From 88b65ea957b286bf7225c63e38b6d83632ee6cce Mon Sep 17 00:00:00 2001
From: Andreas Rumpf <rumpf_a@web.de>
Date: Fri, 11 Aug 2017 20:14:35 +0200
Subject: [PATCH] fixes #6234

---
 compiler/ccgexprs.nim               |  8 ++++----
 lib/system/gc.nim                   |  3 +++
 lib/system/mmdisp.nim               |  2 +-
 tests/ccgbugs/twrongrefcounting.nim | 22 ++++++++++++++++++++++
 4 files changed, 30 insertions(+), 5 deletions(-)
 create mode 100644 tests/ccgbugs/twrongrefcounting.nim

diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index de784acf9c..fbdd5f3182 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -1100,9 +1100,9 @@ proc rawGenNew(p: BProc, a: TLoc, sizeExpr: Rope) =
   if a.s == OnHeap and usesNativeGC():
     # use newObjRC1 as an optimization
     if canFormAcycle(a.t):
-      linefmt(p, cpsStmts, "if ($1) #nimGCunref($1);$n", a.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefRC1($1); $1 = NIM_NIL; }$n", a.rdLoc)
     else:
-      linefmt(p, cpsStmts, "if ($1) #nimGCunrefNoCycle($1);$n", a.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefNoCycle($1); $1 = NIM_NIL; }$n", a.rdLoc)
     b.r = ropecg(p.module, "($1) #newObjRC1($2, $3)", args)
     linefmt(p, cpsStmts, "$1 = $2;$n", a.rdLoc, b.rdLoc)
   else:
@@ -1130,9 +1130,9 @@ proc genNewSeqAux(p: BProc, dest: TLoc, length: Rope) =
   initLoc(call, locExpr, dest.t, OnHeap)
   if dest.s == OnHeap and usesNativeGC():
     if canFormAcycle(dest.t):
-      linefmt(p, cpsStmts, "if ($1) #nimGCunref($1);$n", dest.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefRC1($1); $1 = NIM_NIL; }$n", dest.rdLoc)
     else:
-      linefmt(p, cpsStmts, "if ($1) #nimGCunrefNoCycle($1);$n", dest.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefNoCycle($1); $1 = NIM_NIL; }$n", dest.rdLoc)
     call.r = ropecg(p.module, "($1) #newSeqRC1($2, $3)", args)
     linefmt(p, cpsStmts, "$1 = $2;$n", dest.rdLoc, call.rdLoc)
   else:
diff --git a/lib/system/gc.nim b/lib/system/gc.nim
index 87a07fae98..80aa5cf1be 100644
--- a/lib/system/gc.nim
+++ b/lib/system/gc.nim
@@ -259,6 +259,9 @@ proc nimGCunrefNoCycle(p: pointer) {.compilerProc, inline.} =
     sysAssert(allocInv(gch.region), "end nimGCunrefNoCycle 2")
   sysAssert(allocInv(gch.region), "end nimGCunrefNoCycle 5")
 
+proc nimGCunrefRC1(p: pointer) {.compilerProc, inline.} =
+  decRef(usrToCell(p))
+
 proc asgnRef(dest: PPointer, src: pointer) {.compilerProc, inline.} =
   # the code generator calls this proc!
   gcAssert(not isOnStack(dest), "asgnRef")
diff --git a/lib/system/mmdisp.nim b/lib/system/mmdisp.nim
index 5b5ba94908..35fa0ce9a4 100644
--- a/lib/system/mmdisp.nim
+++ b/lib/system/mmdisp.nim
@@ -16,7 +16,7 @@
 const
   debugGC = false # we wish to debug the GC...
   logGC = false
-  traceGC = defined(smokeCycles) # extensive debugging
+  traceGC = false # extensive debugging
   alwaysCycleGC = defined(smokeCycles)
   alwaysGC = defined(fulldebug) # collect after every memory
                                 # allocation (for debugging)
diff --git a/tests/ccgbugs/twrongrefcounting.nim b/tests/ccgbugs/twrongrefcounting.nim
new file mode 100644
index 0000000000..c0c3e0fc24
--- /dev/null
+++ b/tests/ccgbugs/twrongrefcounting.nim
@@ -0,0 +1,22 @@
+discard """
+  output: '''ok'''
+  cmd: "nim c -r --gc:refc -d:useGcAssert -d:useSysAssert -d:fulldebug -d:smokeCycles $file"
+"""
+# bug #6234
+type
+    Foo = ref object
+        s: seq[Bar]
+    Bar = ref object
+        f: Foo
+
+proc test() =
+    var f = Foo.new()
+    for i in 0 .. 5:
+        f.s = @[]
+        for j in 0 .. 5:
+            var b = Bar.new()
+            b.f = f
+            f.s.add(b)
+
+test()
+echo "ok"