new implementations for --gc:orc (#14121)

* cycle collector: new implementation
* cycle collector: make self-adaptive based on its previous effectiveness
* cycle collector: added Lins's jump stack to improve traversal from 3*N to 2*N
* cycle collector: make tests green
* API extensions and bugfixes
* code cleanup and use --gc:orc for tasyncawait

lib/system/cellseqs_v2.nim

@@ -10,13 +10,13 @@
 # Cell seqs for cyclebreaker and cyclicrefs_v2.
 
 type
-  CellTuple = (ptr pointer, PNimType)
+  CellTuple = (PT, PNimType)
   CellArray = ptr UncheckedArray[CellTuple]
   CellSeq = object
     len, cap: int
     d: CellArray
 
-proc add(s: var CellSeq, c: ptr pointer; t: PNimType) {.inline.} =
+proc add(s: var CellSeq, c: PT; t: PNimType) {.inline.} =
   if s.len >= s.cap:
     s.cap = s.cap * 3 div 2
     when defined(useMalloc):
@@ -42,14 +42,15 @@ proc init(s: var CellSeq, cap: int = 1024) =
     s.d = cast[CellArray](alloc(s.cap * sizeof(CellTuple)))
 
 proc deinit(s: var CellSeq) =
-  when defined(useMalloc):
+  if s.d != nil:
-    c_free(s.d)
+    when defined(useMalloc):
-  else:
+      c_free(s.d)
-    dealloc(s.d)
+    else:
-  s.d = nil
+      dealloc(s.d)
+    s.d = nil
   s.len = 0
   s.cap = 0
 
-proc pop(s: var CellSeq): (ptr pointer, PNimType) =
+proc pop(s: var CellSeq): (PT, PNimType) =
   result = s.d[s.len-1]
   dec s.len

lib/system/cyclebreaker.nim

@@ -52,6 +52,8 @@ That seems acceptable, no leak is produced. This implies that the standard
 depth-first traversal suffices.
 
