pi test compiles, but crashes randomly

compiler/lowerings.nim

@@ -197,18 +197,21 @@ proc createNimCreatePromiseCall(prom, threadParam: PNode): PNode =
   result = newFastAsgnStmt(prom, castExpr)
 
 proc createWrapperProc(f: PNode; threadParam, argsParam: PSym;
-                       varSection, call, barrier, prom: PNode): PSym =
+                       varSection, call, barrier, prom: PNode;
+                       spawnKind: TSpawnResult): PSym =
   var body = newNodeI(nkStmtList, f.info)
   if barrier != nil:
     body.add callCodeGenProc("barrierEnter", barrier)
   body.add varSection
-  if prom != nil:
+  if prom != nil and spawnKind != srByVar:
     body.add createNimCreatePromiseCall(prom, threadParam.newSymNode)
     if barrier == nil:
       body.add callCodeGenProc("nimPromiseCreateCondVar", prom)
 
   body.add callCodeGenProc("nimArgsPassingDone", threadParam.newSymNode)
-  if prom != nil:
+  if spawnKind == srByVar:
+    body.add newAsgnStmt(genDeref(prom), call)
+  elif prom != nil:
     let fk = prom.typ.sons[1].promiseKind
     if fk == promInvalid:
       localError(f.info, "cannot create a promise of type: " & 
@@ -471,9 +474,16 @@ proc wrapProcForSpawn*(owner: PSym; n: PNode; retType: PType;
     objType.addField(field)
     promField = newDotExpr(scratchObj, field)
     promAsExpr = indirectAccess(castExpr, field, n.info)
+  elif spawnKind == srByVar:
+    var field = newSym(skField, getIdent"prom", owner, n.info)
+    field.typ = newType(tyPtr, objType.owner)
+    field.typ.rawAddSon(retType)
+    objType.addField(field)
+    promAsExpr = indirectAccess(castExpr, field, n.info)
+    result.add newFastAsgnStmt(newDotExpr(scratchObj, field), genAddrOf(dest))
 
   let wrapper = createWrapperProc(fn, threadParam, argsParam, varSection, call,
-                                  barrierAsExpr, promAsExpr)
+                                  barrierAsExpr, promAsExpr, spawnKind)
   result.add callCodeGenProc("nimSpawn", wrapper.newSymNode,
                              genAddrOf(scratchObj.newSymNode))
 

compiler/semdata.nim

@@ -91,6 +91,7 @@ type
     generics*: seq[TInstantiationPair] # pending list of instantiated generics to compile
     lastGenericIdx*: int      # used for the generics stack
     hloLoopDetector*: int     # used to prevent endless loops in the HLO
+    inParallelStmt*: int
    
 proc makeInstPair*(s: PSym, inst: PInstantiation): TInstantiationPair =
   result.genericSym = s

compiler/semexprs.nim

@@ -1615,13 +1615,17 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
     result = setMs(n, s)
     var x = n.lastSon
     if x.kind == nkDo: x = x.sons[bodyPos]
+    inc c.inParallelStmt
     result.sons[1] = semStmt(c, x)
+    dec c.inParallelStmt
   of mSpawn:
     result = setMs(n, s)
     result.sons[1] = semExpr(c, n.sons[1])
-    # later passes may transform the type 'Promise[T]' back into 'T'
     if not result[1].typ.isEmptyType:
-      result.typ = createPromise(c, result[1].typ, n.info)
+      if c.inParallelStmt > 0:
+        result.typ = result[1].typ
+      else:
+        result.typ = createPromise(c, result[1].typ, n.info)
   else: result = semDirectOp(c, n, flags)
 
 proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =

tests/parallel/tpi.nim

@@ -0,0 +1,22 @@
+
+import strutils, math, threadpool
+
+proc term(k: float): float = 4 * math.pow(-1, k) / (2*k + 1)
+
+proc piU(n: int): float =
+  var ch = newSeq[Promise[float]](n+1)
+  for k in 0..n:
+    ch[k] = spawn term(float(k))
+  for k in 0..n:
+    result += ^ch[k]
+
+proc piS(n: int): float =
+  var ch = newSeq[float](n+1)
+  parallel:
+    for k in 0..ch.high:
+      ch[k] = spawn term(float(k))
+  for k in 0..ch.high:
+    result += ch[k]
+
+echo formatFloat(piU(5000))
+echo formatFloat(piS(5000))