/rpython/memory/gc/minimark.py

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""" MiniMark GC.

Environment variables can be used to fine-tune the following parameters:

 PYPY_GC_NURSERY         The nursery size.  Defaults to 1/2 of your cache or
                         '4M'.  Small values
                         (like 1 or 1KB) are useful for debugging.

 PYPY_GC_NURSERY_CLEANUP The interval at which nursery is cleaned up. Must
                         be smaller than the nursery size and bigger than the
                         biggest object we can allotate in the nursery.

 PYPY_GC_MAJOR_COLLECT   Major collection memory factor.  Default is '1.82',
                         which means trigger a major collection when the
                         memory consumed equals 1.82 times the memory
                         really used at the end of the previous major
                         collection.

 PYPY_GC_GROWTH          Major collection threshold's max growth rate.
                         Default is '1.4'.  Useful to collect more often
                         than normally on sudden memory growth, e.g. when
                         there is a temporary peak in memory usage.

 PYPY_GC_MAX             The max heap size.  If coming near this limit, it
                         will first collect more often, then raise an
                         RPython MemoryError, and if that is not enough,
                         crash the program with a fatal error.  Try values
                         like '1.6GB'.

 PYPY_GC_MAX_DELTA       The major collection threshold will never be set
                         to more than PYPY_GC_MAX_DELTA the amount really
                         used after a collection.  Defaults to 1/8th of the
                         total RAM size (which is constrained to be at most
                         2/3/4GB on 32-bit systems).  Try values like '200MB'.

 PYPY_GC_MIN             Don't collect while the memory size is below this
                         limit.  Useful to avoid spending all the time in
                         the GC in very small programs.  Defaults to 8
                         times the nursery.

 PYPY_GC_DEBUG           Enable extra checks around collections that are
                         too slow for normal use.  Values are 0 (off),
                         1 (on major collections) or 2 (also on minor
                         collections).
"""
# XXX Should find a way to bound the major collection threshold by the
# XXX total addressable size.  Maybe by keeping some minimarkpage arenas
# XXX pre-reserved, enough for a few nursery collections?  What about
# XXX raw-malloced memory?
import sys
from rpython.rtyper.lltypesystem import lltype, llmemory, llarena, llgroup
from rpython.rtyper.lltypesystem.lloperation import llop
from rpython.rtyper.lltypesystem.llmemory import raw_malloc_usage
from rpython.memory.gc.base import GCBase, MovingGCBase
from rpython.memory.gc import env
from rpython.memory.support import mangle_hash
from rpython.rlib.rarithmetic import ovfcheck, LONG_BIT, intmask, r_uint
from rpython.rlib.rarithmetic import LONG_BIT_SHIFT
from rpython.rlib.debug import ll_assert, debug_print, debug_start, debug_stop
from rpython.rlib.objectmodel import specialize


#
# Handles the objects in 2 generations:
#
#  * young objects: allocated in the nursery if they are not too large, or
#    raw-malloced otherwise.  The nursery is a fixed-size memory buffer of
#    4MB by default.  When full, we do a minor collection;
#    the surviving objects from the nursery are moved outside, and the
#    non-surviving raw-malloced objects are freed.  All surviving objects
#    become old.
#
#  * old objects: never move again.  These objects are either allocated by
#    minimarkpage.py (if they are small), or raw-malloced (if they are not
#    small).  Collected by regular mark-n-sweep during major collections.
#

WORD = LONG_BIT // 8
NULL = llmemory.NULL

first_gcflag = 1 << (LONG_BIT//2)

# The following flag is set on objects if we need to do something to
# track the young pointers that it might contain.  The flag is not set
# on young objects (unless they are large arrays, see below), and we
# simply assume that any young object can point to any other young object.
# For old and prebuilt objects, the flag is usually set, and is cleared
# when we write a young pointer to it.  For large arrays with
# GCFLAG_HAS_CARDS, we rely on card marking to track where the
# young pointers are; the flag GCFLAG_TRACK_YOUNG_PTRS is set in this
# case too, to speed up the write barrier.
GCFLAG_TRACK_YOUNG_PTRS = first_gcflag << 0

# The following flag is set on some prebuilt objects.  The flag is set
# unless the object is already listed in 'prebuilt_root_objects'.
# When a pointer is written inside an object with GCFLAG_NO_HEAP_PTRS
# set, the write_barrier clears the flag and adds the object to
# 'prebuilt_root_objects'.
GCFLAG_NO_HEAP_PTRS = first_gcflag << 1

# The following flag is set on surviving objects during a major collection,
# and on surviving raw-malloced young objects during a minor collection.
GCFLAG_VISITED      = first_gcflag << 2

# The following flag is set on nursery objects of which we asked the id
# or the identityhash.  It means that a space of the size of the object
# has already been allocated in the nonmovable part.  The same flag is
# abused to mark prebuilt objects whose hash has been taken during
# translation and is statically recorded.
GCFLAG_HAS_SHADOW   = first_gcflag << 3

# The following flag is set temporarily on some objects during a major
# collection.  See pypy/doc/discussion/finalizer-order.txt
GCFLAG_FINALIZATION_ORDERING = first_gcflag << 4

# This flag is reserved for RPython.
GCFLAG_EXTRA        = first_gcflag << 5

# The following flag is set on externally raw_malloc'ed arrays of pointers.
# They are allocated with some extra space in front of them for a bitfield,
# one bit per 'card_page_indices' indices.
GCFLAG_HAS_CARDS    = first_gcflag << 6
GCFLAG_CARDS_SET    = first_gcflag << 7     # <- at least one card bit is set
# note that GCFLAG_CARDS_SET is the most significant bit of a byte:
# this is required for the JIT (x86)

