fixes #7833; still to-do: fix setLen

compiler/ccgexprs.nim

@@ -1092,17 +1092,20 @@ proc genSeqElemAppend(p: BProc, e: PNode, d: var TLoc) =
   #    seq = (typeof seq) incrSeq(&seq->Sup, sizeof(x));
   #    seq->data[seq->len-1] = x;
   let seqAppendPattern = if not p.module.compileToCpp:
-                           "$1 = ($2) #incrSeqV3(&($1)->Sup, $3);$n"
+                           "($2) #incrSeqV3(&($1)->Sup, $3)"
                          else:
-                           "$1 = ($2) #incrSeqV3($1, $3);$n"
-  var a, b, dest, tmpL: TLoc
+                           "($2) #incrSeqV3($1, $3)"
+  var a, b, dest, tmpL, call: TLoc
   initLocExpr(p, e.sons[1], a)
   initLocExpr(p, e.sons[2], b)
   let seqType = skipTypes(e.sons[1].typ, {tyVar})
-  lineCg(p, cpsStmts, seqAppendPattern, [
-      rdLoc(a),
-      getTypeDesc(p.module, e.sons[1].typ),
-      genTypeInfo(p.module, seqType, e.info)])
+  initLoc(call, locCall, e, OnHeap)
+  call.r = ropecg(p.module, seqAppendPattern, [rdLoc(a),
+    getTypeDesc(p.module, e.sons[1].typ),
+    genTypeInfo(p.module, seqType, e.info)])
+  # emit the write barrier if required, but we can always move here, so
+  # use 'genRefAssign' for the seq.
+  genRefAssign(p, a, call, {})
   #if bt != b.t:
   #  echo "YES ", e.info, " new: ", typeToString(bt), " old: ", typeToString(b.t)
   initLoc(dest, locExpr, e.sons[2], OnHeap)

compiler/condsyms.nim

@@ -72,3 +72,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimNoZeroTerminator")
   defineSymbol("nimNotNil")
   defineSymbol("nimVmExportFixed")
+  defineSymbol("nimIncrSeqV3")

lib/system/gc.nim

@@ -548,7 +548,10 @@ proc growObj(old: pointer, newsize: int, gch: var GcHeap): pointer =
   gcTrace(res, csAllocated)
   track("growObj old", ol, 0)
   track("growObj new", res, newsize)
-  when reallyDealloc:
+  when defined(nimIncrSeqV3):
+    # since we steal the old seq's contents, we set the old length to 0.
+    cast[PGenericSeq](old).len = 0
+  elif reallyDealloc:
     sysAssert(allocInv(gch.region), "growObj before dealloc")
     if ol.refcount shr rcShift <=% 1:
       # free immediately to save space:

tests/gc/growobjcrash.nim

@@ -14,11 +14,11 @@ proc handleRequest(query: string): StringTableRef =
   let x = foo
   result = x()
 
-const Limit = when compileOption("gc", "markAndSweep"): 5*1024*1024 else: 700_000
+const Limit = 5*1024*1024
 
 proc main =
   var counter = 0
-  for i in 0 .. 100_000:
+  for i in 0 .. 10_000_000:
     for k, v in handleRequest("nick=Elina2&type=activate"):
       inc counter
       if counter mod 100 == 0: