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Nim/lib/system/gc.nim
rumpf_a@web.de 40a5d6c3b9 continued work on html/xmlparser
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#
#
# Nimrod's Runtime Library
# (c) Copyright 2009 Andreas Rumpf
#
# See the file "copying.txt", included in this
# distribution, for details about the copyright.
#
# Garbage Collector
#
# The basic algorithm is *Deferrent Reference Counting* with cycle detection.
# Special care has been taken to avoid recursion as far as possible to avoid
# stack overflows when traversing deep datastructures. This is comparable to
# an incremental and generational GC. It should be well-suited for soft real
# time applications (like games).
#
# Future Improvements:
# * Support for multi-threading. However, locks for the reference counting
# might turn out to be too slow.
const
CycleIncrease = 2 # is a multiplicative increase
InitialCycleThreshold = 4*1024*1024 # X MB because cycle checking is slow
ZctThreshold = 256 # we collect garbage if the ZCT's size
# reaches this threshold
# this seems to be a good value
const
rcIncrement = 0b1000 # so that lowest 3 bits are not touched
# NOTE: Most colors are currently unused
rcBlack = 0b000 # cell is colored black; in use or free
rcGray = 0b001 # possible member of a cycle
rcWhite = 0b010 # member of a garbage cycle
rcPurple = 0b011 # possible root of a cycle
rcZct = 0b100 # in ZCT
rcRed = 0b101 # Candidate cycle undergoing sigma-computation
rcOrange = 0b110 # Candidate cycle awaiting epoch boundary
rcShift = 3 # shift by rcShift to get the reference counter
colorMask = 0b111
type
TWalkOp = enum
waZctDecRef, waPush, waCycleDecRef
TFinalizer {.compilerproc.} = proc (self: pointer)
# A ref type can have a finalizer that is called before the object's
# storage is freed.
TGcStat {.final, pure.} = object
stackScans: int # number of performed stack scans (for statistics)
cycleCollections: int # number of performed full collections
maxThreshold: int # max threshold that has been set
maxStackSize: int # max stack size
maxStackCells: int # max stack cells in ``decStack``
cycleTableSize: int # max entries in cycle table
TGcHeap {.final, pure.} = object # this contains the zero count and
# non-zero count table
zct: TCellSeq # the zero count table
decStack: TCellSeq # cells in the stack that are to decref again
cycleRoots: TCellSet
tempStack: TCellSeq # temporary stack for recursion elimination
stat: TGcStat
var
stackBottom: pointer
gch: TGcHeap
cycleThreshold: int = InitialCycleThreshold
recGcLock: int = 0
# we use a lock to prevend the garbage collector to be triggered in a
# finalizer; the collector should not call itself this way! Thus every
# object allocated by a finalizer will not trigger a garbage collection.
# This is wasteful but safe. This is a lock against recursive garbage
# collection, not a lock for threads!
proc unsureAsgnRef(dest: ppointer, src: pointer) {.compilerproc.}
# unsureAsgnRef updates the reference counters only if dest is not on the
# stack. It is used by the code generator if it cannot decide wether a
# reference is in the stack or not (this can happen for var parameters).
#proc growObj(old: pointer, newsize: int): pointer {.compilerproc.}
proc newObj(typ: PNimType, size: int): pointer {.compilerproc.}
proc newSeq(typ: PNimType, len: int): pointer {.compilerproc.}
proc addZCT(s: var TCellSeq, c: PCell) {.noinline.} =
if (c.refcount and rcZct) == 0:
c.refcount = c.refcount and not colorMask or rcZct
add(s, c)
proc cellToUsr(cell: PCell): pointer {.inline.} =
# convert object (=pointer to refcount) to pointer to userdata
result = cast[pointer](cast[TAddress](cell)+%TAddress(sizeof(TCell)))
proc usrToCell(usr: pointer): PCell {.inline.} =
# convert pointer to userdata to object (=pointer to refcount)
result = cast[PCell](cast[TAddress](usr)-%TAddress(sizeof(TCell)))
proc canbeCycleRoot(c: PCell): bool {.inline.} =
result = ntfAcyclic notin c.typ.flags
proc extGetCellType(c: pointer): PNimType {.compilerproc.} =
# used for code generation concerning debugging
result = usrToCell(c).typ
proc internRefcount(p: pointer): int {.exportc: "getRefcount".} =
result = int(usrToCell(p).refcount) shr rcShift
proc GC_disable() = inc(recGcLock)
proc GC_enable() =
if recGcLock > 0: dec(recGcLock)
proc GC_setStrategy(strategy: TGC_Strategy) =
case strategy
of gcThroughput: nil
of gcResponsiveness: nil
of gcOptimizeSpace: nil
of gcOptimizeTime: nil
proc GC_enableMarkAndSweep() =
cycleThreshold = InitialCycleThreshold
proc GC_disableMarkAndSweep() =
cycleThreshold = high(cycleThreshold)-1
# set to the max value to suppress the cycle detector
# this that has to equals zero, otherwise we have to round up UnitsPerPage:
when BitsPerPage mod (sizeof(int)*8) != 0:
{.error: "(BitsPerPage mod BitsPerUnit) should be zero!".}
when debugGC:
proc writeCell(msg: CString, c: PCell) =
var kind = -1
if c.typ != nil: kind = ord(c.typ.kind)
when debugGC:
c_fprintf(c_stdout, "[GC] %s: %p %d rc=%ld from %s(%ld)\n",
msg, c, kind, c.refcount shr rcShift, c.filename, c.line)
else:
c_fprintf(c_stdout, "[GC] %s: %p %d rc=%ld\n",
msg, c, kind, c.refcount shr rcShift)
when traceGC:
# traceGC is a special switch to enable extensive debugging
type
TCellState = enum
csAllocated, csZctFreed, csCycFreed
var
states: array[TCellState, TCellSet]
proc traceCell(c: PCell, state: TCellState) =
case state
of csAllocated:
if c in states[csAllocated]:
writeCell("attempt to alloc an already allocated cell", c)
assert(false)
excl(states[csCycFreed], c)
excl(states[csZctFreed], c)
of csZctFreed:
if c in states[csZctFreed]:
writeCell("attempt to free zct cell twice", c)
assert(false)
if c in states[csCycFreed]:
writeCell("attempt to free with zct, but already freed with cyc", c)
assert(false)
if c notin states[csAllocated]:
writeCell("attempt to free not an allocated cell", c)
assert(false)
excl(states[csAllocated], c)
of csCycFreed:
if c notin states[csAllocated]:
writeCell("attempt to free a not allocated cell", c)
assert(false)
if c in states[csCycFreed]:
writeCell("attempt to free cyc cell twice", c)
assert(false)
if c in states[csZctFreed]:
writeCell("attempt to free with cyc, but already freed with zct", c)
assert(false)
excl(states[csAllocated], c)
incl(states[state], c)
proc writeLeakage() =
var z = 0
var y = 0
var e = 0
for c in elements(states[csAllocated]):
inc(e)
if c in states[csZctFreed]: inc(z)
elif c in states[csCycFreed]: inc(z)
else: writeCell("leak", c)
cfprintf(cstdout, "Allocations: %ld; ZCT freed: %ld; CYC freed: %ld\n",
e, z, y)
template gcTrace(cell, state: expr): stmt =
when traceGC: traceCell(cell, state)
# -----------------------------------------------------------------------------
# forward declarations:
proc collectCT(gch: var TGcHeap)
proc IsOnStack(p: pointer): bool {.noinline.}
proc forAllChildren(cell: PCell, op: TWalkOp)
proc doOperation(p: pointer, op: TWalkOp)
proc forAllChildrenAux(dest: Pointer, mt: PNimType, op: TWalkOp)
# we need the prototype here for debugging purposes
proc prepareDealloc(cell: PCell) =
if cell.typ.finalizer != nil:
