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; may look like this:; 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.

HISTORY

Initial version by leo 2003.11.03 - 2003.11.07.