Partition variables into different graphs. Used for Nim's write tracking, borrow checking and also for the cursor inference. The algorithm is a reinvention / variation of Steensgaard's algorithm. The used data structure is "union find" with path compression.We perform two passes over the AST:
- Pass one (computeLiveRanges): collect livetimes of local variables and whether they are potentially re-assigned.
- Pass two (traverse): combine local variables to abstract "graphs". Strict func checking: Ensure that graphs that are connected to const parameters are not mutated.
- Cursor inference: Ensure that potential cursors are not
- borrowed from locations that are connected to a graph that is mutated during the liveness of the cursor. (We track all possible mutations of a graph.)
See https://nim-lang.github.io/Nim/manual_experimental.html#view-types-algorithm for a high-level description of how borrow checking works.
Types
MutationInfo = object param*: PSym mutatedHere*, connectedVia*: TLineInfo flags: set[SubgraphFlag] maxMutation, minConnection: AbstractTime mutations: seq[AbstractTime]
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Partitions = object abstractTime: AbstractTime loopStart: AbstractTime s: seq[VarIndex] graphs: seq[MutationInfo] goals: set[Goal] unanalysableMutation: bool inAsgnSource, inConstructor, inNoSideEffectSection: int inLoop: int owner: PSym g: ModuleGraph
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Procs
proc `$`(config: ConfigRef; g: MutationInfo): string {. ...raises: [KeyError, Exception], tags: [ReadDirEffect, RootEffect].}
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proc checkBorrowedLocations(par: var Partitions; body: PNode; config: ConfigRef) {. ...raises: [Exception, ERecoverableError], tags: [RootEffect].}
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proc computeCursors(s: PSym; n: PNode; g: ModuleGraph) {. ...raises: [Exception, ERecoverableError, KeyError], tags: [RootEffect, ReadDirEffect].}
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proc computeGraphPartitions(s: PSym; n: PNode; g: ModuleGraph; goals: set[Goal]): Partitions {. ...raises: [Exception, ERecoverableError, KeyError], tags: [RootEffect, ReadDirEffect].}
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proc hasSideEffect(c: var Partitions; info: var MutationInfo): bool {. ...raises: [], tags: [].}
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