# the finalizer could invoke something that
# allocates memory; this could trigger a garbage
# collection. Since we are already collecting we
# prevend recursive entering here by a lock.
# XXX: we should set the cell's children to nil!
inc(recGcLock)
(cast[TFinalizer](cell.typ.finalizer))(cellToUsr(cell))
dec(recGcLock)
proc setStackBottom(theStackBottom: pointer) {.compilerproc.} =
stackBottom = theStackBottom
proc PossibleRoot(gch: var TGcHeap, c: PCell) {.inline.} =
if canbeCycleRoot(c): incl(gch.cycleRoots, c)
proc decRef(c: PCell) {.inline.} =
when stressGC:
if c.refcount <% rcIncrement:
writeCell("broken cell", c)
assert(c.refcount >=% rcIncrement)
c.refcount = c.refcount -% rcIncrement
if c.refcount <% rcIncrement:
addZCT(gch.zct, c)
elif canBeCycleRoot(c):
incl(gch.cycleRoots, c)
proc incRef(c: PCell) {.inline.} =
c.refcount = c.refcount +% rcIncrement
if canBeCycleRoot(c):
incl(gch.cycleRoots, c)
proc nimGCref(p: pointer) {.compilerproc, inline.} = incRef(usrToCell(p))
proc nimGCunref(p: pointer) {.compilerproc, inline.} = decRef(usrToCell(p))
proc asgnRef(dest: ppointer, src: pointer) {.compilerproc, inline.} =
# the code generator calls this proc!
assert(not isOnStack(dest))
# BUGFIX: first incRef then decRef!
if src != nil: incRef(usrToCell(src))
if dest^ != nil: decRef(usrToCell(dest^))
dest^ = src
proc asgnRefNoCycle(dest: ppointer, src: pointer) {.compilerproc, inline.} =
# the code generator calls this proc if it is known at compile time that no
# cycle is possible.
if src != nil:
var c = usrToCell(src)
c.refcount = c.refcount +% rcIncrement
if dest^ != nil:
var c = usrToCell(dest^)
c.refcount = c.refcount -% rcIncrement
if c.refcount <% rcIncrement:
addZCT(gch.zct, c)
dest^ = src
proc unsureAsgnRef(dest: ppointer, src: pointer) =
if not IsOnStack(dest):
if src != nil: incRef(usrToCell(src))
if dest^ != nil: decRef(usrToCell(dest^))
dest^ = src
proc initGC() =
when traceGC:
for i in low(TCellState)..high(TCellState): Init(states[i])
gch.stat.stackScans = 0
gch.stat.cycleCollections = 0
gch.stat.maxThreshold = 0
gch.stat.maxStackSize = 0
gch.stat.maxStackCells = 0
gch.stat.cycleTableSize = 0
# init the rt
init(gch.zct)
init(gch.tempStack)
Init(gch.cycleRoots)
Init(gch.decStack)
new(gOutOfMem) # reserve space for the EOutOfMemory exception here!
proc forAllSlotsAux(dest: pointer, n: ptr TNimNode, op: TWalkOp) =
var d = cast[TAddress](dest)
case n.kind
of nkNone: assert(false)
of nkSlot: forAllChildrenAux(cast[pointer](d +% n.offset), n.typ, op)
of nkList:
for i in 0..n.len-1: forAllSlotsAux(dest, n.sons[i], op)
of nkCase:
var m = selectBranch(dest, n)
if m != nil: forAllSlotsAux(dest, m, op)
proc forAllChildrenAux(dest: Pointer, mt: PNimType, op: TWalkOp) =
var d = cast[TAddress](dest)
if dest == nil: return # nothing to do
if ntfNoRefs notin mt.flags:
case mt.Kind
of tyArray, tyArrayConstr, tyOpenArray:
for i in 0..(mt.size div mt.base.size)-1:
forAllChildrenAux(cast[pointer](d +% i *% mt.base.size), mt.base, op)
of tyRef, tyString, tySequence: # leaf:
doOperation(cast[ppointer](d)^, op)
of tyObject, tyTuple, tyPureObject:
forAllSlotsAux(dest, mt.node, op)
else: nil
proc forAllChildren(cell: PCell, op: TWalkOp) =
assert(cell != nil)
assert(cell.typ != nil)
case cell.typ.Kind
of tyRef: # common case
forAllChildrenAux(cellToUsr(cell), cell.typ.base, op)
of tySequence:
var d = cast[TAddress](cellToUsr(cell))
var s = cast[PGenericSeq](d)
if s != nil:
for i in 0..s.len-1:
forAllChildrenAux(cast[pointer](d +% i *% cell.typ.base.size +%
GenericSeqSize), cell.typ.base, op)
of tyString: nil
else: assert(false)
proc checkCollection {.inline.} =
# checks if a collection should be done
if recGcLock == 0:
collectCT(gch)
proc newObj(typ: PNimType, size: int): pointer =
# generates a new object and sets its reference counter to 0
assert(typ.kind in {tyRef, tyString, tySequence})
checkCollection()
var res = cast[PCell](rawAlloc(allocator, size + sizeof(TCell)))
zeroMem(res, size+sizeof(TCell))
assert((cast[TAddress](res) and (MemAlign-1)) == 0)