_GCFLAG_FIRST_UNUSED = first_gcflag << 8    # the first unused bit


FORWARDSTUB = lltype.GcStruct('forwarding_stub',
                              ('forw', llmemory.Address))
FORWARDSTUBPTR = lltype.Ptr(FORWARDSTUB)
NURSARRAY = lltype.Array(llmemory.Address)

# ____________________________________________________________

class MiniMarkGC(MovingGCBase):
    _alloc_flavor_ = "raw"
    inline_simple_malloc = True
    inline_simple_malloc_varsize = True
    needs_write_barrier = True
    prebuilt_gc_objects_are_static_roots = False
    malloc_zero_filled = True    # xxx experiment with False
    gcflag_extra = GCFLAG_EXTRA

    # All objects start with a HDR, i.e. with a field 'tid' which contains
    # a word.  This word is divided in two halves: the lower half contains
    # the typeid, and the upper half contains various flags, as defined
    # by GCFLAG_xxx above.
    HDR = lltype.Struct('header', ('tid', lltype.Signed))
    typeid_is_in_field = 'tid'
    withhash_flag_is_in_field = 'tid', GCFLAG_HAS_SHADOW
    # ^^^ prebuilt objects may have the flag GCFLAG_HAS_SHADOW;
    #     then they are one word longer, the extra word storing the hash.

    _ADDRARRAY = lltype.Array(llmemory.Address, hints={'nolength': True})


    # During a minor collection, the objects in the nursery that are
    # moved outside are changed in-place: their header is replaced with
    # the value -42, and the following word is set to the address of
    # where the object was moved.  This means that all objects in the
    # nursery need to be at least 2 words long, but objects outside the
    # nursery don't need to.
    minimal_size_in_nursery = (
        llmemory.sizeof(HDR) + llmemory.sizeof(llmemory.Address))


    TRANSLATION_PARAMS = {
        # Automatically adjust the size of the nursery and the
        # 'major_collection_threshold' from the environment.
        # See docstring at the start of the file.
        "read_from_env": True,

        # The size of the nursery.  Note that this is only used as a
        # fall-back number.
        "nursery_size": 896*1024,

        # The system page size.  Like malloc, we assume that it is 4K
        # for 32-bit systems; unlike malloc, we assume that it is 8K
        # for 64-bit systems, for consistent results.
        "page_size": 1024*WORD,

        # The size of an arena.  Arenas are groups of pages allocated
        # together.
        "arena_size": 65536*WORD,

        # The maximum size of an object allocated compactly.  All objects
        # that are larger are just allocated with raw_malloc().  Note that
        # the size limit for being first allocated in the nursery is much
        # larger; see below.
        "small_request_threshold": 35*WORD,

        # Full collection threshold: after a major collection, we record
        # the total size consumed; and after every minor collection, if the
        # total size is now more than 'major_collection_threshold' times,
        # we trigger the next major collection.
        "major_collection_threshold": 1.82,

        # Threshold to avoid that the total heap size grows by a factor of
        # major_collection_threshold at every collection: it can only
        # grow at most by the following factor from one collection to the
        # next.  Used e.g. when there is a sudden, temporary peak in memory
        # usage; this avoids that the upper bound grows too fast.
        "growth_rate_max": 1.4,

        # The number of array indices that are mapped to a single bit in
        # write_barrier_from_array().  Must be a power of two.  The default
        # value of 128 means that card pages are 512 bytes (1024 on 64-bits)
        # in regular arrays of pointers; more in arrays whose items are
        # larger.  A value of 0 disables card marking.
        "card_page_indices": 128,

        # Objects whose total size is at least 'large_object' bytes are
        # allocated out of the nursery immediately, as old objects.  The
        # minimal allocated size of the nursery is 2x the following
        # number (by default, at least 132KB on 32-bit and 264KB on 64-bit).
        "large_object": (16384+512)*WORD,

        # This is the chunk that we cleanup in the nursery. The point is
        # to avoid having to trash all the caches just to zero the nursery,
        # so we trade it by cleaning it bit-by-bit, as we progress through
        # nursery. Has to fit at least one large object
        "nursery_cleanup": 32768 * WORD,
        }

    def __init__(self, config,
                 read_from_env=False,
                 nursery_size=32*WORD,
                 nursery_cleanup=9*WORD,
                 page_size=16*WORD,
                 arena_size=64*WORD,
                 small_request_threshold=5*WORD,
                 major_collection_threshold=2.5,
                 growth_rate_max=2.5,   # for tests
                 card_page_indices=0,
                 large_object=8*WORD,
                 ArenaCollectionClass=None,
                 **kwds):
        MovingGCBase.__init__(self, config, **kwds)
        assert small_request_threshold % WORD == 0
        self.read_from_env = read_from_env
        self.nursery_size = nursery_size
        self.nursery_cleanup = nursery_cleanup
        self.small_request_threshold = small_request_threshold
        self.major_collection_threshold = major_collection_threshold
        self.growth_rate_max = growth_rate_max
        self.num_major_collects = 0
        self.min_heap_size = 0.0
        self.max_heap_size = 0.0
        self.max_heap_size_already_raised = False
        self.max_delta = float(r_uint(-1))
        #
        self.card_page_indices = card_page_indices
        if self.card_page_indices > 0:
            self.card_page_shift = 0
            while (1 << self.card_page_shift) < self.card_page_indices:
                self.card_page_shift += 1
        #
        # 'large_object' limit how big objects can be in the nursery, so
        # it gives a lower bound on the allowed size of the nursery.
        self.nonlarge_max = large_object - 1
        #
        self.nursery      = NULL
        self.nursery_free = NULL
        self.nursery_top  = NULL
        self.nursery_real_top = NULL
        self.debug_tiny_nursery = -1
        self.debug_rotating_nurseries = lltype.nullptr(NURSARRAY)
        self.extra_threshold = 0
        #
        # The ArenaCollection() handles the nonmovable objects allocation.
        if ArenaCollectionClass is None:
            from rpython.memory.gc import minimarkpage
            ArenaCollectionClass = minimarkpage.ArenaCollection
        self.ac = ArenaCollectionClass(arena_size, page_size,
                                       small_request_threshold)
        #
        # Used by minor collection: a list of (mostly non-young) objects that
        # (may) contain a pointer to a young object.  Populated by
        # the write barrier: when we clear GCFLAG_TRACK_YOUNG_PTRS, we
        # add it to this list.
        # Note that young array objects may (by temporary "mistake") be added
        # to this list, but will be removed again at the start of the next
        # minor collection.
        self.old_objects_pointing_to_young = self.AddressStack()
        #
        # Similar to 'old_objects_pointing_to_young', but lists objects
        # that have the GCFLAG_CARDS_SET bit.  For large arrays.  Note
        # that it is possible for an object to be listed both in here
        # and in 'old_objects_pointing_to_young', in which case we
        # should just clear the cards and trace it fully, as usual.
        # Note also that young array objects are never listed here.
        self.old_objects_with_cards_set = self.AddressStack()
        #
        # A list of all prebuilt GC objects that contain pointers to the heap
        self.prebuilt_root_objects = self.AddressStack()
        #
        self._init_writebarrier_logic()


