| 1 | # analyseFunctions |
| 2 | |
| 3 | ## File |
| 4 | `src/Inference/AnalyseFunctions.ts` |
| 5 | |
| 6 | ## Purpose |
| 7 | Recursively analyzes all nested function expressions and object methods in a function to infer their aliasing effect signatures, which describe how the function affects its captured variables when invoked. |
| 8 | |
| 9 | ## Input Invariants |
| 10 | - The HIR has been through SSA conversion and type inference |
| 11 | - FunctionExpression and ObjectMethod instructions have an empty `aliasingEffects` array (`@aliasingEffects=[]`) |
| 12 | - Context variables (captured variables from outer scope) exist on `fn.context` but do not have their effect populated |
| 13 | |
| 14 | ## Output Guarantees |
| 15 | - Every FunctionExpression and ObjectMethod has its `aliasingEffects` array populated with the effects the function performs when called (mutations, captures, aliasing to return value, etc.) |
| 16 | - Each context variable's `effect` property is set to either `Effect.Capture` (if the variable is captured or mutated by the inner function) or `Effect.Read` (if only read) |
| 17 | - Context variable mutable ranges are reset to `{start: 0, end: 0}` and scopes are set to `null` to prepare for the outer function's subsequent `inferMutationAliasingRanges` pass |
| 18 | |
| 19 | ## Algorithm |
| 20 | 1. **Recursive traversal**: Iterates through all blocks and instructions looking for `FunctionExpression` or `ObjectMethod` instructions |
| 21 | 2. **Depth-first processing**: For each function expression found, calls `lowerWithMutationAliasing()` which: |
| 22 | - Recursively calls `analyseFunctions()` on the inner function (handles nested functions) |
| 23 | - Runs `inferMutationAliasingEffects()` on the inner function to determine effects |
| 24 | - Runs `deadCodeElimination()` to clean up |
| 25 | - Runs `inferMutationAliasingRanges()` to compute mutable ranges and extract externally-visible effects |
| 26 | - Runs `rewriteInstructionKindsBasedOnReassignment()` and `inferReactiveScopeVariables()` |
| 27 | - Stores the computed effects in `fn.aliasingEffects` |
| 28 | 3. **Context variable effect classification**: Scans the computed effects to determine which context variables are captured/mutated vs only read: |
| 29 | - Effects like `Capture`, `Alias`, `Assign`, `MaybeAlias`, `CreateFrom` mark the source as captured |
| 30 | - Mutation effects (`Mutate`, `MutateTransitive`, etc.) mark the target as captured |
| 31 | - Sets `operand.effect = Effect.Capture` or `Effect.Read` accordingly |
| 32 | 4. **Range reset**: Resets mutable ranges and scopes on context variables to prepare for outer function analysis |
| 33 | |
| 34 | ## Key Data Structures |
| 35 | - **HIRFunction.aliasingEffects**: Array of `AliasingEffect` storing the externally-visible behavior of a function when called |
| 36 | - **Place.effect**: Effect enum value (`Capture` or `Read`) describing how a context variable is used |
| 37 | - **AliasingEffect**: Union type describing data flow (Capture, Alias, Assign, etc.) and mutations (Mutate, MutateTransitive, etc.) |
| 38 | - **FunctionExpression/ObjectMethod.loweredFunc.func**: The inner HIRFunction to analyze |
| 39 | |
| 40 | ## Edge Cases |
| 41 | - **Nested functions**: Handled via recursive call to `analyseFunctions()` before processing the current function - innermost functions are analyzed first |
| 42 | - **ObjectMethod**: Treated identically to FunctionExpression |
| 43 | - **Apply effects invariant**: The pass asserts that no `Apply` effects remain in the function's signature - these should have been resolved to more precise effects by `inferMutationAliasingRanges()` |
| 44 | - **Conditional mutations**: Effects like `MutateTransitiveConditionally` are tracked - a function that conditionally mutates a captured variable will have that effect in its signature |
| 45 | - **Immutable captures**: `ImmutableCapture`, `Freeze`, `Create`, `Impure`, `Render` effects do not contribute to marking context variables as `Capture` |
| 46 | |
| 47 | ## TODOs |
| 48 | - No TODO comments in the pass itself |
| 49 | |
| 50 | ## Example |
| 51 | Consider a function that captures and conditionally mutates a variable: |
| 52 | |
| 53 | ```javascript |
| 54 | function useHook(a, b) { |
| 55 | let z = {a}; |
| 56 | let y = b; |
| 57 | let x = function () { |
| 58 | if (y) { |
| 59 | maybeMutate(z); // Unknown function, may mutate z |
| 60 | } |
| 61 | }; |
| 62 | return x; |
| 63 | } |
| 64 | ``` |
| 65 | |
| 66 | **Before AnalyseFunctions:** |
| 67 | ``` |
| 68 | Function @context[y$28, z$25] @aliasingEffects=[] |
| 69 | ``` |
| 70 | |
| 71 | **After AnalyseFunctions:** |
| 72 | ``` |
| 73 | Function @context[read y$28, capture z$25] @aliasingEffects=[ |
| 74 | MutateTransitiveConditionally z$25, |
| 75 | Create $14 = primitive |
| 76 | ] |
| 77 | ``` |
| 78 | |
| 79 | The pass infers: |
| 80 | - `y` is only read (used in the condition) |
| 81 | - `z` is captured into the function and conditionally mutated transitively (because `maybeMutate()` is unknown) |
| 82 | - The inner function's signature includes `MutateTransitiveConditionally z$25` to indicate this potential mutation |
| 83 | |
| 84 | This signature is then used by `InferMutationAliasingEffects` on the outer function to understand that creating this function captures `z`, and calling the function may mutate `z`. |