fixes #2070

lib/system/gc.nim

@@ -528,11 +528,18 @@ proc growObj(old: pointer, newsize: int, gch: var TGcHeap): pointer =
   zeroMem(cast[pointer](cast[ByteAddress](res)+% oldsize +% sizeof(TCell)),
           newsize-oldsize)
   sysAssert((cast[ByteAddress](res) and (MemAlign-1)) == 0, "growObj: 3")
-  sysAssert(res.refcount shr rcShift <=% 1, "growObj: 4")
-  #if res.refcount <% rcIncrement:
-  #  add(gch.zct, res)
-  #else: # XXX: what to do here?
-  #  decRef(ol)
+  # This can be wrong for intermediate temps that are nevertheless on the
+  # heap because of lambda lifting:
+  #gcAssert(res.refcount shr rcShift <=% 1, "growObj: 4")
+  when logGC:
+    writeCell("growObj old cell", ol)
+    writeCell("growObj new cell", res)
+  gcTrace(ol, csZctFreed)
+  gcTrace(res, csAllocated)
+  when reallyDealloc: 
+    sysAssert(allocInv(gch.region), "growObj before dealloc")
+    if ol.refcount shr rcShift <=% 1:
+      # free immediately to save space:
       if (ol.refcount and ZctFlag) != 0:
         var j = gch.zct.len-1
         var d = gch.zct.d
@@ -542,14 +549,15 @@ proc growObj(old: pointer, newsize: int, gch: var TGcHeap): pointer =
             break
           dec(j)
       if canbeCycleRoot(ol): excl(gch.cycleRoots, ol)
-  when logGC:
-    writeCell("growObj old cell", ol)
-    writeCell("growObj new cell", res)
-  gcTrace(ol, csZctFreed)
-  gcTrace(res, csAllocated)
-  when reallyDealloc: 
-    sysAssert(allocInv(gch.region), "growObj before dealloc")
       rawDealloc(gch.region, ol)
+    else:
+      # we split the old refcount in 2 parts. XXX This is still not entirely
+      # correct if the pointer that receives growObj's result is on the stack.
+      # A better fix would be to emit the location specific write barrier for
+      # 'growObj', but this is lost of more work and who knows what new problems
+      # this would create.
+      res.refcount = rcIncrement
+      decRef(ol)
   else:
     sysAssert(ol.typ != nil, "growObj: 5")
     zeroMem(ol, sizeof(TCell))

lib/system/gc_ms.nim

@@ -297,6 +297,8 @@ proc growObj(old: pointer, newsize: int, gch: var TGcHeap): pointer =
   zeroMem(cast[pointer](cast[ByteAddress](res)+% oldsize +% sizeof(TCell)),
           newsize-oldsize)
   sysAssert((cast[ByteAddress](res) and (MemAlign-1)) == 0, "growObj: 3")
+  when false:
+    # this is wrong since seqs can be shared via 'shallow':
     when withBitvectors: excl(gch.allocated, ol)
     when reallyDealloc: rawDealloc(gch.region, ol)
     else: