#
#
# Nim's Runtime Library
# (c) Copyright 2013 Andreas Rumpf
#
# See the file "copying.txt", included in this
# distribution, for details about the copyright.
#
#[
Efficient set of pointers for the GC (and repr)
-----------------------------------------------
The GC depends on an extremely efficient datastructure for storing a
set of pointers - this is called a `CellSet` in the source code.
Inserting, deleting and searching are done in constant time. However,
modifying a `CellSet` during traversal leads to undefined behaviour.
All operations on a CellSet have to perform efficiently. Because a Cellset can
become huge a hash table alone is not suitable for this.
We use a mixture of bitset and hash table for this. The hash table maps *pages*
to a page descriptor. The page descriptor contains a bit for any possible cell
address within this page. So including a cell is done as follows:
- Find the page descriptor for the page the cell belongs to.
- Set the appropriate bit in the page descriptor indicating that the
cell points to the start of a memory block.
Removing a cell is analogous - the bit has to be set to zero.
Single page descriptors are never deleted from the hash table. This is not
needed as the data structures needs to be rebuilt periodically anyway.
Complete traversal is done in this way::
for each page descriptor d:
for each bit in d:
if bit == 1:
traverse the pointer belonging to this bit
]#
when defined(gcOrc) or defined(gcArc):
type
PCell = Cell
when not declaredInScope(PageShift):
include bitmasks
else:
type
RefCount = int
Cell {.pure.} = object
refcount: RefCount # the refcount and some flags
typ: PNimType
when trackAllocationSource:
filename: cstring
line: int
when useCellIds:
id: int
PCell = ptr Cell
type
PPageDesc = ptr PageDesc
BitIndex = range[0..UnitsPerPage-1]
PageDesc {.final, pure.} = object
next: PPageDesc # all nodes are connected with this pointer
key: uint # start address at bit 0
bits: array[BitIndex, int] # a bit vector
PPageDescArray = ptr UncheckedArray[PPageDesc]
CellSet {.final, pure.} = object
counter, max: int
head: PPageDesc
data: PPageDescArray
when defined(gcOrc) or defined(gcArc):
discard
else:
include cellseqs_v1
# ------------------- cell set handling ---------------------------------------
const
InitCellSetSize = 1024 # must be a power of two!
proc init(s: var CellSet) =
s.data = cast[PPageDescArray](alloc0(InitCellSetSize * sizeof(PPageDesc)))
s.max = InitCellSetSize-1
s.counter = 0
s.head = nil
proc deinit(s: var CellSet) =
var it = s.head
while it != nil:
var n = it.next
dealloc(it)
it = n
s.head = nil # play it safe here
dealloc(s.data)