 ]#
+
+type PT = ptr pointer
 include cellseqs_v2
 
 const

lib/system/cyclicrefs_bacon.nim

@@ -0,0 +1,364 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2020 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+# Cycle collector based on
+# https://researcher.watson.ibm.com/researcher/files/us-bacon/Bacon01Concurrent.pdf
+# And ideas from Lins' in 2008 by the notion of "critical links", see
+# "Cyclic reference counting" by Rafael Dueire Lins
+# R.D. Lins / Information Processing Letters 109 (2008) 71–78
+#
+
+type PT = Cell
+include cellseqs_v2
+
+const
+  colBlack = 0b000
+  colGray = 0b001
+  colWhite = 0b010
+  colPurple = 0b011
+  isCycleCandidate = 0b100 # cell is marked as a cycle candidate
+  jumpStackFlag = 0b1000
+  rcShift = 4      # shift by rcShift to get the reference counter
+  colorMask = 0b011
+
+type
+  TraceProc = proc (p, env: pointer) {.nimcall, benign.}
+  DisposeProc = proc (p: pointer) {.nimcall, benign.}
+
+template color(c): untyped = c.rc and colorMask
+template setColor(c, col) =
+  when col == colBlack:
+    c.rc = c.rc and not colorMask
+  else:
+    c.rc = c.rc and not colorMask or col
+
+proc nimIncRefCyclic(p: pointer) {.compilerRtl, inl.} =
+  let h = head(p)
+  inc h.rc, rcIncrement
+  h.setColor colPurple # mark as potential cycle!
+
+const
+  useJumpStack = false # for thavlak the jump stack doesn't improve the performance at all
+
+type
+  GcEnv = object
+    traceStack: CellSeq
+    when useJumpStack:
+      jumpStack: CellSeq   # Lins' jump stack in order to speed up traversals
+    freed, touched: int
+
+proc trace(s: Cell; desc: PNimType; j: var GcEnv) {.inline.} =
+  if desc.traceImpl != nil:
+    var p = s +! sizeof(RefHeader)
+    cast[TraceProc](desc.traceImpl)(p, addr(j))
+
+when true:
+  template debug(str: cstring; s: Cell) = discard
+else:
+  proc debug(str: cstring; s: Cell) =
+    let p = s +! sizeof(RefHeader)
+    cprintf("[%s] name %s RC %ld\n", str, p, s.rc shr rcShift)
+
+proc free(s: Cell; desc: PNimType) {.inline.} =
+  when traceCollector:
+    cprintf("[From ] %p rc %ld color %ld\n", s, s.rc shr rcShift, s.color)
+  let p = s +! sizeof(RefHeader)
+
+  debug("free", s)
+
+  if desc.disposeImpl != nil:
+    cast[DisposeProc](desc.disposeImpl)(p)
+  nimRawDispose(p)
+
+proc nimTraceRef(q: pointer; desc: PNimType; env: pointer) {.compilerRtl.} =
+  let p = cast[ptr pointer](q)
+  if p[] != nil:
+    var j = cast[ptr GcEnv](env)
+    j.traceStack.add(head p[], desc)
+
+proc nimTraceRefDyn(q: pointer; env: pointer) {.compilerRtl.} =
+  let p = cast[ptr pointer](q)
+  if p[] != nil:
+    var j = cast[ptr GcEnv](env)
+    j.traceStack.add(head p[], cast[ptr PNimType](p[])[])
+
+var
+  roots: CellSeq
+
+proc unregisterCycle(s: Cell) =
+  # swap with the last element. O(1)
+  let idx = s.rootIdx
+  if idx >= roots.len or idx < 0:
+    cprintf("[Bug!] %ld\n", idx)
+    quit 1
+
+  roots.d[idx] = roots.d[roots.len-1]
+  roots.d[idx][0].rootIdx = idx
+  dec roots.len
+
+proc scanBlack(s: Cell; desc: PNimType; j: var GcEnv) =
+  #[
+  proc scanBlack(s: Cell) =
+    setColor(s, colBlack)
+    for t in sons(s):
+      t.rc = t.rc + rcIncrement
+      if t.color != colBlack:
+        scanBlack(t)
+  ]#
+  debug "scanBlack", s
+  s.setColor colBlack
+  trace(s, desc, j)
+  while j.traceStack.len > 0:
+    let (t, desc) = j.traceStack.pop()
+    inc t.rc, rcIncrement
+    debug "incRef", t
+    if t.color != colBlack:
+      t.setColor colBlack
+      trace(t, desc, j)
+
+proc markGray(s: Cell; desc: PNimType; j: var GcEnv) =
+  #[
+  proc markGray(s: Cell) =
+    if s.color != colGray:
+      setColor(s, colGray)
+      for t in sons(s):
+        t.rc = t.rc - rcIncrement
+        if t.color != colGray:
+          markGray(t)
+  ]#
+  if s.color != colGray:
+    s.setColor colGray
+    inc j.touched
+    trace(s, desc, j)
+    while j.traceStack.len > 0:
+      let (t, desc) = j.traceStack.pop()
+      dec t.rc, rcIncrement
+      when useJumpStack:
+        if (t.rc shr rcShift) >= 0 and (t.rc and jumpStackFlag) == 0:
+          t.rc = t.rc or jumpStackFlag
+          when traceCollector:
+            cprintf("[Now in jumpstack] %p %ld color %ld in jumpstack %ld\n", t, t.rc shr rcShift, t.color, t.rc and jumpStackFlag)
+          j.jumpStack.add(t, desc)
+      if t.color != colGray:
+        t.setColor colGray
+        inc j.touched
+        trace(t, desc, j)
+
+proc scan(s: Cell; desc: PNimType; j: var GcEnv) =
+  #[
+  proc scan(s: Cell) =
+    if s.color == colGray:
+      if s.rc > 0:
+        scanBlack(s)
+      else:
+        s.setColor(colWhite)
+        for t in sons(s): scan(t)
+  ]#
+  if s.color == colGray:
+    if (s.rc shr rcShift) >= 0:
+      scanBlack(s, desc, j)
+      # XXX this should be done according to Lins' paper but currently breaks
+      #when useJumpStack:
+      #  s.setColor colPurple
+    else:
+      when useJumpStack:
+        # first we have to repair all the nodes we have seen
+        # that are still alive; we also need to mark what they
+        # refer to as alive:
+        while j.jumpStack.len > 0:
+          let (t, desc) = j.jumpStack.pop
+          # not in jump stack anymore!
+          t.rc = t.rc and not jumpStackFlag
+          if t.color == colGray and (t.rc shr rcShift) >= 0:
+            scanBlack(t, desc, j)
+            # XXX this should be done according to Lins' paper but currently breaks
+            #t.setColor colPurple
+            when traceCollector:
+              cprintf("[jump stack] %p %ld\n", t, t.rc shr rcShift)
+
+      s.setColor(colWhite)
+      trace(s, desc, j)
+      while j.traceStack.len > 0:
+        let (t, desc) = j.traceStack.pop()
+        if t.color == colGray:
