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diff --git a/lib/pure/concurrency/threadpool.nim b/lib/pure/concurrency/threadpool.nim
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+#
+#
+#            Nimrod's Runtime Library
+#        (c) Copyright 2014 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## Implements Nimrod's 'spawn'.
+
+import cpuinfo, cpuload, locks
+
+{.push stackTrace:off.}
+
+type
+  CondVar = object
+    c: TCond
+    L: TLock
+    counter: int
+
+proc createCondVar(): CondVar =
+  initCond(result.c)
+  initLock(result.L)
+
+proc destroyCondVar(cv: var CondVar) {.inline.} =
+  deinitCond(cv.c)
+  deinitLock(cv.L)
+
+proc await(cv: var CondVar) =
+  acquire(cv.L)
+  while cv.counter <= 0:
+    wait(cv.c, cv.L)
+  dec cv.counter
+  release(cv.L)
+
+proc signal(cv: var CondVar) =
+  acquire(cv.L)
+  inc cv.counter
+  release(cv.L)
+  signal(cv.c)
+
+type
+  Barrier* {.compilerProc.} = object
+    counter: int
+    cv: CondVar
+
+proc barrierEnter*(b: ptr Barrier) {.compilerProc.} =
+  atomicInc b.counter
+
+proc barrierLeave*(b: ptr Barrier) {.compilerProc.} =
+  atomicDec b.counter
+  if b.counter <= 0: signal(b.cv)
+
+proc openBarrier*(b: ptr Barrier) {.compilerProc.} =
+  b.counter = 0
+  b.cv = createCondVar()
+
+proc closeBarrier*(b: ptr Barrier) {.compilerProc.} =
+  await(b.cv)
+  destroyCondVar(b.cv)
+
+{.pop.}
+
+# ----------------------------------------------------------------------------
+
+type
+  WorkerProc = proc (thread, args: pointer) {.nimcall, gcsafe.}
+  Worker = object
+    taskArrived: CondVar
+    taskStarted: CondVar #\
+    # task data:
+    f: WorkerProc
+    data: pointer
+    ready: bool # put it here for correct alignment!
+    initialized: bool # whether it has even been initialized
+
+proc nimArgsPassingDone(p: pointer) {.compilerProc.} =
+  let w = cast[ptr Worker](p)
+  signal(w.taskStarted)
+
+var
+  gSomeReady = createCondVar()
+  readyWorker: ptr Worker
+
+proc slave(w: ptr Worker) {.thread.} =
+  while true:
+    w.ready = true
+    readyWorker = w
+    signal(gSomeReady)
+    await(w.taskArrived)
+    assert(not w.ready)
+    w.f(w, w.data)
+
+const
+  MaxThreadPoolSize* = 256 ## maximal size of the thread pool. 256 threads
+                           ## should be good enough for anybody ;-)
+
+var
+  currentPoolSize: int
+  maxPoolSize = MaxThreadPoolSize
+  minPoolSize = 4
+
+proc setMinPoolSize*(size: range[1..MaxThreadPoolSize]) =
+  ## sets the minimal thread pool size. The default value of this is 4.
+  minPoolSize = size
+
+proc setMaxPoolSize*(size: range[1..MaxThreadPoolSize]) =
+  ## sets the minimal thread pool size. The default value of this
+  ## is ``MaxThreadPoolSize``.
+  maxPoolSize = size
+
+var
+  workers: array[MaxThreadPoolSize, TThread[ptr Worker]]
+  workersData: array[MaxThreadPoolSize, Worker]
+
+proc activateThread(i: int) {.noinline.} =
+  workersData[i].taskArrived = createCondVar()
+  workersData[i].taskStarted = createCondVar()
+  workersData[i].initialized = true
+  createThread(workers[i], slave, addr(workersData[i]))
+
+proc setup() =
+  currentPoolSize = min(countProcessors(), MaxThreadPoolSize)
+  readyWorker = addr(workersData[0])
+  for i in 0.. <currentPoolSize: activateThread(i)
+
+proc preferSpawn*(): bool =
+  ## Use this proc to determine quickly if a 'spawn' or a direct call is
+  ## preferable. If it returns 'true' a 'spawn' may make sense. In general
+  ## it is not necessary to call this directly; use 'spawnX' instead.
+  result = gSomeReady.counter > 0
+
+proc spawn*(call: stmt) {.magic: "Spawn".}
+  ## always spawns a new task, so that the 'call' is never executed on
+  ## the calling thread. 'call' has to be proc call 'p(...)' where 'p'
+  ## is gcsafe and has 'void' as the return type.
+
+template spawnX*(call: stmt) =
+  ## spawns a new task if a CPU core is ready, otherwise executes the
+  ## call in the calling thread. Usually it is advised to
+  ## use 'spawn' in order to not block the producer for an unknown
+  ## amount of time. 'call' has to be proc call 'p(...)' where 'p'
+  ## is gcsafe and has 'void' as the return type.
+  if preferSpawn(): spawn call
+  else: call
+
+proc parallel*(body: stmt) {.magic: "Parallel".}
+  ## a parallel section can be used to execute a block in parallel. ``body``
+  ## has to be in a DSL that is a particular subset of the language. Please
+  ## refer to the manual for further information.
+
+var
+  state: ThreadPoolState
+  stateLock: TLock
+
+initLock stateLock
+
+proc selectWorker(w: ptr Worker; fn: WorkerProc; data: pointer): bool =
+  if cas(addr w.ready, true, false):
+    w.data = data
+    w.f = fn
+    signal(w.taskArrived)
+    await(w.taskStarted)
+    result = true
+
+proc nimSpawn(fn: WorkerProc; data: pointer) {.compilerProc.} =
+  # implementation of 'spawn' that is used by the code generator.
+  while true:
+    if selectWorker(readyWorker, fn, data): return
+    for i in 0.. <currentPoolSize:
+      if selectWorker(addr(workersData[i]), fn, data): return
+    # determine what to do, but keep in mind this is expensive too:
+    # state.calls < maxPoolSize: warmup phase
+    # (state.calls and 127) == 0: periodic check
+    if state.calls < maxPoolSize or (state.calls and 127) == 0:
+      # ensure the call to 'advice' is atomic:
+      if tryAcquire(stateLock):
+        case advice(state)
+        of doNothing: discard
+        of doCreateThread:
+          if currentPoolSize < maxPoolSize:
+            if not workersData[currentPoolSize].initialized:
+              activateThread(currentPoolSize)
+            let w = addr(workersData[currentPoolSize])
+            inc currentPoolSize
+            if selectWorker(w, fn, data):
+              release(stateLock)
+              return
+            # else we didn't succeed but some other thread, so do nothing.
+        of doShutdownThread:
+          if currentPoolSize > minPoolSize: dec currentPoolSize
+          # we don't free anything here. Too dangerous.
+        release(stateLock)
+      # else the acquire failed, but this means some
+      # other thread succeeded, so we don't need to do anything here.
+    await(gSomeReady)
+
+proc sync*() =
+  ## a simple barrier to wait for all spawn'ed tasks. If you need more elaborate
+  ## waiting, you have to use an explicit barrier.
+  while true:
+    var allReady = true
+    for i in 0 .. <currentPoolSize:
+      if not allReady: break
+      allReady = allReady and workersData[i].ready
+    if allReady: break
+    await(gSomeReady)
+
+setup()