1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
|
#include <stdio.h>
#include <stdlib.h>
#define STACK_MAX 256
#define INITIAL_GC_THRESHOLD 8
typedef enum {
OBJ_INT,
OBJ_PAIR
} ObjectType;
typedef struct sObject {
/* The next object in the list of all objects */
struct sObject* next;
unsigned char marked;
ObjectType type;
union {
/* OBJ_INT */
int value;
/* OBJ_PAIR */
struct {
struct sObject* head;
struct sObject* tail;
};
};
} Object;
typedef struct {
/* The total number of currently allocated objects. */
int numObjects;
/* The number of objects required to trigger a GC. */
int maxObjects;
/* The first object in the list of all objects. */
Object* firstObject;
Object* stack[STACK_MAX];
int stackSize;
} VM;
VM* newVM() {
VM* vm = malloc(sizeof(VM));
vm->stackSize = 0;
vm->numObjects = 0;
vm->maxObjects = INITIAL_GC_THRESHOLD;
vm->firstObject = NULL;
return vm;
}
void mark(Object* object) {
/* If already marked we're done. Check this first
* to avoid recursing on cycles in the object graph */
if (object->marked) return;
object->marked = 1;
if (object->type == OBJ_PAIR) {
mark(object->head);
mark(object->tail);
}
}
void markAll(VM* vm) {
for (int i = 0; i < vm->stackSize; i++) {
mark(vm->stack[i]);
}
}
void sweep(VM* vm) {
Object** object = &vm->firstObject;
while (*object) {
if (!(*object)->marked) {
/* Remove unmarked object. */
Object* unreached = *object;
*object = unreached->next;
free(unreached);
vm->numObjects--;
} else {
/* Reset mark for next traversal */
(*object)->marked = 0;
object = &(*object)->next;
}
}
}
void assert(int condition, const char* message) {
if (!condition) {
printf("%s\n", message);
exit(1);
}
}
void push(VM* vm, Object* value) {
assert(vm->stackSize < STACK_MAX, "Stack Overflow!");
vm->stack[vm->stackSize++] = value;
}
Object* pop(VM* vm) {
assert(vm->stackSize > 0, "Stack underflow!");
return vm->stack[--vm->stackSize];
}
void gc(VM* vm) {
int numObjects = vm->numObjects;
markAll(vm);
sweep(vm);
vm->maxObjects = vm->numObjects * 2;
}
Object* newObject(VM* vm, ObjectType type) {
if (vm->numObjects == vm->maxObjects) gc(vm);
Object* object = malloc(sizeof(Object));
object->type = type;
/* Insert into list of all objects. */
object->next = vm->firstObject;
vm->firstObject = object;
vm->numObjects++;
return object;
}
void pushInt(VM* vm, int intValue) {
Object* object = newObject(vm, OBJ_INT);
object->value = intValue;
push(vm, object);
}
Object* pushPair(VM* vm) {
Object* object = newObject(vm, OBJ_PAIR);
object->tail = pop(vm);
object->head = pop(vm);
push(vm, object);
return object;
}
void freeVM(VM *vm) {
vm->stackSize = 0;
gc(vm);
free(vm);
}
void test1() {
printf("Test 1: Objects on stack are preserved.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
gc(vm);
assert(vm->numObjects == 2, "Should have preserved objects.");
freeVM(vm);
}
void test2() {
printf("Test 2: Unreached objects are collected.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
pop(vm);
pop(vm);
gc(vm);
assert(vm->numObjects == 0, "Should have collected objects.");
freeVM(vm);
}
void test3() {
printf("Test 3: Reach nested objects.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
pushPair(vm);
pushInt(vm, 3);
pushInt(vm, 4);
pushPair(vm);
pushPair(vm);
gc(vm);
assert(vm->numObjects == 7, "Should have reached objects.");
freeVM(vm);
}
static void test4() {
printf("Test 4: Handle cycles.\n");
VM* vm = newVM();
pushInt(vm, 1);
pushInt(vm, 2);
Object* a = pushPair(vm);
pushInt(vm, 3);
pushInt(vm, 4);
Object* b = pushPair(vm);
/* Set up a cycle, and also make 2 and 4 unreachable and collectible. */
a->tail = b;
b->tail = a;
gc(vm);
assert(vm->numObjects == 4, "Should have collected objects.");
freeVM(vm);
}
static void perfTest() {
printf("Performance Test.\n");
VM* vm = newVM();
for (int i = 0; i < 1000; i++) {
for (int j = 0; j < 20; j++) {
pushInt(vm, i);
}
for (int k = 0; k < 20; k++) {
pop(vm);
}
}
freeVM(vm);
}
int main(int argc, const char * argv[]) {
test1();
test2();
test3();
test4();
perfTest();
return 0;
}
|
