gc.c 12.1 KB
Newer Older
Damien's avatar
Damien committed
1
2
3
4
5
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

6
#include "mpconfig.h"
Damien's avatar
Damien committed
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
#include "gc.h"

// a machine word is big enough to hold a pointer
/*
#define BYTES_PER_WORD (8)
typedef unsigned long machine_uint_t;
*/
typedef unsigned char byte;

#define BITS_PER_BYTE (8)
#define BITS_PER_WORD (BITS_PER_BYTE * BYTES_PER_WORD)
#define WORDS_PER_BLOCK (4)
#define BYTES_PER_BLOCK (WORDS_PER_BLOCK * BYTES_PER_WORD)
#define STACK_SIZE (64) // tunable; minimum is 1

static byte *gc_alloc_table_start;
static machine_uint_t gc_alloc_table_byte_len;
static machine_uint_t *gc_pool_start;
static machine_uint_t *gc_pool_end;

static int gc_stack_overflow;
static machine_uint_t gc_stack[STACK_SIZE];
static machine_uint_t *gc_sp;

// ATB = allocation table byte
// 0b00 = FREE -- free block
// 0b01 = HEAD -- head of a chain of blocks
// 0b10 = TAIL -- in the tail of a chain of blocks
// 0b11 = MARK -- marked head block

#define AT_FREE (0)
#define AT_HEAD (1)
#define AT_TAIL (2)
#define AT_MARK (3)

#define BLOCKS_PER_ATB (4)
#define ATB_MASK_0 (0x03)
#define ATB_MASK_1 (0x0c)
#define ATB_MASK_2 (0x30)
#define ATB_MASK_3 (0xc0)

#define ATB_0_IS_FREE(a) (((a) & ATB_MASK_0) == 0)
#define ATB_1_IS_FREE(a) (((a) & ATB_MASK_1) == 0)
#define ATB_2_IS_FREE(a) (((a) & ATB_MASK_2) == 0)
#define ATB_3_IS_FREE(a) (((a) & ATB_MASK_3) == 0)

#define BLOCK_SHIFT(block) (2 * ((block) & (BLOCKS_PER_ATB - 1)))
#define ATB_GET_KIND(block) ((gc_alloc_table_start[(block) / BLOCKS_PER_ATB] >> BLOCK_SHIFT(block)) & 3)
#define ATB_ANY_TO_FREE(block) do { gc_alloc_table_start[(block) / BLOCKS_PER_ATB] &= (~(AT_MARK << BLOCK_SHIFT(block))); } while (0)
#define ATB_FREE_TO_HEAD(block) do { gc_alloc_table_start[(block) / BLOCKS_PER_ATB] |= (AT_HEAD << BLOCK_SHIFT(block)); } while (0)
#define ATB_FREE_TO_TAIL(block) do { gc_alloc_table_start[(block) / BLOCKS_PER_ATB] |= (AT_TAIL << BLOCK_SHIFT(block)); } while (0)
#define ATB_HEAD_TO_MARK(block) do { gc_alloc_table_start[(block) / BLOCKS_PER_ATB] |= (AT_MARK << BLOCK_SHIFT(block)); } while (0)
#define ATB_MARK_TO_HEAD(block) do { gc_alloc_table_start[(block) / BLOCKS_PER_ATB] &= (~(AT_TAIL << BLOCK_SHIFT(block))); } while (0)

#define BLOCK_FROM_PTR(ptr) (((ptr) - (machine_uint_t)gc_pool_start) / BYTES_PER_BLOCK)
#define PTR_FROM_BLOCK(block) (((block) * BYTES_PER_BLOCK + (machine_uint_t)gc_pool_start))
#define ATB_FROM_BLOCK(bl) ((bl) / BLOCKS_PER_ATB)

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
// TODO waste less memory; currently requires that all entries in alloc_table have a corresponding block in pool
void gc_init(void *start, void *end) {
    // align end pointer on block boundary
    end = (void*)((machine_uint_t)end & (~(BYTES_PER_BLOCK - 1)));

    // calculate parameters for GC
    machine_uint_t total_word_len = (machine_uint_t*)end - (machine_uint_t*)start;
    gc_alloc_table_byte_len = total_word_len * BYTES_PER_WORD / (1 + BITS_PER_BYTE / 2 * BYTES_PER_BLOCK);
    gc_alloc_table_start = (byte*)start;
    machine_uint_t gc_pool_block_len = gc_alloc_table_byte_len * BITS_PER_BYTE / 2;
    machine_uint_t gc_pool_word_len = gc_pool_block_len * WORDS_PER_BLOCK;
    gc_pool_start = (machine_uint_t*)end - gc_pool_word_len;
    gc_pool_end = end;

    // clear ATBs
    memset(gc_alloc_table_start, 0, gc_alloc_table_byte_len);

    // allocate first block because gc_pool_start points there and it will never
    // be freed, so allocating 1 block with null pointers will minimise memory loss
    ATB_FREE_TO_HEAD(0);
    for (int i = 0; i < WORDS_PER_BLOCK; i++) {
        gc_pool_start[i] = 0;
    }

    /*
    printf("GC layout:\n");
    printf("  alloc table at %p, length %u bytes\n", gc_alloc_table_start, gc_alloc_table_byte_len);
    printf("  pool at %p, length %u blocks = %u words = %u bytes\n", gc_pool_start, gc_pool_block_len, gc_pool_word_len, gc_pool_word_len * BYTES_PER_WORD);
    */
}