    def setup(self):
        """Called at run-time to initialize the GC."""
        #
        # Hack: MovingGCBase.setup() sets up stuff related to id(), which
        # we implement differently anyway.  So directly call GCBase.setup().
        GCBase.setup(self)
        #
        # Two lists of all raw_malloced objects (the objects too large)
        self.young_rawmalloced_objects = self.null_address_dict()
        self.old_rawmalloced_objects = self.AddressStack()
        self.rawmalloced_total_size = r_uint(0)
        #
        # Two lists of all objects with finalizers.  Actually they are lists
        # of pairs (finalization_queue_nr, object).  "probably young objects"
        # are all traced and moved to the "old" list by the next minor
        # collection.
        self.probably_young_objects_with_finalizers = self.AddressDeque()
        self.old_objects_with_finalizers = self.AddressDeque()
        p = lltype.malloc(self._ADDRARRAY, 1, flavor='raw',
                          track_allocation=False)
        self.singleaddr = llmemory.cast_ptr_to_adr(p)
        #
        # Two lists of all objects with destructors.
        self.young_objects_with_destructors = self.AddressStack()
        self.old_objects_with_destructors = self.AddressStack()
        #
        # Two lists of the objects with weakrefs.  No weakref can be an
        # old object weakly pointing to a young object: indeed, weakrefs
        # are immutable so they cannot point to an object that was
        # created after it.
        self.young_objects_with_weakrefs = self.AddressStack()
        self.old_objects_with_weakrefs = self.AddressStack()
        #
        # Support for id and identityhash: map nursery objects with
        # GCFLAG_HAS_SHADOW to their future location at the next
        # minor collection.
        self.nursery_objects_shadows = self.AddressDict()
        #
        # Allocate a nursery.  In case of auto_nursery_size, start by
        # allocating a very small nursery, enough to do things like look
        # up the env var, which requires the GC; and then really
        # allocate the nursery of the final size.
        if not self.read_from_env:
            self.allocate_nursery()
        else:
            #
            defaultsize = self.nursery_size
            minsize = 2 * (self.nonlarge_max + 1)
            self.nursery_size = minsize
            self.allocate_nursery()
            #
            # From there on, the GC is fully initialized and the code
            # below can use it
            newsize = env.read_from_env('PYPY_GC_NURSERY')
            # PYPY_GC_NURSERY=smallvalue means that minor collects occur
            # very frequently; the extreme case is PYPY_GC_NURSERY=1, which
            # forces a minor collect for every malloc.  Useful to debug
            # external factors, like trackgcroot or the handling of the write
            # barrier.  Implemented by still using 'minsize' for the nursery
            # size (needed to handle mallocs just below 'large_objects') but
            # hacking at the current nursery position in collect_and_reserve().
            if newsize <= 0:
                newsize = env.estimate_best_nursery_size()
                if newsize <= 0:
                    newsize = defaultsize
            if newsize < minsize:
                self.debug_tiny_nursery = newsize & ~(WORD-1)
                newsize = minsize

            nurs_cleanup = env.read_from_env('PYPY_GC_NURSERY_CLEANUP')
            if nurs_cleanup > 0:
                self.nursery_cleanup = nurs_cleanup
            #
            major_coll = env.read_float_from_env('PYPY_GC_MAJOR_COLLECT')
            if major_coll > 1.0:
                self.major_collection_threshold = major_coll
            #
            growth = env.read_float_from_env('PYPY_GC_GROWTH')
            if growth > 1.0:
                self.growth_rate_max = growth
            #
            min_heap_size = env.read_uint_from_env('PYPY_GC_MIN')
            if min_heap_size > 0:
                self.min_heap_size = float(min_heap_size)
            else:
                # defaults to 8 times the nursery
                self.min_heap_size = newsize * 8
            #
            max_heap_size = env.read_uint_from_env('PYPY_GC_MAX')
            if max_heap_size > 0:
                self.max_heap_size = float(max_heap_size)
            #
            max_delta = env.read_uint_from_env('PYPY_GC_MAX_DELTA')
            if max_delta > 0:
                self.max_delta = float(max_delta)
            else:
                self.max_delta = 0.125 * env.get_total_memory()
            #
            self.minor_collection()    # to empty the nursery
            llarena.arena_free(self.nursery)
            self.nursery_size = newsize
            self.allocate_nursery()
        #
        if self.nursery_cleanup < self.nonlarge_max + 1:
            self.nursery_cleanup = self.nonlarge_max + 1
        # We need exactly initial_cleanup + N*nursery_cleanup = nursery_size.
        # We choose the value of initial_cleanup to be between 1x and 2x the
        # value of nursery_cleanup.
        self.initial_cleanup = self.nursery_cleanup + (
                self.nursery_size % self.nursery_cleanup)
        if (r_uint(self.initial_cleanup) > r_uint(self.nursery_size) or
            self.debug_tiny_nursery >= 0):
            self.initial_cleanup = self.nursery_size

