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(Parrot_Interp interpreter, STRING *s, const void *b, size_t len): 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(Parrot_Interp interpreter, IMAGE_IO *io, INTVAL v): Pushes an ASCII version of the integer v onto the end of the *io "stream".
static void push_ascii_number(Parrot_Interp interpreter, IMAGE_IO *io, FLOATVAL v): Pushes an ASCII version of the number v onto the end of the *io "stream".
static void push_ascii_string(Parrot_Interp interpreter, IMAGE_IO *io, STRING *s): 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(Parrot_Interp interpreter, IMAGE_IO *io, PMC *v): Pushes an ASCII version of the PMC *v onto the end of the *io "stream".
static INTVAL shift_ascii_integer(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns an integer from the start of the *io "stream".
static FLOATVAL shift_ascii_number(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns an number from the start of the *io "stream".
static STRING *shift_ascii_string(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns an string from the start of the *io "stream".
static PMC *shift_ascii_pmc(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns a PMC from the start of the *io "stream".

`opcode_t` IO Functions

static PARROT_INLINE void op_check_size(Parrot_Interp interpreter, STRING *s, size_t len): 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(Parrot_Interp interpreter, STRING *s, opcode_t b, size_t len): Appends the opcode b to the string *s.
static void push_opcode_integer(Parrot_Interp interpreter, IMAGE_IO *io, INTVAL v): Pushes the integer v onto the end of the *io "stream".; XXX assumes sizeof(opcode_t) == sizeof(INTVAL).
static void push_opcode_number(Parrot_Interp interpreter, IMAGE_IO *io, FLOATVAL v): Pushes the number v onto the end of the *io "stream".
static void push_opcode_string(Parrot_Interp interpreter, IMAGE_IO *io, STRING *v): Pushes the string *v onto the end of the *io "stream".
static void push_opcode_pmc(Parrot_Interp interpreter, IMAGE_IO *io, PMC *v): Pushes the PMC *v onto the end of the *io "stream".
static INTVAL shift_opcode_integer(Parrot_Interp interpreter, IMAGE_IO *io): 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(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns an PMC from the start of the *io "stream".; Note that this actually reads a PMC id, not a PMC.
tatic FLOATVAL shift_opcode_number(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns an number from the start of the *io "stream".
static STRING *shift_opcode_string(Parrot_Interp interpreter, IMAGE_IO *io): Removes and returns a string from the start of the *io "stream".

Helper Functions

static void pmc_add_ext(Parrot_Interp interpreter, PMC *pmc): Adds a PMC_EXT to *pmc.
static void cleanup_next_for_GC_pool(Parrot_Interp interpreter, struct Small_Object_Pool *pool): Sets all the next_for_GC pointers to NULL.
static void cleanup_next_for_GC(Parrot_Interp interpreter): Cleans up the next_for_GC pointers.
static void ft_init(Parrot_Interp interpreter, visit_info *info): Initializes the freeze/thaw subsystem.
static void todo_list_init(Parrot_Interp interpreter, visit_info *info): Initializes the *info lists.
PARROT_INLINE static void freeze_pmc(Parrot_Interp interpreter, PMC *pmc, visit_info *info, int seen, UINTVAL id)
PARROT_INLINE static int thaw_pmc(Parrot_Interp interpreter, visit_info *info, UINTVAL *id, INTVAL *type): 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.
PARROT_INLINE static void do_action(Parrot_Interp interpreter, PMC *pmc, visit_info *info, int seen, UINTVAL id): 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.
PARROT_INLINE static PMC *thaw_create_pmc(Parrot_Interp interpreter, PMC *pmc, visit_info *info, INTVAL type): Called from do_thaw() to attach the vtable etc. to *pmc.
PARROT_INLINE static void do_thaw(Parrot_Interp interpreter, PMC *pmc, visit_info *info): 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(Parrot_Interp interpreter, PMC *pmc): Creates and returns a unique id (tag) for a PMC. This is the object number in the PMCs arena(s) shifted left by 2. Starts at 1<<2. 0 is a NULLPMC.
static void add_pmc_next_for_GC(Parrot_Interp interpreter, PMC *pmc, visit_info *info): Remembers the PMC for later processing.
PARROT_INLINE static int next_for_GC_seen(Parrot_Interp interpreter, PMC *pmc, visit_info *info, UINTVAL *id): 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(Parrot_Interp interpreter, PMC *pmc, visit_info *info): Remembers the PMC to be processed later.
PARROT_INLINE static int todo_list_seen(Parrot_Interp interpreter, PMC *pmc, visit_info *info, UINTVAL *id): 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.
tatic void visit_next_for_GC(Parrot_Interp interpreter, PMC *pmc, visit_info *info): 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(Parrot_Interp interpreter, PMC *pmc, visit_info *info): Checks the seen PMC via the todo list.
static void visit_todo_list_thaw(Parrot_Interp interpreter, PMC *old, visit_info *info): Callback for thaw - action first.; Todo-list and seen handling is all in do_thaw().
static void visit_loop_next_for_GC(Parrot_Interp interpreter, PMC *current, visit_info *info): Put first item on todo list, then run as long as there are items to be done.
static PMC *visit_loop_todo_list(Parrot_Interp interpreter, PMC *current, visit_info *info): The thaw loop.
static void create_image(Parrot_Interp interpreter, PMC *pmc, visit_info *info): Allocate image to some estimated size.
static PMC *run_thaw(Parrot_Interp interpreter, STRING *image, visit_enum_type what): 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(Parrot_Interp interpreter, PMC *pmc): 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(Parrot_Interp interpreter, PMC *pmc): Freeze using either method.
PMC *Parrot_thaw(Parrot_Interp interpreter, STRING *image): Thaw a PMC, called from the thaw opcode.
PMC *Parrot_thaw_constants(Parrot_Interp interpreter, STRING *image): Thaw the constants. This is used by PackFile for unpacking PMC constants.
PMC *Parrot_clone(Parrot_Interp interpreter, PMC *pmc): 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.