+          if (t.rc shr rcShift) >= 0:
+            scanBlack(t, desc, j)
+          else:
+            when useJumpStack:
+              # first we have to repair all the nodes we have seen
+              # that are still alive; we also need to mark what they
+              # refer to as alive:
+              while j.jumpStack.len > 0:
+                let (t, desc) = j.jumpStack.pop
+                # not in jump stack anymore!
+                t.rc = t.rc and not jumpStackFlag
+                if t.color == colGray and (t.rc shr rcShift) >= 0:
+                  scanBlack(t, desc, j)
+                  # XXX this should be done according to Lins' paper but currently breaks
+                  #t.setColor colPurple
+                  when traceCollector:
+                    cprintf("[jump stack] %p %ld\n", t, t.rc shr rcShift)
+
+            t.setColor(colWhite)
+            trace(t, desc, j)
+
+proc collectWhite(s: Cell; desc: PNimType; j: var GcEnv) =
+  #[
+  proc collectWhite(s: Cell) =
+    if s.color == colWhite and not buffered(s):
+      s.setColor(colBlack)
+      for t in sons(s):
+        collectWhite(t)
+      free(s) # watch out, a bug here!
+  ]#
+  if s.color == colWhite and (s.rc and isCycleCandidate) == 0:
+    s.setColor(colBlack)
+    when false:
+      # optimized version (does not work)
+      j.traceStack.add(s, desc)
+      # this way of writing the loop means we can free all the nodes
+      # afterwards avoiding the "use after free" bug in the paper.
+      var i = 0
+      while i < j.traceStack.len:
+        let (t, desc) = j.traceStack.d[j.traceStack.len-1]
+        inc i
+        if t.color == colWhite and (t.rc and isCycleCandidate) == 0:
+          t.setColor(colBlack)
+          trace(t, desc, j)
+
+      for i in 0 ..< j.traceStack.len:
+        free(j.traceStack.d[i][0], j.traceStack.d[i][1])
+      j.traceStack.len = 0
+    else:
+      var subgraph: CellSeq
+      init subgraph
+      subgraph.add(s, desc)
+      trace(s, desc, j)
+      while j.traceStack.len > 0:
+        let (t, desc) = j.traceStack.pop()
+        if t.color == colWhite and (t.rc and isCycleCandidate) == 0:
+          subgraph.add(t, desc)
+          t.setColor(colBlack)
+          trace(t, desc, j)
+      for i in 0 ..< subgraph.len:
+        free(subgraph.d[i][0], subgraph.d[i][1])
+      inc j.freed, subgraph.len
+      deinit subgraph
+
+proc collectCyclesBacon(j: var GcEnv) =
+  # pretty direct translation from
+  # https://researcher.watson.ibm.com/researcher/files/us-bacon/Bacon01Concurrent.pdf
+  # Fig. 2. Synchronous Cycle Collection
+  #[
+    for s in roots:
+      markGray(s)
+    for s in roots:
+      scan(s)
+    for s in roots:
+      remove s from roots
+      s.buffered = false
+      collectWhite(s)
+  ]#
+  for i in 0 ..< roots.len:
+    markGray(roots.d[i][0], roots.d[i][1], j)
+  for i in 0 ..< roots.len:
+    scan(roots.d[i][0], roots.d[i][1], j)
+
+  for i in 0 ..< roots.len:
+    let s = roots.d[i][0]
+    s.rc = s.rc and not isCycleCandidate
+    collectWhite(s, roots.d[i][1], j)
+  #roots.len = 0
+
+const
+  defaultThreshold = 10_000
+
+var
+  rootsThreshold = defaultThreshold
+
+proc collectCycles() =
+  ## Collect cycles.
+  var j: GcEnv
+  init j.traceStack
+  when useJumpStack:
+    init j.jumpStack
+    collectCyclesBacon(j)
+    while j.jumpStack.len > 0:
+      let (t, desc) = j.jumpStack.pop
+      # not in jump stack anymore!
+      t.rc = t.rc and not jumpStackFlag
+    deinit j.jumpStack
+  else:
+    collectCyclesBacon(j)
+
+  deinit j.traceStack
+  deinit roots
+  # compute the threshold based on the previous history
+  # of the cycle collector's effectiveness:
+  # we're effective when we collected 50% or more of the nodes
+  # we touched. If we're effective, we can reset the threshold:
+  if j.freed * 2 >= j.touched:
+    rootsThreshold = defaultThreshold
+  else:
+    rootsThreshold = rootsThreshold * 3 div 2
+  when false:
+    cprintf("[collectCycles] freed %ld new threshold %ld\n", j.freed, rootsThreshold)
+
+proc registerCycle(s: Cell; desc: PNimType) =
+  if roots.d == nil: init(roots)
+  s.rootIdx = roots.len
+  add(roots, s, desc)
+  if roots.len >= rootsThreshold:
+    collectCycles()
+
+proc GC_fullCollect* =
+  ## Forces a full garbage collection pass. With ``--gc:orc`` triggers the cycle
+  ## collector.
+  collectCycles()
+
+proc GC_enableMarkAndSweep() =
+  rootsThreshold = defaultThreshold
+
+proc GC_disableMarkAndSweep() =
+  rootsThreshold = high(int)
+
+proc rememberCycle(isDestroyAction: bool; s: Cell; desc: PNimType) {.noinline.} =
+  if isDestroyAction:
+    if (s.rc and isCycleCandidate) != 0:
+      s.rc = s.rc and not isCycleCandidate
+      unregisterCycle(s)
+  else:
+    # do not call 'rememberCycle' again unless this cell
+    # got an 'incRef' event:
+    #s.setColor colGreen  # XXX This is wrong!
+    if (s.rc and isCycleCandidate) == 0:
+      s.rc = s.rc or isCycleCandidate
+      registerCycle(s, desc)
+
+proc nimDecRefIsLastCyclicDyn(p: pointer): bool {.compilerRtl, inl.} =
+  if p != nil:
+    var cell = head(p)
+    if (cell.rc and not rcMask) == 0:
+      result = true
+      #cprintf("[DESTROY] %p\n", p)
+    else:
+      dec cell.rc, rcIncrement
+    if cell.color == colPurple:
+      rememberCycle(result, cell, cast[ptr PNimType](p)[])
+
+proc nimDecRefIsLastCyclicStatic(p: pointer; desc: PNimType): bool {.compilerRtl, inl.} =
+  if p != nil:
+    var cell = head(p)
+    if (cell.rc and not rcMask) == 0:
+      result = true
+      #cprintf("[DESTROY] %p %s\n", p, desc.name)
+    else:
+      dec cell.rc, rcIncrement
+    if cell.color == colPurple:
+      rememberCycle(result, cell, desc)