# now it is buffered in the ZCT
res.typ = typ
when debugGC:
if framePtr != nil and framePtr.prev != nil:
res.filename = framePtr.prev.filename
res.line = framePtr.prev.line
res.refcount = rcZct # refcount is zero, but mark it to be in the ZCT
assert(isAllocatedPtr(allocator, res))
# its refcount is zero, so add it to the ZCT:
block addToZCT:
# we check the last 8 entries (cache line) for a slot
# that could be reused
var L = gch.zct.len
var d = gch.zct.d
for i in countdown(L-1, max(0, L-8)):
var c = d[i]
if c.refcount >=% rcIncrement:
c.refcount = c.refcount and not colorMask
d[i] = res
break addToZCT
add(gch.zct, res)
when logGC: writeCell("new cell", res)
gcTrace(res, csAllocated)
result = cellToUsr(res)
proc newSeq(typ: PNimType, len: int): pointer =
result = newObj(typ, addInt(mulInt(len, typ.base.size), GenericSeqSize))
cast[PGenericSeq](result).len = len
cast[PGenericSeq](result).space = len
proc growObj(old: pointer, newsize: int): pointer =
checkCollection()
var ol = usrToCell(old)
assert(ol.typ != nil)
assert(ol.typ.kind in {tyString, tySequence})
var res = cast[PCell](rawAlloc(allocator, newsize + sizeof(TCell)))
var elemSize = 1
if ol.typ.kind != tyString:
elemSize = ol.typ.base.size
var oldsize = cast[PGenericSeq](old).len*elemSize + GenericSeqSize
copyMem(res, ol, oldsize + sizeof(TCell))
zeroMem(cast[pointer](cast[TAddress](res)+% oldsize +% sizeof(TCell)),
newsize-oldsize)
assert((cast[TAddress](res) and (MemAlign-1)) == 0)
assert(res.refcount shr rcShift <=% 1)
#if res.refcount <% rcIncrement:
# add(gch.zct, res)
#else: # XXX: what to do here?
# decRef(ol)
if (ol.refcount and colorMask) == rcZct:
var j = gch.zct.len-1
var d = gch.zct.d
while j >= 0:
if d[j] == ol:
d[j] = res
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: rawDealloc(allocator, ol)
else:
assert(ol.typ != nil)
zeroMem(ol, sizeof(TCell))
result = cellToUsr(res)
# ---------------- cycle collector -------------------------------------------
proc doOperation(p: pointer, op: TWalkOp) =
if p == nil: return
var c: PCell = usrToCell(p)
assert(c != nil)
case op # faster than function pointers because of easy prediction
of waZctDecRef:
assert(c.refcount >=% rcIncrement)
c.refcount = c.refcount -% rcIncrement
when logGC: writeCell("decref (from doOperation)", c)
if c.refcount <% rcIncrement: addZCT(gch.zct, c)
of waPush:
add(gch.tempStack, c)
of waCycleDecRef:
assert(c.refcount >=% rcIncrement)
c.refcount = c.refcount -% rcIncrement
# we now use a much simpler and non-recursive algorithm for cycle removal
proc collectCycles(gch: var TGcHeap) =
var tabSize = 0
for c in elements(gch.cycleRoots):
inc(tabSize)
forallChildren(c, waCycleDecRef)
gch.stat.cycleTableSize = max(gch.stat.cycleTableSize, tabSize)
# restore reference counts (a depth-first traversal is needed):
var marker: TCellSet
Init(marker)
for c in elements(gch.cycleRoots):
if c.refcount >=% rcIncrement:
if not containsOrIncl(marker, c):
gch.tempStack.len = 0
forAllChildren(c, waPush)
while gch.tempStack.len > 0:
dec(gch.tempStack.len)
var d = gch.tempStack.d[gch.tempStack.len]
d.refcount = d.refcount +% rcIncrement
if d in gch.cycleRoots and not containsOrIncl(marker, d):
forAllChildren(d, waPush)
# remove cycles:
for c in elements(gch.cycleRoots):
if c.refcount <% rcIncrement:
gch.tempStack.len = 0
forAllChildren(c, waPush)
while gch.tempStack.len > 0:
dec(gch.tempStack.len)
var d = gch.tempStack.d[gch.tempStack.len]
if d.refcount <% rcIncrement:
if d notin gch.cycleRoots: # d is leaf of c and not part of cycle
addZCT(gch.zct, d)
when logGC: writeCell("add to ZCT (from cycle collector)", d)
prepareDealloc(c)
gcTrace(c, csCycFreed)
when logGC: writeCell("cycle collector dealloc cell", c)
when reallyDealloc: rawDealloc(allocator, c)
else:
assert(c.typ != nil)
zeroMem(c, sizeof(TCell))
Deinit(gch.cycleRoots)
Init(gch.cycleRoots)
proc gcMark(p: pointer) {.inline.} =
# the addresses are not as cells on the stack, so turn them to cells:
var cell = usrToCell(p)
var c = cast[TAddress](cell)
if c >% PageSize and (c and (MemAlign-1)) == 0:
