
| 33 | 1. **State Tracking**: A `State` object tracks whether a return statement was encountered: |
| 34 | ```typescript |
| 35 | type State = { |
| 36 | hasReturnStatement: boolean; |
| 37 | }; |
| 38 | ``` |
| 39 | |
| 40 | 2. **Terminal Visitor** (`visitTerminal`): Checks if any terminal is a `return` statement |
| 41 | |
| 42 | 3. **Scope Transform** (`transformScope`): For each scope: |
| 43 | - Creates a fresh state for this scope |
| 44 | - Recursively visits the scope's contents |
| 45 | - Checks pruning criteria: |
| 46 | - `!scopeState.hasReturnStatement` - no early return |
| 47 | - `scope.reassignments.size === 0` - no reassignments |
| 48 | - `scope.declarations.size === 0` OR `!hasOwnDeclaration(scopeBlock)` - no outputs |
| 49 | |
| 50 | 4. **hasOwnDeclaration Helper**: Determines if a scope has "own" declarations vs declarations propagated from nested scopes |
| 51 | |
| 52 | ## Edge Cases |
| 53 | |
| 54 | ### Return Statements |
| 55 | Scopes containing return statements are preserved because early returns need memoization to avoid re-executing the return check on every render. |
| 56 | |
| 57 | ### Bubbled-Up Declarations |
| 58 | When nested scopes are flattened or merged, their declarations may be propagated to parent scopes. The `hasOwnDeclaration` check ensures that parent scopes with only inherited declarations can still be pruned. |
| 59 | |
| 60 | ### Reassignments |
| 61 | Scopes with reassignments are kept because the reassignment represents a side effect that needs to be tracked for memoization. |
| 62 | |
| 63 | ### Already-Pruned Scopes |
| 64 | The pass operates on `ReactiveScopeBlock` (kind: 'scope'), not `PrunedReactiveScopeBlock`. Scopes already pruned by earlier passes are not revisited. |
| 65 | |
| 66 | ### Interaction with Subsequent Passes |
| 67 | The `MergeReactiveScopesThatInvalidateTogether` pass explicitly handles pruned scopes - it does not merge across them. |
| 68 | |
| 69 | ## TODOs |
| 70 | None in the source file. |
| 71 | |
| 72 | ## Example |
| 73 | |
| 74 | ### Fixture: `prune-scopes-whose-deps-invalidate-array.js` |
| 75 | |
| 76 | **Input:** |
| 77 | ```javascript |
| 78 | function Component(props) { |
| 79 | const x = []; |
| 80 | useHook(); |
| 81 | x.push(props.value); |
| 82 | const y = [x]; |
| 83 | return [y]; |
| 84 | } |
| 85 | ``` |
| 86 | |
| 87 | What happens: |
| 88 | - The scope for `x` cannot be memoized because `useHook()` is called inside it |
| 89 | - `FlattenScopesWithHooksOrUseHIR` marks scope @0 as `pruned-scope` |
| 90 | - `PruneUnusedScopes` doesn't change it further since it's already pruned |
| 91 | |
| 92 | **Output (no memoization for x):** |
| 93 | ```javascript |
| 94 | function Component(props) { |
| 95 | const x = []; |
| 96 | useHook(); |
| 97 | x.push(props.value); |
| 98 | const y = [x]; |
| 99 | return [y]; |
| 100 | } |
| 101 | ``` |
| 102 | |
| 103 | ### Key Insight |
| 104 | |
| 105 | The `pruneUnusedScopes` pass is part of a multi-pass pruning strategy: |
| 106 | 1. `FlattenScopesWithHooksOrUseHIR` - Prunes scopes that contain hook/use calls |
| 107 | 2. `pruneNonEscapingScopes` - Prunes scopes whose outputs don't escape |
| 108 | 3. `pruneNonReactiveDependencies` - Removes non-reactive dependencies |
| 109 | 4. **`pruneUnusedScopes`** - Prunes scopes with no remaining outputs |
| 110 | |
| 111 | This pass acts as a cleanup for scopes that became "empty" after previous pruning passes removed their outputs. |