    def _nursery_memory_size(self):
        extra = self.nonlarge_max + 1
        return self.nursery_size + extra

    def _alloc_nursery(self):
        # the start of the nursery: we actually allocate a bit more for
        # the nursery than really needed, to simplify pointer arithmetic
        # in malloc_fixedsize_clear().  The few extra pages are never used
        # anyway so it doesn't even count.
        nursery = llarena.arena_malloc(self._nursery_memory_size(), 2)
        if not nursery:
            raise MemoryError("cannot allocate nursery")
        return nursery

    def allocate_nursery(self):
        debug_start("gc-set-nursery-size")
        debug_print("nursery size:", self.nursery_size)
        self.nursery = self._alloc_nursery()
        # the current position in the nursery:
        self.nursery_free = self.nursery
        # the end of the nursery:
        self.nursery_top = self.nursery + self.nursery_size
        self.nursery_real_top = self.nursery_top
        # initialize the threshold
        self.min_heap_size = max(self.min_heap_size, self.nursery_size *
                                              self.major_collection_threshold)
        # the following two values are usually equal, but during raw mallocs
        # of arrays, next_major_collection_threshold is decremented to make
        # the next major collection arrive earlier.
        # See translator/c/test/test_newgc, test_nongc_attached_to_gc
        self.next_major_collection_initial = self.min_heap_size
        self.next_major_collection_threshold = self.min_heap_size
        self.set_major_threshold_from(0.0)
        ll_assert(self.extra_threshold == 0, "extra_threshold set too early")
        self.initial_cleanup = self.nursery_size
        debug_stop("gc-set-nursery-size")


    def set_major_threshold_from(self, threshold, reserving_size=0):
        # Set the next_major_collection_threshold.
        threshold_max = (self.next_major_collection_initial *
                         self.growth_rate_max)
        if threshold > threshold_max:
            threshold = threshold_max
        #
        threshold += reserving_size
        if threshold < self.min_heap_size:
            threshold = self.min_heap_size
        #
        if self.max_heap_size > 0.0 and threshold > self.max_heap_size:
            threshold = self.max_heap_size
            bounded = True
        else:
            bounded = False
        #
        self.next_major_collection_initial = threshold
        self.next_major_collection_threshold = threshold
        return bounded


    def post_setup(self):
        # set up extra stuff for PYPY_GC_DEBUG.
        MovingGCBase.post_setup(self)
        if self.DEBUG and llarena.has_protect:
            # gc debug mode: allocate 23 nurseries instead of just 1,
            # and use them alternatively, while mprotect()ing the unused
            # ones to detect invalid access.
            debug_start("gc-debug")
            self.debug_rotating_nurseries = lltype.malloc(
                NURSARRAY, 22, flavor='raw', track_allocation=False)
            i = 0
            while i < 22:
                nurs = self._alloc_nursery()
                llarena.arena_protect(nurs, self._nursery_memory_size(), True)
                self.debug_rotating_nurseries[i] = nurs
                i += 1
            debug_print("allocated", len(self.debug_rotating_nurseries),
                        "extra nurseries")
            debug_stop("gc-debug")

    def debug_rotate_nursery(self):
        if self.debug_rotating_nurseries:
            debug_start("gc-debug")
            oldnurs = self.nursery
            llarena.arena_protect(oldnurs, self._nursery_memory_size(), True)
            #
            newnurs = self.debug_rotating_nurseries[0]
            i = 0
            while i < len(self.debug_rotating_nurseries) - 1:
                self.debug_rotating_nurseries[i] = (
                    self.debug_rotating_nurseries[i + 1])
                i += 1
            self.debug_rotating_nurseries[i] = oldnurs
            #
            llarena.arena_protect(newnurs, self._nursery_memory_size(), False)
            self.nursery = newnurs
            self.nursery_top = self.nursery + self.initial_cleanup
            self.nursery_real_top = self.nursery + self.nursery_size
            debug_print("switching from nursery", oldnurs,
                        "to nursery", self.nursery,
                        "size", self.nursery_size)
            debug_stop("gc-debug")