# fast check: does it look like a cell?
if isAllocatedPtr(allocator, cell):
# mark the cell:
cell.refcount = cell.refcount +% rcIncrement
add(gch.decStack, cell)
# ----------------- stack management --------------------------------------
# inspired from Smart Eiffel
proc stackSize(): int {.noinline.} =
var stackTop: array[0..1, pointer]
result = abs(cast[int](addr(stackTop[0])) - cast[int](stackBottom))
when defined(sparc): # For SPARC architecture.
proc isOnStack(p: pointer): bool =
var stackTop: array [0..1, pointer]
var b = cast[TAddress](stackBottom)
var a = cast[TAddress](addr(stackTop[0]))
var x = cast[TAddress](p)
result = x >=% a and x <=% b
proc markStackAndRegisters(gch: var TGcHeap) {.noinline, cdecl.} =
when defined(sparcv9):
asm """"flushw \n" """
else:
asm """"ta 0x3 ! ST_FLUSH_WINDOWS\n" """
var
max = stackBottom
sp: PPointer
stackTop: array[0..1, pointer]
sp = addr(stackTop[0])
# Addresses decrease as the stack grows.
while sp <= max:
gcMark(sp^)
sp = cast[ppointer](cast[TAddress](sp) +% sizeof(pointer))
elif defined(ELATE):
{.error: "stack marking code is to be written for this architecture".}
elif defined(hppa) or defined(hp9000) or defined(hp9000s300) or
defined(hp9000s700) or defined(hp9000s800) or defined(hp9000s820):
# ---------------------------------------------------------------------------
# Generic code for architectures where addresses increase as the stack grows.
# ---------------------------------------------------------------------------
proc isOnStack(p: pointer): bool =
var stackTop: array [0..1, pointer]
var a = cast[TAddress](stackBottom)
var b = cast[TAddress](addr(stackTop[0]))
var x = cast[TAddress](p)
result = x >=% a and x <=% b
var
jmpbufSize {.importc: "sizeof(jmp_buf)", nodecl.}: int
# a little hack to get the size of a TJmpBuf in the generated C code
# in a platform independant way
proc markStackAndRegisters(gch: var TGcHeap) {.noinline, cdecl.} =
var registers: C_JmpBuf
if c_setjmp(registers) == 0'i32: # To fill the C stack with registers.
var max = cast[TAddress](stackBottom)
var sp = cast[TAddress](addr(registers)) +% jmpbufSize -% sizeof(pointer)
# sp will traverse the JMP_BUF as well (jmp_buf size is added,
# otherwise sp would be below the registers structure).
while sp >=% max:
gcMark(cast[ppointer](sp)^)
sp = sp -% sizeof(pointer)
else:
# ---------------------------------------------------------------------------
# Generic code for architectures where addresses decrease as the stack grows.
# ---------------------------------------------------------------------------
proc isOnStack(p: pointer): bool =
var stackTop: array [0..1, pointer]
var b = cast[TAddress](stackBottom)
var a = cast[TAddress](addr(stackTop[0]))
var x = cast[TAddress](p)
result = x >=% a and x <=% b
proc markStackAndRegisters(gch: var TGcHeap) {.noinline, cdecl.} =
# We use a jmp_buf buffer that is in the C stack.
# Used to traverse the stack and registers assuming
# that 'setjmp' will save registers in the C stack.
var registers: C_JmpBuf
if c_setjmp(registers) == 0'i32: # To fill the C stack with registers.