lib/system/cyclicrefs_v2.nim

@@ -14,6 +14,7 @@
 # "Cyclic reference counting" by Rafael Dueire Lins
 # R.D. Lins / Information Processing Letters 109 (2008) 71–78
 
+type PT = ptr pointer
 include cellseqs_v2
 
 const
@@ -60,12 +61,25 @@ proc free(s: Cell; desc: PNimType) {.inline.} =
   when traceCollector:
     cprintf("[From ] %p rc %ld color %ld in jumpstack %ld\n", s, s.rc shr rcShift,
             s.color, s.rc and jumpStackFlag)
-  var p = s +! sizeof(RefHeader)
+  let p = s +! sizeof(RefHeader)
   if desc.disposeImpl != nil:
     cast[DisposeProc](desc.disposeImpl)(p)
   nimRawDispose(p)
 
 proc collect(s: Cell; desc: PNimType; j: var GcEnv) =
+  #[ Algorithm from the paper:
+
+    Collect(S) =
+      If (Color(S) == red) then
+        RC(S) = 1;
+        Color(S) = green;
+        for T in Sons(S) do
+          Remove(<S, T>);
+          if (Color(T) == red) then Collect(T);
+      free_list = S;
+
+  (Whatever that really means...)
+  ]#
   if s.color == colRed:
     s.setColor colGreen
     trace(s, desc, j)

lib/system/mm/malloc.nim

@@ -38,10 +38,13 @@ proc deallocSharedImpl(p: pointer) = deallocImpl(p)
 
 proc GC_disable() = discard
 proc GC_enable() = discard
-proc GC_fullCollect() = discard
+
+when not defined(gcOrc):
+  proc GC_fullCollect() = discard
+  proc GC_enableMarkAndSweep() = discard
+  proc GC_disableMarkAndSweep() = discard
+
 proc GC_setStrategy(strategy: GC_Strategy) = discard
-proc GC_enableMarkAndSweep() = discard
-proc GC_disableMarkAndSweep() = discard
 
 proc getOccupiedMem(): int = discard
 proc getFreeMem(): int = discard

lib/system/refs_v2.nim

@@ -34,15 +34,25 @@ hash of ``package & "." & module & "." & name`` to save space.
 