    def malloc_fixedsize_clear(self, typeid, size,
                               needs_finalizer=False,
                               is_finalizer_light=False,
                               contains_weakptr=False):
        size_gc_header = self.gcheaderbuilder.size_gc_header
        totalsize = size_gc_header + size
        rawtotalsize = raw_malloc_usage(totalsize)
        #
        # If the object needs a finalizer, ask for a rawmalloc.
        # The following check should be constant-folded.
        if needs_finalizer and not is_finalizer_light:
            # old-style finalizers only!
            ll_assert(not contains_weakptr,
                     "'needs_finalizer' and 'contains_weakptr' both specified")
            obj = self.external_malloc(typeid, 0, alloc_young=False)
            res = llmemory.cast_adr_to_ptr(obj, llmemory.GCREF)
            self.register_finalizer(-1, res)
            return res
        #
        # If totalsize is greater than nonlarge_max (which should never be
        # the case in practice), ask for a rawmalloc.  The following check
        # should be constant-folded.
        if rawtotalsize > self.nonlarge_max:
            ll_assert(not contains_weakptr,
                      "'contains_weakptr' specified for a large object")
            obj = self.external_malloc(typeid, 0, alloc_young=True)
            #
        else:
            # If totalsize is smaller than minimal_size_in_nursery, round it
            # up.  The following check should also be constant-folded.
            min_size = raw_malloc_usage(self.minimal_size_in_nursery)
            if rawtotalsize < min_size:
                totalsize = rawtotalsize = min_size
            #
            # Get the memory from the nursery.  If there is not enough space
            # there, do a collect first.
            result = self.nursery_free
            self.nursery_free = result + totalsize
            if self.nursery_free > self.nursery_top:
                result = self.collect_and_reserve(result, totalsize)
            #
            # Build the object.
            llarena.arena_reserve(result, totalsize)
            obj = result + size_gc_header
            self.init_gc_object(result, typeid, flags=0)
        #
        # If it is a weakref or has a lightweight destructor, record it
        # (checks constant-folded).
        if needs_finalizer:
            self.young_objects_with_destructors.append(obj)
        if contains_weakptr:
            self.young_objects_with_weakrefs.append(obj)
        return llmemory.cast_adr_to_ptr(obj, llmemory.GCREF)


    def malloc_varsize_clear(self, typeid, length, size, itemsize,
                             offset_to_length):
        size_gc_header = self.gcheaderbuilder.size_gc_header
        nonvarsize = size_gc_header + size
        #
        # Compute the maximal length that makes the object still
        # below 'nonlarge_max'.  All the following logic is usually
        # constant-folded because self.nonlarge_max, size and itemsize
        # are all constants (the arguments are constant due to
        # inlining).
        maxsize = self.nonlarge_max - raw_malloc_usage(nonvarsize)
        if maxsize < 0:
            toobig = r_uint(0)    # the nonvarsize alone is too big
        elif raw_malloc_usage(itemsize):
            toobig = r_uint(maxsize // raw_malloc_usage(itemsize)) + 1
        else:
            toobig = r_uint(sys.maxint) + 1

        if r_uint(length) >= r_uint(toobig):
            #
            # If the total size of the object would be larger than
            # 'nonlarge_max', then allocate it externally.  We also
            # go there if 'length' is actually negative.
            obj = self.external_malloc(typeid, length, alloc_young=True)
            #
        else:
            # With the above checks we know now that totalsize cannot be more
            # than 'nonlarge_max'; in particular, the + and * cannot overflow.
            totalsize = nonvarsize + itemsize * length
            totalsize = llarena.round_up_for_allocation(totalsize)
            #
            # 'totalsize' should contain at least the GC header and
            # the length word, so it should never be smaller than
            # 'minimal_size_in_nursery'
            ll_assert(raw_malloc_usage(totalsize) >=
                      raw_malloc_usage(self.minimal_size_in_nursery),
                      "malloc_varsize_clear(): totalsize < minimalsize")
            #
            # Get the memory from the nursery.  If there is not enough space
            # there, do a collect first.
            result = self.nursery_free
            self.nursery_free = result + totalsize
            if self.nursery_free > self.nursery_top:
                result = self.collect_and_reserve(result, totalsize)
            #
            # Build the object.
            llarena.arena_reserve(result, totalsize)
            self.init_gc_object(result, typeid, flags=0)
            #
            # Set the length and return the object.
            obj = result + size_gc_header
            (obj + offset_to_length).signed[0] = length
        #
        return llmemory.cast_adr_to_ptr(obj, llmemory.GCREF)


    def malloc_fixed_or_varsize_nonmovable(self, typeid, length):
        # length==0 for fixedsize
        obj = self.external_malloc(typeid, length, alloc_young=True)
        return llmemory.cast_adr_to_ptr(obj, llmemory.GCREF)


    def collect(self, gen=1):
        """Do a minor (gen=0) or major (gen>0) collection."""
        self.minor_collection()
        if gen > 0:
            self.major_collection()

    def move_nursery_top(self, totalsize):
        size = self.nursery_cleanup
        ll_assert(self.nursery_real_top - self.nursery_top >= size,
            "nursery_cleanup not a divisor of nursery_size - initial_cleanup")
        ll_assert(llmemory.raw_malloc_usage(totalsize) <= size,
            "totalsize > nursery_cleanup")
        llarena.arena_reset(self.nursery_top, size, 2)
        self.nursery_top += size
    move_nursery_top._always_inline_ = True

    def collect_and_reserve(self, prev_result, totalsize):
        """To call when nursery_free overflows nursery_top.
        First check if the nursery_top is the real top, otherwise we
        can just move the top of one cleanup and continue

        Do a minor collection, and possibly also a major collection,
        and finally reserve 'totalsize' bytes at the start of the
        now-empty nursery.
        """
        if self.nursery_top < self.nursery_real_top:
            self.move_nursery_top(totalsize)
            return prev_result
        self.minor_collection()
        #
        if self.get_total_memory_used() > self.next_major_collection_threshold:
            self.major_collection()
            #
            # The nursery might not be empty now, because of
            # execute_finalizers().  If it is almost full again,
            # we need to fix it with another call to minor_collection().
            if self.nursery_free + totalsize > self.nursery_top:
                #
                if self.nursery_free + totalsize > self.nursery_real_top:
                    self.minor_collection()
                    # then the nursery is empty
                else:
                    # we just need to clean up a bit more of the nursery
                    self.move_nursery_top(totalsize)
        #
        result = self.nursery_free
        self.nursery_free = result + totalsize
        ll_assert(self.nursery_free <= self.nursery_top, "nursery overflow")
        #
        if self.debug_tiny_nursery >= 0:   # for debugging
            if self.nursery_top - self.nursery_free > self.debug_tiny_nursery:
                self.nursery_free = self.nursery_top - self.debug_tiny_nursery
        #
        return result
    collect_and_reserve._dont_inline_ = True


