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#include "../gc.h"
#include "../util.h"
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct object_header {
bool mark : 1;
bool is_compiled_function : 1;
int rsvd : 1;
uint16_t untraced_slot_count : 13;
uint16_t value_slot_count;
struct object_header *next;
uintptr_t slots[];
};
/**
* A singly-linked list of live objects.
*/
static struct object_header *live_objects = NULL;
/**
* The number of bytes of live data, including object headers.
*/
static size_t live_bytes = 0;
/**
* The number of bytes of live data to reach before collecting.
*/
static size_t limit_bytes = 80 * 1024 * 1024;
/**
* The root stack is an arbitrary 4KiB. It shouldn't get anywhere near this
* deep, so this is simply a convenient size.
*/
static struct value *root_stack[4096 / sizeof(uintptr_t)] = {0};
static size_t root_stack_depth = 0;
static struct object_header *hdr(void *ptr) {
return &((struct object_header *)ptr)[-1];
}
static const struct object_header *hdrc(const void *ptr) {
return &((const struct object_header *)ptr)[-1];
}
void gc_init(void) {}
static void gc_mark(const struct value initial_value) {
// If the initial value just wasn't a pointer, we don't need to mark it.
if (!is_ptr(initial_value))
return;
struct object *obj = untag_ptr(initial_value);
if (!obj)
return;
// Otherwise, we do a traversal starting at the value, doing pointer reversal
// to avoid recursion.
//
// In simple recursive marking, we store where we came from in the call
// stack. Each stack frame effectively stores:
//
// 1. the pointer to the object we came from
// 2. the index of the slot we came from in that object
//
// In pointer reversal, instead of using that space, we store where we came
// from in the previous object itself. This relies on having the
// TAG_GC_INTERNAL tag.
struct object *prev = NULL;
while (obj) {
// Check if the object has already been marked. If so, we don't need to
// traverse it.
struct object_header *header = hdr(obj);
if (!header->mark) {
// Invariant: We've just started traversing this object, so none of its
// fields should have TAG_GC_INTERNAL.
// Check the object's fields, looking for one that
todo("gc_mark 1");
}
todo("gc_mark 2");
}
/*
struct object_header *header = hdr(obj);
if (header->mark)
return;
header->mark = true;
struct value *slots = (struct value *)&header->slots[0];
for (size_t i = 0; i < header->value_slot_count; i++)
gc_mark(slots[i]);
*/
}
static void gc_sweep(void) {
struct object_header **head = &live_objects, *header;
while ((header = *head)) {
if ((*head)->mark) {
(*head)->mark = false;
head = &header->next;
} else {
*head = header->next;
const size_t total_bytes =
sizeof(struct object_header) +
(header->value_slot_count + header->untraced_slot_count) *
sizeof(uintptr_t);
free(header);
live_bytes -= total_bytes;
}
}
}
void gc_collect(void) {
fprintf(stderr, "gc_collect()... ");
for (size_t i = 0; i < root_stack_depth; i++)
gc_mark(*root_stack[i]);
const size_t before = live_bytes;
gc_sweep();
const size_t after = live_bytes;
fprintf(stderr, "collected %zu bytes\n", before - after);
assume(live_bytes < limit_bytes);
}
struct object *gc_alloc(size_t value_slot_count, size_t untraced_slot_count) {
assume(value_slot_count < (1 << 16));
assume(untraced_slot_count < (1 << 13));
const size_t total_bytes =
sizeof(struct object_header) +
(value_slot_count + untraced_slot_count) * sizeof(uintptr_t);
if ((live_bytes += total_bytes) >= limit_bytes)
gc_collect();
struct object_header *header = malloc(total_bytes);
if (!header) {
gc_collect();
header = malloc(total_bytes);
if (!header)
todo("OOM");
}
memset(header, 0, total_bytes);
header->value_slot_count = value_slot_count;
header->untraced_slot_count = untraced_slot_count;
header->next = live_objects;
live_objects = header;
return (struct object *)&header->slots[0];
}
struct object *gc_alloc_compiled_function(size_t value_slot_count,
size_t untraced_slot_count) {
struct object *obj = gc_alloc(value_slot_count, untraced_slot_count);
hdr(obj)->is_compiled_function = true;
return obj;
}
struct object *gc_alloc_hashtable_eq(size_t value_slot_count,
size_t untraced_slot_count) {
return gc_alloc(value_slot_count, untraced_slot_count);
}
struct value gc_read_value_slot(const struct object *obj, size_t slot_index) {
assume(slot_index < hdrc(obj)->value_slot_count);
struct value *slots = (struct value *)obj;
struct value value = slots[slot_index];
return value;
}
uintptr_t gc_read_untraced_slot(const struct object *obj, size_t slot_index) {
assume(slot_index < hdrc(obj)->untraced_slot_count);
todo("gc_read_untraced_slot");
}
uint8_t gc_read_untraced_byte(const struct object *obj, size_t byte_index) {
todo("gc_read_untraced_byte");
}
void gc_write_value_slot(struct object *obj, size_t slot_index,
struct value value) {
struct value *slots = (struct value *)obj;
slots[slot_index] = value;
}
void gc_write_untraced_slot(struct object *obj, size_t slot_index,
uintptr_t value) {
assume(slot_index < hdrc(obj)->untraced_slot_count);
todo("gc_write_untraced_slot");
}
void gc_write_untraced_byte(struct object *obj, size_t byte_index,
uint8_t byte) {
todo("gc_write_untraced_byte");
}
void gc_root_push(struct value *root) {
assume(root_stack_depth < sizeof(root_stack));
root_stack[root_stack_depth++] = root;
}
void gc_root_pop(void) {
assume(root_stack_depth > 0);
root_stack_depth--;
}
void gc_debug(void) { fprintf(stderr, "live bytes = %zu\n", live_bytes); }
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