compiler/varpartitions

  Source   Edit

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

Goal = enum
  constParameters, borrowChecking, cursorInference
  Source   Edit
MutationInfo = object
  param: PSym
  mutatedHere, connectedVia: TLineInfo
  flags: set[SubgraphFlag]
  maxMutation, minConnection: AbstractTime
  mutations: seq[AbstractTime]
  Source   Edit
Partitions = object
  abstractTime: AbstractTime
  s: seq[VarIndex]
  graphs: seq[MutationInfo]
  goals: set[Goal]
  unanalysableMutation: bool
  inAsgnSource, inConstructor, inNoSideEffectSection: int
  inConditional, inLoop: int
  owner: PSym
  g: ModuleGraph
  Source   Edit

Procs

proc `$`(config: ConfigRef; g: MutationInfo): string {.
    ...raises: [Exception, KeyError], tags: [RootEffect, ReadDirEffect].}
  Source   Edit
proc checkBorrowedLocations(par: var Partitions; body: PNode; config: ConfigRef) {.
    ...raises: [Exception, KeyError, ValueError, IOError, ERecoverableError], tags: [
    RootEffect, ReadDirEffect, WriteIOEffect, ReadIOEffect, ReadEnvEffect].}
  Source   Edit
proc computeCursors(s: PSym; n: PNode; g: ModuleGraph) {.
    ...raises: [Exception, ValueError, KeyError, IOError, ERecoverableError], tags: [
    RootEffect, ReadDirEffect, WriteIOEffect, ReadIOEffect, ReadEnvEffect].}
  Source   Edit
proc computeGraphPartitions(s: PSym; n: PNode; g: ModuleGraph; goals: set[Goal]): Partitions {.
    ...raises: [Exception, ValueError, KeyError, IOError, ERecoverableError], tags: [
    RootEffect, ReadDirEffect, WriteIOEffect, ReadIOEffect, ReadEnvEffect].}
  Source   Edit
proc hasSideEffect(c: var Partitions; info: var MutationInfo): bool {.
    ...raises: [], tags: [].}
  Source   Edit