    # XXX kill alloc_young and make it always True
    def external_malloc(self, typeid, length, alloc_young):
        """Allocate a large object using the ArenaCollection or
        raw_malloc(), possibly as an object with card marking enabled,
        if it has gc pointers in its var-sized part.  'length' should be
        specified as 0 if the object is not varsized.  The returned
        object is fully initialized and zero-filled."""
        #
        # Here we really need a valid 'typeid', not 0 (as the JIT might
        # try to send us if there is still a bug).
        ll_assert(bool(self.combine(typeid, 0)),
                  "external_malloc: typeid == 0")
        #
        # Compute the total size, carefully checking for overflows.
        size_gc_header = self.gcheaderbuilder.size_gc_header
        nonvarsize = size_gc_header + self.fixed_size(typeid)
        if length == 0:
            # this includes the case of fixed-size objects, for which we
            # should not even ask for the varsize_item_sizes().
            totalsize = nonvarsize
        elif length > 0:
            # var-sized allocation with at least one item
            itemsize = self.varsize_item_sizes(typeid)
            try:
                varsize = ovfcheck(itemsize * length)
                totalsize = ovfcheck(nonvarsize + varsize)
            except OverflowError:
                raise MemoryError
        else:
            # negative length!  This likely comes from an overflow
            # earlier.  We will just raise MemoryError here.
            raise MemoryError
        #
        # If somebody calls this function a lot, we must eventually
        # force a full collection.
        if (float(self.get_total_memory_used()) + raw_malloc_usage(totalsize) >
                self.next_major_collection_threshold):
            self.minor_collection()
            self.major_collection(raw_malloc_usage(totalsize))
        #
        # Check if the object would fit in the ArenaCollection.
        # Also, an object allocated from ArenaCollection must be old.
        if (raw_malloc_usage(totalsize) <= self.small_request_threshold
            and not alloc_young):
            #
            # Yes.  Round up 'totalsize' (it cannot overflow and it
            # must remain <= self.small_request_threshold.)
            totalsize = llarena.round_up_for_allocation(totalsize)
            ll_assert(raw_malloc_usage(totalsize) <=
                      self.small_request_threshold,
                      "rounding up made totalsize > small_request_threshold")
            #
            # Allocate from the ArenaCollection and clear the memory returned.
            result = self.ac.malloc(totalsize)
            llmemory.raw_memclear(result, totalsize)
            #
            extra_flags = GCFLAG_TRACK_YOUNG_PTRS
            #
        else:
            # No, so proceed to allocate it externally with raw_malloc().
            # Check if we need to introduce the card marker bits area.
            if (self.card_page_indices <= 0  # <- this check is constant-folded
                or not self.has_gcptr_in_varsize(typeid) or
                raw_malloc_usage(totalsize) <= self.nonlarge_max):
                #
                # In these cases, we don't want a card marker bits area.
                # This case also includes all fixed-size objects.
                cardheadersize = 0
                extra_flags = 0
                #
            else:
                # Reserve N extra words containing card bits before the object.
                extra_words = self.card_marking_words_for_length(length)
                cardheadersize = WORD * extra_words
                extra_flags = GCFLAG_HAS_CARDS | GCFLAG_TRACK_YOUNG_PTRS
                # if 'alloc_young', then we also immediately set
                # GCFLAG_CARDS_SET, but without adding the object to
                # 'old_objects_with_cards_set'.  In this way it should
                # never be added to that list as long as it is young.
                if alloc_young:
                    extra_flags |= GCFLAG_CARDS_SET
            #
            # Detect very rare cases of overflows
            if raw_malloc_usage(totalsize) > (sys.maxint - (WORD-1)
                                              - cardheadersize):
                raise MemoryError("rare case of overflow")
            #
            # Now we know that the following computations cannot overflow.
            # Note that round_up_for_allocation() is also needed to get the
            # correct number added to 'rawmalloced_total_size'.
            allocsize = (cardheadersize + raw_malloc_usage(
                            llarena.round_up_for_allocation(totalsize)))
            #
            # Allocate the object using arena_malloc(), which we assume here
            # is just the same as raw_malloc(), but allows the extra
            # flexibility of saying that we have extra words in the header.
            # The memory returned is cleared by a raw_memclear().
            arena = llarena.arena_malloc(allocsize, 2)
            if not arena:
                raise MemoryError("cannot allocate large object")
            #
            # Reserve the card mark bits as a list of single bytes
            # (the loop is empty in C).
            i = 0
            while i < cardheadersize:
                llarena.arena_reserve(arena + i, llmemory.sizeof(lltype.Char))
                i += 1
            #
            # Reserve the actual object.  (This is also a no-op in C).
            result = arena + cardheadersize
            llarena.arena_reserve(result, totalsize)
            #
            # Record the newly allocated object and its full malloced size.
            # The object is young or old depending on the argument.
            self.rawmalloced_total_size += r_uint(allocsize)
            if alloc_young:
                if not self.young_rawmalloced_objects:
                    self.young_rawmalloced_objects = self.AddressDict()
                self.young_rawmalloced_objects.add(result + size_gc_header)
            else:
                self.old_rawmalloced_objects.append(result + size_gc_header)
                extra_flags |= GCFLAG_TRACK_YOUNG_PTRS
        #
        # Common code to fill the header and length of the object.
        self.init_gc_object(result, typeid, extra_flags)
        if self.is_varsize(typeid):
            offset_to_length = self.varsize_offset_to_length(typeid)
            (result + size_gc_header + offset_to_length).signed[0] = length
        return result + size_gc_header


