fixes a serious codegen bug that caused to emit wrong barriers in rare cases

compiler/ccgexprs.nim

@@ -685,19 +685,23 @@ proc genDeref(p: BProc, e: PNode, d: var TLoc; enforceDeref=false) =
   else:
     var a: TLoc
     initLocExprSingleUse(p, e.sons[0], a)
-    let typ = skipTypes(a.t, abstractInst)
-    case typ.kind
-    of tyRef:
-      d.s = OnHeap
-    of tyVar:
-      d.s = OnUnknown
-      if tfVarIsPtr notin typ.flags and p.module.compileToCpp and
-          e.kind == nkHiddenDeref:
-        putIntoDest(p, d, e.typ, rdLoc(a))
-        return
-    of tyPtr:
-      d.s = OnUnknown         # BUGFIX!
-    else: internalError(e.info, "genDeref " & $a.t.kind)
+    if d.k == locNone:
+      let typ = skipTypes(a.t, abstractInst)
+      # dest = *a;  <-- We do not know that 'dest' is on the heap!
+      # It is completely wrong to set 'd.s' here, unless it's not yet
+      # been assigned to.
+      case typ.kind
+      of tyRef:
+        d.s = OnHeap
+      of tyVar:
+        d.s = OnUnknown
+        if tfVarIsPtr notin typ.flags and p.module.compileToCpp and
+            e.kind == nkHiddenDeref:
+          putIntoDest(p, d, e.typ, rdLoc(a))
+          return
+      of tyPtr:
+        d.s = OnUnknown         # BUGFIX!
+      else: internalError(e.info, "genDeref " & $a.t.kind)
     if enforceDeref and mt == ctPtrToArray:
       # we lie about the type for better C interop: 'ptr array[3,T]' is
       # translated to 'ptr T', but for deref'ing this produces wrong code.

tests/parallel/twrong_refcounts.nim

@@ -0,0 +1,53 @@
+discard """
+  output: "Success"
+"""
+
+import math, threadPool
+
+# ---
+
+type
+  Person = object
+    age: int
+    friend: ref Person
+
+var
+  people: seq[ref Person] = @[]
+
+proc newPerson(age:int): ref Person =
+  result.new()
+  result.age = age
+
+proc greet(p:Person) =
+  #echo p.age, ", ", p.friend.age
+  p.friend.age += 1
+
+# ---
+
+proc setup =
+  for i in 0 .. <20:
+    people.add newPerson(i + 1)
+  for i in 0 .. <20:
+    people[i].friend = people[random(20)]
+
+proc update =
+  var countA: array[20, int]
+  var countB: array[20, int]
+
+  for i, p in people:
+    countA[i] = getRefCount(p)
+  parallel:
+    for i in 0 .. people.high:
+      spawn greet(people[i][])
+  for i, p in people:
+    countB[i] = getRefCount(p)
+
+  for i in 0 .. <20:
+    doAssert countA[i] == countB[i]
+  echo "Success"
+
+# ---
+
+when isMainModule:
+  setup()
+  update()

todo.txt

@@ -2,6 +2,7 @@ version 0.10.4
 ==============
 
 - make 'nil' work for 'add' and 'len'
+- enable parameter info for nimsuggest
 
 version 1.0
 ===========