NAME ^

src/pmc_freeze.c - Freeze and thaw functionality

DESCRIPTION ^

Freeze uses the next_for_GC pointer() to remember seen PMCs. PMCs are written as IDs (or tags), which are calculated from their arena address. This PMC number is multiplied by four. The 2 low bits indicate a seen PMC or a PMC of the same type as the previous one respectively.

Thawing PMCs uses a list with (maximum) size of the amount of PMCs to keep track of retrieved PMCs.

The individual information of PMCs is frozen/thawed by their vtables.

To avoid recursion, the whole functionality is driven by pmc->vtable->visit, which is called for the first PMC initially. Container PMCs call a "todo-callback" for all contained PMCs. The individual action vtable (freeze/thaw) is then called for all todo-PMCs.

In the current implementation IMAGE_IO is a stand-in for some kind of serializer PMC which will eventually be written. It associates a Parrot STRING with a vtable.

Image Stream Functions ^

static void str_append
Appends len bytes from buffer *b to string *s.Plain ascii - for testing only:For speed reasons we mess around with the string buffers directly.No encoding of strings, no transcoding.
static void push_ascii_integer
Pushes an ASCII version of the integer v onto the end of the *io "stream".
static void push_ascii_number
Pushes an ASCII version of the number v onto the end of the *io "stream".
static void push_ascii_string
Pushes an ASCII version of the string *s onto the end of the *io "stream".For testing only - no encodings and such.XXX no string delimiters - so no space allowed.
static void push_ascii_pmc
Pushes an ASCII version of the PMC *v onto the end of the *io "stream".
static INTVAL shift_ascii_integer
Removes and returns an integer from the start of the *io "stream".
static FLOATVAL shift_ascii_number
Removes and returns an number from the start of the *io "stream".
static STRING *shift_ascii_string
Removes and returns an string from the start of the *io "stream".
static PMC *shift_ascii_pmc
Removes and returns a PMC from the start of the *io "stream".

opcode_t IO Functions ^

static void op_check_size
Checks the size of the "stream" buffer to see if it can accommodate len more bytes. If not then the buffer is expanded.
static void op_append
Appends the opcode b to the string *s.
static void push_opcode_integer
Pushes the integer v onto the end of the *io "stream".XXX assumes sizeof (opcode_t) == sizeof (INTVAL).
static void push_opcode_number
Pushes the number v onto the end of the *io "stream".
static void push_opcode_string
Pushes the string *v onto the end of the *io "stream".
static void push_opcode_pmc
Pushes the PMC *v onto the end of the *io "stream".
static INTVAL shift_opcode_integer
Removes and returns an integer from the start of the *io "stream".TODO - The shift functions aren't portable yet. We need to have a packfile header for wordsize and endianess.
static PMC *shift_opcode_pmc
Removes and returns an PMC from the start of the *io "stream".Note that this actually reads a PMC id, not a PMC.
static FLOATVAL shift_opcode_number
Removes and returns an number from the start of the *io "stream".
static STRING *shift_opcode_string
Removes and returns a string from the start of the *io "stream".

Helper Functions ^

static void pmc_add_ext
Adds a PMC_EXT to *pmc.
static void cleanup_next_for_GC_pool
Sets all the next_for_GC pointers to NULL.
static void cleanup_next_for_GC
Cleans up the next_for_GC pointers.
static void ft_init
Initializes the freeze/thaw subsystem.
static void todo_list_init
Initializes the *info lists.
static void freeze_pmc
Freeze PMC, setting type, seen, and "same-as-last" indicators as appropriate.
static int thaw_pmc
Freeze and thaw a PMC (id).For example, the ASCII representation of the Array 
    P0 = [P1=666, P2=777, P0]
may look like this: 
    0xdf4 30 3 0xdf8 33 666 0xdf2 777 0xdf5
where 30 is class_enum_Array, 33 is class_enum_Integer, the type of the second Integer is suppressed, the repeated P0 has bit 0 set.
static void do_action
Called from visit_next_for_GC() and visit_todo_list() to perform the action specified in info->what.Currently only VISIT_FREEZE_NORMAL is implemented.
static PMC *thaw_create_pmc
Called from do_thaw() to attach the vtable etc. to *pmc.
static void do_thaw
Called by visit_todo_list_thaw() to thaw and return a PMC.seen is false if this is the first time the PMC has been encountered.
static UINTVAL id_from_pmc
Find a PMC in an arena, and return an id (left-shifted 2 bits), based on its position.If not found, throw an exception.
static void add_pmc_next_for_GC
Remembers the PMC for later processing.
static int next_for_GC_seen
Remembers next child to visit via the next_for_GC pointer generate a unique ID per PMC and freeze the ID (not the PMC address) so thaw the hash-lookup can be replaced by an array lookup then which is a lot faster.
static void add_pmc_todo_list
Remembers the PMC to be processed later.
static int todo_list_seen
Returns true if the PMC was seen, otherwise it put it on the todo list, generates an ID (tag) for PMC, offset by 4 as are addresses, low bits are flags.
static void visit_next_for_GC
visit_child callbacks:Checks if the PMC was seen, generate an ID for it if not, then do the appropriate action.
static void visit_todo_list
Checks the seen PMC via the todo list.
static void visit_todo_list_thaw
Callback for thaw - action first.Todo-list and seen handling is all in do_thaw().
static void visit_loop_next_for_GC
Put first item on todo list, then run as long as there are items to be done.
static void visit_loop_todo_list
The thaw loop.
static void create_image
Allocate image to some estimated size.
static PMC *run_thaw
Performs thawing. what indicates what to be thawed.Thaw could use the next_for_GC pointers as todo-list too, but this would need 2 runs through the arenas to clean the next_for_GC pointers.For now it seems cheaper to use a list for remembering contained aggregates. We could of course decide dynamically, which strategy to use, e.g.: given a big image, the first thawed item is a small aggregate. This implies, it probably contains (or some big strings) more nested containers, for which the next_for_GC approach could be a win.

Public Interface ^

STRING *Parrot_freeze_at_destruct
This function must not consume any resources (except the image itself). It uses the next_for_GC pointer, so its not reentrant and must not be interrupted by a DOD run.
STRING *Parrot_freeze
Freeze using either method.
PMC *Parrot_thaw
Thaw a PMC, called from the thaw opcode.
PMC *Parrot_thaw_constants
Thaw the constants. This is used by PackFile for unpacking PMC constants.
PMC *Parrot_clone
There are for sure shortcuts to clone faster, e.g. always thaw the image immediately or use a special callback. But for now we just thaw a frozen PMC.

TODO ^

The seen-hash version for freezing might go away sometimes.

SEE ALSO ^

Lot of discussion on p6i and docs/dev/pmc_freeze.pod.

HISTORY ^

Initial version by leo 2003.11.03 - 2003.11.07.


 parrot