s.data = nil
s.counter = 0
proc nextTry(h, maxHash: int): int {.inline.} =
result = ((5*h) + 1) and maxHash
# For any initial h in range(maxHash), repeating that maxHash times
# generates each int in range(maxHash) exactly once (see any text on
# random-number generation for proof).
proc cellSetGet(t: CellSet, key: uint): PPageDesc =
var h = cast[int](key) and t.max
while t.data[h] != nil:
if t.data[h].key == key: return t.data[h]
h = nextTry(h, t.max)
return nil
proc cellSetRawInsert(t: CellSet, data: PPageDescArray, desc: PPageDesc) =
var h = cast[int](desc.key) and t.max
while data[h] != nil:
sysAssert(data[h] != desc, "CellSetRawInsert 1")
h = nextTry(h, t.max)
sysAssert(data[h] == nil, "CellSetRawInsert 2")
data[h] = desc
proc cellSetEnlarge(t: var CellSet) =
var oldMax = t.max
t.max = ((t.max+1)*2)-1
var n = cast[PPageDescArray](alloc0((t.max + 1) * sizeof(PPageDesc)))
for i in 0 .. oldMax:
if t.data[i] != nil:
cellSetRawInsert(t, n, t.data[i])
dealloc(t.data)
t.data = n
proc cellSetPut(t: var CellSet, key: uint): PPageDesc =
var h = cast[int](key) and t.max
while true:
var x = t.data[h]
if x == nil: break
if x.key == key: return x
h = nextTry(h, t.max)
if ((t.max+1)*2 < t.counter*3) or ((t.max+1)-t.counter < 4):
cellSetEnlarge(t)
inc(t.counter)
h = cast[int](key) and t.max
while t.data[h] != nil: h = nextTry(h, t.max)
sysAssert(t.data[h] == nil, "CellSetPut")
# the new page descriptor goes into result
result = cast[PPageDesc](alloc0(sizeof(PageDesc)))
result.next = t.head
result.key = key
t.head = result
t.data[h] = result
# ---------- slightly higher level procs --------------------------------------
proc contains(s: CellSet, cell: PCell): bool =
var u = cast[uint](cell)
var t = cellSetGet(s, u shr PageShift)
if t != nil:
u = (u mod PageSize) div MemAlign
result = (t.bits[u shr IntShift] and (1 shl (u and IntMask))) != 0
else:
result = false
proc incl(s: var CellSet, cell: PCell) =
var u = cast[uint](cell)
var t = cellSetPut(s, u shr PageShift)
u = (u mod PageSize) div MemAlign
t.bits[u shr IntShift] = t.bits[u shr IntShift] or (1 shl (u and IntMask))
proc excl(s: var CellSet, cell: PCell) =
var u = cast[uint](cell)
var t = cellSetGet(s, u shr PageShift)
if t != nil:
u = (u mod PageSize) div MemAlign
t.bits[u shr IntShift] = (t.bits[u shr IntShift] and
not (1 shl (u and IntMask)))
proc containsOrIncl(s: var CellSet, cell: PCell): bool =
var u = cast[uint](cell)
var t = cellSetGet(s, u shr PageShift)
if t != nil:
u = (u mod PageSize) div MemAlign
result = (t.bits[u shr IntShift] and (1 shl (u and IntMask))) != 0
if not result:
t.bits[u shr IntShift] = t.bits[u shr IntShift] or
(1 shl (u and IntMask))
else:
incl(s, cell)
result = false
iterator elements(t: CellSet): PCell {.inline.} =
# while traversing it is forbidden to add pointers to the tree!
var r = t.head
while r != nil:
var i: uint = 0
while int(i) <= high(r.bits):
var w = r.bits[i] # taking a copy of r.bits[i] here is correct, because
# modifying operations are not allowed during traversation
var j: uint = 0
while w != 0: # test all remaining bits for zero
if (w and 1) != 0: # the bit is set!
yield cast[PCell]((r.key shl PageShift) or
(i shl IntShift + j) * MemAlign)
inc(j)
w = w shr 1
inc(i)
r = r.next
when false:
type
CellSetIter = object
p: PPageDesc
i, w, j: int
proc next(it: var CellSetIter): PCell =
while true:
while it.w != 0: # test all remaining bits for zero
if (it.w and 1) != 0: # the bit is set!
result = cast[PCell]((it.p.key shl PageShift) or
(it.i shl IntShift +% it.j) *% MemAlign)
inc(it.j)
it.w = it.w shr 1
return
else:
inc(it.j)
it.w = it.w shr 1
# load next w:
if it.i >= high(it.p.bits):
it.i = 0
it.j = 0
it.p = it.p.next
if it.p == nil: return nil
else:
inc it.i
it.w = it.p.bits[i]
proc init(it: var CellSetIter; t: CellSet): PCell =
it.p = t.head
it.i = -1
it.w = 0
result = it.next
iterator elementsExcept(t, s: CellSet): PCell {.inline.} =
var r = t.head
while r != nil:
let ss = cellSetGet(s, r.key)
var i:uint = 0
while int(i) <= high(r.bits):
var w = r.bits[i]
if ss != nil:
w = w and not ss.bits[i]
var j:uint = 0
while w != 0:
if (w and 1) != 0:
yield cast[PCell]((r.key shl PageShift) or
(i shl IntShift + j) * MemAlign)
inc(j)
w = w shr 1
inc(i)
r = r.next