    # ----------
    # Other functions in the GC API

    def set_max_heap_size(self, size):
        self.max_heap_size = float(size)
        if self.max_heap_size > 0.0:
            if self.max_heap_size < self.next_major_collection_initial:
                self.next_major_collection_initial = self.max_heap_size
            if self.max_heap_size < self.next_major_collection_threshold:
                self.next_major_collection_threshold = self.max_heap_size

    def raw_malloc_memory_pressure(self, sizehint):
        self.next_major_collection_threshold -= sizehint
        if self.next_major_collection_threshold < 0:
            # cannot trigger a full collection now, but we can ensure
            # that one will occur very soon
            self.nursery_top = self.nursery_real_top
            self.nursery_free = self.nursery_real_top

    def can_optimize_clean_setarrayitems(self):
        if self.card_page_indices > 0:
            return False
        return MovingGCBase.can_optimize_clean_setarrayitems(self)

    def can_move(self, obj):
        """Overrides the parent can_move()."""
        return self.is_in_nursery(obj)


    def shrink_array(self, obj, smallerlength):
        #
        # Only objects in the nursery can be "resized".  Resizing them
        # means recording that they have a smaller size, so that when
        # moved out of the nursery, they will consume less memory.
        # In particular, an array with GCFLAG_HAS_CARDS is never resized.
        # Also, a nursery object with GCFLAG_HAS_SHADOW is not resized
        # either, as this would potentially loose part of the memory in
        # the already-allocated shadow.
        if not self.is_in_nursery(obj):
            return False
        if self.header(obj).tid & GCFLAG_HAS_SHADOW:
            return False
        #
        size_gc_header = self.gcheaderbuilder.size_gc_header
        typeid = self.get_type_id(obj)
        totalsmallersize = (
            size_gc_header + self.fixed_size(typeid) +
            self.varsize_item_sizes(typeid) * smallerlength)
        llarena.arena_shrink_obj(obj - size_gc_header, totalsmallersize)
        #
        offset_to_length = self.varsize_offset_to_length(typeid)
        (obj + offset_to_length).signed[0] = smallerlength
        return True


    # ----------
    # Simple helpers

    def get_type_id(self, obj):
        tid = self.header(obj).tid
        return llop.extract_ushort(llgroup.HALFWORD, tid)

    def combine(self, typeid16, flags):
        return llop.combine_ushort(lltype.Signed, typeid16, flags)

    def init_gc_object(self, addr, typeid16, flags=0):
        # The default 'flags' is zero.  The flags GCFLAG_NO_xxx_PTRS
        # have been chosen to allow 'flags' to be zero in the common
        # case (hence the 'NO' in their name).
        hdr = llmemory.cast_adr_to_ptr(addr, lltype.Ptr(self.HDR))
        hdr.tid = self.combine(typeid16, flags)

    def init_gc_object_immortal(self, addr, typeid16, flags=0):
        # For prebuilt GC objects, the flags must contain
        # GCFLAG_NO_xxx_PTRS, at least initially.
        flags |= GCFLAG_NO_HEAP_PTRS | GCFLAG_TRACK_YOUNG_PTRS
        self.init_gc_object(addr, typeid16, flags)

    def is_in_nursery(self, addr):
        ll_assert(llmemory.cast_adr_to_int(addr) & 1 == 0,
                  "odd-valued (i.e. tagged) pointer unexpected here")
        return self.nursery <= addr < self.nursery_real_top

    def appears_to_be_young(self, addr):
        # "is a valid addr to a young object?"
        # but it's ok to occasionally return True accidentally.
        # Maybe the best implementation would be a bloom filter
        # of some kind instead of the dictionary lookup that is
        # sometimes done below.  But the expected common answer
        # is "Yes" because addr points to the nursery, so it may
        # not be useful to optimize the other case too much.
        #
        # First, if 'addr' appears to be a pointer to some place within
        # the nursery, return True
        if not self.translated_to_c:
            # When non-translated, filter out tagged pointers explicitly.
            # When translated, it may occasionally give a wrong answer
            # of True if 'addr' is a tagged pointer with just the wrong value.
            if not self.is_valid_gc_object(addr):
                return False

        if self.nursery <= addr < self.nursery_real_top:
            return True      # addr is in the nursery
        #
        # Else, it may be in the set 'young_rawmalloced_objects'
        return (bool(self.young_rawmalloced_objects) and
                self.young_rawmalloced_objects.contains(addr))
    appears_to_be_young._always_inline_ = True

    def debug_is_old_object(self, addr):
        return (self.is_valid_gc_object(addr)
                and not self.appears_to_be_young(addr))

    def is_forwarded(self, obj):
        """Returns True if the nursery obj is marked as forwarded.
        Implemented a bit obscurely by checking an unrelated flag
        that can never be set on a young object -- except if tid == -42.
        """
        assert self.is_in_nursery(obj)
        tid = self.header(obj).tid
        result = (tid & GCFLAG_FINALIZATION_ORDERING != 0)
        if result:
            ll_assert(tid == -42, "bogus header for young obj")
        else:
            ll_assert(bool(tid), "bogus header (1)")
            ll_assert(tid & -_GCFLAG_FIRST_UNUSED == 0, "bogus header (2)")
        return result

    def get_forwarding_address(self, obj):
        return llmemory.cast_adr_to_ptr(obj, FORWARDSTUBPTR).forw

    def get_possibly_forwarded_type_id(self, obj):
        if self.is_in_nursery(obj) and self.is_forwarded(obj):
            obj = self.get_forwarding_address(obj)
        return self.get_type_id(obj)

    def get_total_memory_used(self):
        """Return the total memory used, not counting any object in the
        nursery: only objects in the ArenaCollection or raw-malloced.
        """
        return self.ac.total_memory_used + self.rawmalloced_total_size