var max = cast[TAddress](stackBottom)
var sp = cast[TAddress](addr(registers))
while sp <=% max:
gcMark(cast[ppointer](sp)^)
sp = sp +% sizeof(pointer)
# ----------------------------------------------------------------------------
# end of non-portable code
# ----------------------------------------------------------------------------
proc CollectZCT(gch: var TGcHeap) =
# Note: Freeing may add child objects to the ZCT! So essentially we do
# deep freeing, which is bad for incremental operation. In order to
# avoid a deep stack, we move objects to keep the ZCT small.
# This is performance critical!
var L = addr(gch.zct.len)
while L^ > 0:
var c = gch.zct.d[0]
# remove from ZCT:
assert((c.refcount and colorMask) == rcZct)
c.refcount = c.refcount and not colorMask
gch.zct.d[0] = gch.zct.d[L^ - 1]
dec(L^)
if c.refcount <% rcIncrement:
# It may have a RC > 0, if it is in the hardware stack or
# it has not been removed yet from the ZCT. This is because
# ``incref`` does not bother to remove the cell from the ZCT
# as this might be too slow.
# In any case, it should be removed from the ZCT. But not
# freed. **KEEP THIS IN MIND WHEN MAKING THIS INCREMENTAL!**
if canBeCycleRoot(c): excl(gch.cycleRoots, c)
when logGC: writeCell("zct dealloc cell", c)
gcTrace(c, csZctFreed)
# We are about to free the object, call the finalizer BEFORE its
# children are deleted as well, because otherwise the finalizer may
# access invalid memory. This is done by prepareDealloc():
prepareDealloc(c)
forAllChildren(c, waZctDecRef)
when reallyDealloc: rawDealloc(allocator, c)
else:
assert(c.typ != nil)
zeroMem(c, sizeof(TCell))
proc unmarkStackAndRegisters(gch: var TGcHeap) =
var d = gch.decStack.d
for i in 0..gch.decStack.len-1:
assert isAllocatedPtr(allocator, d[i])
decRef(d[i]) # OPT: cannot create a cycle!
gch.decStack.len = 0
proc collectCT(gch: var TGcHeap) =
if gch.zct.len >= ZctThreshold or (cycleGC and
getOccupiedMem() >= cycleThreshold) or stressGC:
gch.stat.maxStackSize = max(gch.stat.maxStackSize, stackSize())
assert(gch.decStack.len == 0)
markStackAndRegisters(gch)
gch.stat.maxStackCells = max(gch.stat.maxStackCells, gch.decStack.len)
inc(gch.stat.stackScans)
collectZCT(gch)
when cycleGC:
if getOccupiedMem() >= cycleThreshold or stressGC:
collectCycles(gch)
collectZCT(gch)
inc(gch.stat.cycleCollections)
cycleThreshold = max(InitialCycleThreshold, getOccupiedMem() *
cycleIncrease)
gch.stat.maxThreshold = max(gch.stat.maxThreshold, cycleThreshold)
unmarkStackAndRegisters(gch)
proc GC_fullCollect() =
var oldThreshold = cycleThreshold
cycleThreshold = 0 # forces cycle collection
collectCT(gch)
cycleThreshold = oldThreshold
proc GC_getStatistics(): string =
GC_disable()
result = "[GC] total memory: " & $(getTotalMem()) & "\n" &
"[GC] occupied memory: " & $(getOccupiedMem()) & "\n" &
"[GC] stack scans: " & $gch.stat.stackScans & "\n" &
"[GC] stack cells: " & $gch.stat.maxStackCells & "\n" &
"[GC] cycle collections: " & $gch.stat.cycleCollections & "\n" &
"[GC] max threshold: " & $gch.stat.maxThreshold & "\n" &
"[GC] zct capacity: " & $gch.zct.cap & "\n" &
"[GC] max cycle table size: " & $gch.stat.cycleTableSize & "\n" &
"[GC] max stack size: " & $gch.stat.maxStackSize
when traceGC: writeLeakage()
GC_enable()
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