inexplicable_segfault.nim

# The following code produces a segfault from within gc.nim (see bottom)
# The segfault does not occur when using --gc:markAndSweep

type
  TNode = tuple
    prev: ptr TNode

var
  front = cast[ptr TNode](alloc0(sizeof(TNode)))
  back = front

echo "allocated: " & $cast[int](front)

for i in 0..3:
  back.prev = cast[ptr TNode](alloc0(sizeof(TNode)))
  echo "allocated: " & $cast[int](back.prev)
  back = back.prev

for i in 0..4:
  var tmp = front
  front = front.prev
  echo "deallocating: " & $cast[int](tmp)
  dealloc(tmp)

var str = cast[ptr string](alloc(sizeof(string)))

str[] = "hi" # SIGSEGV OCCURS ON THIS LINE (in gc.nim, see below)

#
# This is the result of running the program:
#allocated: -1220075480
#allocated: -1220075464
#allocated: -1220075448
#allocated: -1220075432
#allocated: -1220075416
#freed: -1220075480
#freed: -1220075464
#freed: -1220075448
#freed: -1220075432
#freed: -1220075416
#Traceback (most recent call last)
# testcase.nim(28)  testcase
# gc.nim(269)       unsureAsgnRef
# gc.nim(208)       decRef
# gc.nim(121)       canbeCycleRoot
# SIGSEGV: Illegal storage access. (Attempt to read from nil?)

This error disappears when you use alloc0(sizeof(string)), I surmise that one of the ptr TNode is left on the stack and becomes the value given by alloc() just a guess though