    def card_marking_words_for_length(self, length):
        # --- Unoptimized version:
        #num_bits = ((length-1) >> self.card_page_shift) + 1
        #return (num_bits + (LONG_BIT - 1)) >> LONG_BIT_SHIFT
        # --- Optimized version:
        return intmask(
          ((r_uint(length) + r_uint((LONG_BIT << self.card_page_shift) - 1)) >>
           (self.card_page_shift + LONG_BIT_SHIFT)))

    def card_marking_bytes_for_length(self, length):
        # --- Unoptimized version:
        #num_bits = ((length-1) >> self.card_page_shift) + 1
        #return (num_bits + 7) >> 3
        # --- Optimized version:
        return intmask(
            ((r_uint(length) + r_uint((8 << self.card_page_shift) - 1)) >>
             (self.card_page_shift + 3)))

    def debug_check_consistency(self):
        if self.DEBUG:
            ll_assert(not self.young_rawmalloced_objects,
                      "young raw-malloced objects in a major collection")
            ll_assert(not self.young_objects_with_weakrefs.non_empty(),
                      "young objects with weakrefs in a major collection")
            MovingGCBase.debug_check_consistency(self)

    def debug_check_object(self, obj):
        # after a minor or major collection, no object should be in the nursery
        ll_assert(not self.is_in_nursery(obj),
                  "object in nursery after collection")
        # similarily, all objects should have this flag, except if they
        # don't have any GC pointer
        typeid = self.get_type_id(obj)
        if self.has_gcptr(typeid):
            ll_assert(self.header(obj).tid & GCFLAG_TRACK_YOUNG_PTRS != 0,
                      "missing GCFLAG_TRACK_YOUNG_PTRS")
        # the GCFLAG_VISITED should not be set between collections
        ll_assert(self.header(obj).tid & GCFLAG_VISITED == 0,
                  "unexpected GCFLAG_VISITED")
        # the GCFLAG_FINALIZATION_ORDERING should not be set between coll.
        ll_assert(self.header(obj).tid & GCFLAG_FINALIZATION_ORDERING == 0,
                  "unexpected GCFLAG_FINALIZATION_ORDERING")
        # the GCFLAG_CARDS_SET should not be set between collections
        ll_assert(self.header(obj).tid & GCFLAG_CARDS_SET == 0,
                  "unexpected GCFLAG_CARDS_SET")
        # if the GCFLAG_HAS_CARDS is set, check that all bits are zero now
        if self.header(obj).tid & GCFLAG_HAS_CARDS:
            if self.card_page_indices <= 0:
                ll_assert(False, "GCFLAG_HAS_CARDS but not using card marking")
                return
            typeid = self.get_type_id(obj)
            ll_assert(self.has_gcptr_in_varsize(typeid),
                      "GCFLAG_HAS_CARDS but not has_gcptr_in_varsize")
            ll_assert(self.header(obj).tid & GCFLAG_NO_HEAP_PTRS == 0,
                      "GCFLAG_HAS_CARDS && GCFLAG_NO_HEAP_PTRS")
            offset_to_length = self.varsize_offset_to_length(typeid)
            length = (obj + offset_to_length).signed[0]
            extra_words = self.card_marking_words_for_length(length)
            #
            size_gc_header = self.gcheaderbuilder.size_gc_header
            p = llarena.getfakearenaaddress(obj - size_gc_header)
            i = extra_words * WORD
            while i > 0:
                p -= 1
                ll_assert(p.char[0] == '\x00',
                          "the card marker bits are not cleared")
                i -= 1

    # ----------
    # Write barrier

    # for the JIT: a minimal description of the write_barrier() method
    # (the JIT assumes it is of the shape
    #  "if addr_struct.int0 & JIT_WB_IF_FLAG: remember_young_pointer()")
    JIT_WB_IF_FLAG = GCFLAG_TRACK_YOUNG_PTRS

    # for the JIT to generate custom code corresponding to the array
    # write barrier for the simplest case of cards.  If JIT_CARDS_SET
    # is already set on an object, it will execute code like this:
    #    MOV eax, index
    #    SHR eax, JIT_WB_CARD_PAGE_SHIFT
    #    XOR eax, -8
    #    BTS [object], eax
    if TRANSLATION_PARAMS['card_page_indices'] > 0:
        JIT_WB_CARDS_SET = GCFLAG_CARDS_SET
        JIT_WB_CARD_PAGE_SHIFT = 1
        while ((1 << JIT_WB_CARD_PAGE_SHIFT) !=
               TRANSLATION_PARAMS['card_page_indices']):
            JIT_WB_CARD_PAGE_SHIFT += 1

    @classmethod
    def JIT_max_size_of_young_obj(cls):
        return cls.TRANSLATION_PARAMS['large_object']

    @classmethod
    def JIT_minimal_size_in_nursery(cls):
        return cls.minimal_size_in_nursery

    def write_barrier(self, addr_struct):
        if self.header(addr_struct).tid & GCFLAG_TRACK_YOUNG_PTRS:
            self.remember_young_pointer(addr_struct)

    def write_barrier_from_array(self, addr_array, index):
        if self.header(addr_array).tid & GCFLAG_TRACK_YOUNG_PTRS:
            if self.card_page_indices > 0:     # <- constant-folded
                self.remember_young_pointer_from_array2(addr_array, index)
            else:
                self.remember_young_pointer(addr_array)

    def _init_writebarrier_logic(self):
        DEBUG = self.DEBUG
        # The purpose of attaching remember_young_pointer to the instance
        # instead of keeping it as a regular method is to
        # make the code in write_barrier() marginally smaller
        # (which is important because it is inlined *everywhere*).
        def remember_young_pointer(addr_struct):
            # 'addr_struct' is the address of the object in which we write.
            # We know that 'addr_struct' has GCFLAG_TRACK_YOUNG_PTRS so far.
   …