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# frozen_string_literal: true
require 'test/unit'
require 'fiddle'
class TestGCCompact < Test::Unit::TestCase
def memory_location(obj)
(Fiddle.dlwrap(obj) >> 1)
end
def assert_object_ids(list)
same_count = list.find_all { |obj|
memory_location(obj) == obj.object_id
}.count
list.count - same_count
end
def big_list
1000.times.map {
# try to make some empty slots by allocating an object and discarding
Object.new
Object.new
} # likely next to each other
end
# Find an object that's allocated in a slot that had a previous
# tenant, and that tenant moved and is still alive
def find_object_in_recycled_slot(addresses)
new_object = nil
loop do
new_object = Object.new
if addresses.include? memory_location(new_object)
break
end
end
new_object
end
def test_find_collided_object
list_of_objects = big_list
ids = list_of_objects.map(&:object_id) # store id in map
addresses = list_of_objects.map(&self.:memory_location)
assert_equal ids, addresses
# All object ids should be equal
assert_equal 0, assert_object_ids(list_of_objects) # should be 0
GC.verify_compaction_references
# Some should have moved
id_count = assert_object_ids(list_of_objects)
skip "couldn't get objects to move" if id_count == 0
assert_operator id_count, :>, 0
new_ids = list_of_objects.map(&:object_id)
# Object ids should not change after compaction
assert_equal ids, new_ids
new_tenant = find_object_in_recycled_slot(addresses)
assert new_tenant
# This is the object that used to be in new_object's position
previous_tenant = list_of_objects[addresses.index(memory_location(new_tenant))]
assert_not_equal previous_tenant.object_id, new_tenant.object_id
# Should be able to look up object by object_id
assert_equal new_tenant, ObjectSpace._id2ref(new_tenant.object_id)
# Should be able to look up object by object_id
assert_equal previous_tenant, ObjectSpace._id2ref(previous_tenant.object_id)
int = (new_tenant.object_id >> 1)
# These two should be the same! but they are not :(
assert_equal int, ObjectSpace._id2ref(int.object_id)
end
def test_many_collisions
list_of_objects = big_list
ids = list_of_objects.map(&:object_id)
addresses = list_of_objects.map(&self.:memory_location)
GC.verify_compaction_references
new_tenants = 10.times.map {
find_object_in_recycled_slot(addresses)
}
collisions = GC.stat(:object_id_collisions)
skip "couldn't get objects to collide" if collisions == 0
assert_operator collisions, :>, 0
end
end
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