 ]#
 
-const
+when defined(gcOrc):
-  rcIncrement = 0b1000 # so that lowest 3 bits are not touched
+  const
-  rcMask = 0b111
+    rcIncrement = 0b10000 # so that lowest 4 bits are not touched
+    rcMask = 0b1111
+
+else:
+  const
+    rcIncrement = 0b1000 # so that lowest 3 bits are not touched
+    rcMask = 0b111
 
 type
   RefHeader = object
     rc: int # the object header is now a single RC field.
             # we could remove it in non-debug builds for the 'owned ref'
             # design but this seems unwise.
+    when defined(gcOrc):
+      rootIdx: int # thanks to this we can delete potential cycle roots
+                   # in O(1) without doubly linked lists
+
   Cell = ptr RefHeader
 
 template `+!`(p: pointer, s: int): pointer =
@@ -85,6 +95,8 @@ proc nimNewObjUninit(size: int): pointer {.compilerRtl.} =
   else:
     var orig = cast[ptr RefHeader](alloc(s))
   orig.rc = 0
+  when defined(gcOrc):
+    orig.rootIdx = 0
   result = orig +! sizeof(RefHeader)
   when traceCollector:
     cprintf("[Allocated] %p result: %p\n", result -! sizeof(RefHeader), result)
@@ -131,7 +143,9 @@ when defined(gcOrc):
   when defined(nimThinout):
     include cyclebreaker
   else:
-    include cyclicrefs_v2
+    include cyclicrefs_bacon
+    #include cyclecollector
+    #include cyclicrefs_v2
 
 proc nimDecRefIsLast(p: pointer): bool {.compilerRtl, inl.} =
   if p != nil:
@@ -157,9 +171,10 @@ proc GC_ref*[T](x: ref T) =
   ## New runtime only supports this operation for 'ref T'.
   if x != nil: nimIncRef(cast[pointer](x))
 
-template GC_fullCollect* =
+when not defined(gcOrc):
-  ## Forces a full garbage collection pass. With ``--gc:arc`` a nop.
+  template GC_fullCollect* =
-  discard
+    ## Forces a full garbage collection pass. With ``--gc:arc`` a nop.
+    discard
 
 template setupForeignThreadGc* =
   ## With ``--gc:arc`` a nop.

tests/arc/tasyncawait.nim

@@ -1,6 +1,6 @@
 discard """
   outputsub: "result: 5000"
-  cmd: "nim c --gc:arc $file"
+  cmd: "nim c --gc:orc $file"
 """
 
 import asyncdispatch, asyncnet, nativesockets, net, strutils, os
@@ -71,4 +71,5 @@ main()
 
 assert msgCount == swarmSize * messagesToSend
 echo "result: ", msgCount
+GC_fullCollect()
 echo "memory: ", formatSize(getOccupiedMem() - mem)

tests/destructor/tarc2.nim

@@ -27,4 +27,5 @@ proc main(rootName: string) =
 
 let mem = getOccupiedMem()
 main("yeah")
+GC_fullCollect()
 echo "leak: ", getOccupiedMem() - mem > 0

tests/destructor/tasync_prototype_cyclic.nim

@@ -51,4 +51,5 @@ proc main =
   discard serve(PAsyncHttpServer(value: "asdas"))
 
 main()
+GC_fullCollect()
 echo "MEMORY ", getOccupiedMem() - mem

tests/destructor/tcycle1.nim

@@ -50,4 +50,5 @@ proc main =
 
 let mem = getOccupiedMem()
 main()
+GC_fullCollect()
 echo "MEM ", getOccupiedMem() - mem

tests/destructor/tcycle2.nim

@@ -15,4 +15,6 @@ proc main(x: int) =
 
 let mem = getOccupiedMem()
 main(90)
+GC_fullCollect()
+
 echo "MEM ", getOccupiedMem() - mem

tests/destructor/tcycle3.nim

@@ -61,4 +61,5 @@ proc main =
 
 let mem = getOccupiedMem()
 main()
+GC_fullCollect()
 echo getOccupiedMem() - mem

tests/gc/thavlak.nim

@@ -394,7 +394,7 @@ proc run(self: var LoopTesterApp) =
   echo "Constructing CFG..."
   var n = 2
 
-  when not defined(gcOrc):
+  when true: # not defined(gcOrc):
     # currently cycle detection is so slow that we disable this part
     for parlooptrees in 1..10:
       discard self.cfg.createNode(n + 1)
@@ -437,4 +437,5 @@ proc main =
 let mem = getOccupiedMem()
 main()
 when defined(gcOrc):
+  GC_fullCollect()
   doAssert getOccupiedMem() == mem