96
97
98
99
100
101
#define VERIFY_PTR(ptr) ( \
        (ptr & (BYTES_PER_BLOCK - 1)) == 0          /* must be aligned on a block */ \
        && ptr >= (machine_uint_t)gc_pool_start     /* must be above start of pool */ \
        && ptr < (machine_uint_t)gc_pool_end        /* must be below end of pool */ \
    )

Damien's avatar
Damien committed
102
103
#define VERIFY_MARK_AND_PUSH(ptr) \
    do { \
104
        if (VERIFY_PTR(ptr)) { \
Damien's avatar
Damien committed
105
106
107
108
109
110
111
112
113
114
115
116
117
            machine_uint_t _block = BLOCK_FROM_PTR(ptr); \
            if (ATB_GET_KIND(_block) == AT_HEAD) { \
                /* an unmarked head, mark it, and push it on gc stack */ \
                ATB_HEAD_TO_MARK(_block); \
                if (gc_sp < &gc_stack[STACK_SIZE]) { \
                    *gc_sp++ = _block; \
                } else { \
                    gc_stack_overflow = 1; \
                } \
            } \
        } \
    } while (0)

118
static void gc_drain_stack(void) {
Damien's avatar
Damien committed
119
120
121
122
    while (gc_sp > gc_stack) {
        // pop the next block off the stack
        machine_uint_t block = *--gc_sp;

123
        // work out number of consecutive blocks in the chain starting with this one
Damien's avatar
Damien committed
124
125
126
127
128
129
130
131
132
133
134
135
136
137
        machine_uint_t n_blocks = 0;
        do {
            n_blocks += 1;
        } while (ATB_GET_KIND(block + n_blocks) == AT_TAIL);

        // check this block's children
        machine_uint_t *scan = (machine_uint_t*)PTR_FROM_BLOCK(block);
        for (machine_uint_t i = n_blocks * WORDS_PER_BLOCK; i > 0; i--, scan++) {
            machine_uint_t ptr2 = *scan;
            VERIFY_MARK_AND_PUSH(ptr2);
        }
    }
}

138
static void gc_deal_with_stack_overflow(void) {
Damien's avatar
Damien committed
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
    while (gc_stack_overflow) {
        gc_stack_overflow = 0;
        gc_sp = gc_stack;

        // scan entire memory looking for blocks which have been marked but not their children
        for (machine_uint_t block = 0; block < gc_alloc_table_byte_len * BLOCKS_PER_ATB; block++) {
            // trace (again) if mark bit set
            if (ATB_GET_KIND(block) == AT_MARK) {
                *gc_sp++ = block;
                gc_drain_stack();
            }
        }
    }
}

154
static void gc_sweep(void) {
Damien's avatar
Damien committed
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
    // free unmarked heads and their tails
    int free_tail = 0;
    for (machine_uint_t block = 0; block < gc_alloc_table_byte_len * BLOCKS_PER_ATB; block++) {
        switch (ATB_GET_KIND(block)) {
            case AT_HEAD:
                free_tail = 1;
                // fall through to free the head

            case AT_TAIL:
                if (free_tail) {
                    ATB_ANY_TO_FREE(block);
                }
                break;

            case AT_MARK:
                ATB_MARK_TO_HEAD(block);
                free_tail = 0;
                break;
        }
    }
}

177
void gc_collect_start(void) {
Damien's avatar
Damien committed
178
179
180
181
182
183
184
185
186
187
188
189
    gc_stack_overflow = 0;
    gc_sp = gc_stack;
}

void gc_collect_root(void **ptrs, machine_uint_t len) {
    for (machine_uint_t i = 0; i < len; i++) {
        machine_uint_t ptr = (machine_uint_t)ptrs[i];
        VERIFY_MARK_AND_PUSH(ptr);
        gc_drain_stack();
    }
}

190
void gc_collect_end(void) {
Damien's avatar
Damien committed
191
192
    gc_deal_with_stack_overflow();
    gc_sweep();
193
}
Damien's avatar
Damien committed
194

195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
void gc_info(gc_info_t *info) {
    info->total = (gc_pool_end - gc_pool_start) * sizeof(machine_uint_t);
    info->used = 0;
    info->free = 0;
    info->num_1block = 0;
    info->num_2block = 0;
    info->max_block = 0;
    for (machine_uint_t block = 0, len = 0; block < gc_alloc_table_byte_len * BLOCKS_PER_ATB; block++) {
        machine_uint_t kind = ATB_GET_KIND(block);
        if (kind == AT_FREE || kind == AT_HEAD) {
            if (len == 1) {
                info->num_1block += 1;
            } else if (len == 2) {
                info->num_2block += 1;
            }
            if (len > info->max_block) {
                info->max_block = len;
            }
        }
        switch (kind) {
Damien's avatar
Damien committed
215
            case AT_FREE:
216
217
                info->free += 1;
                len = 0;
Damien's avatar
Damien committed
218
219
220
                break;

            case AT_HEAD:
221
222
223
224
                info->used += 1;
                len = 1;
                break;

Damien's avatar
Damien committed
225
            case AT_TAIL:
226
227
                info->used += 1;
                len += 1;
Damien's avatar
Damien committed
228
229
230
                break;

            case AT_MARK:
231
                // shouldn't happen
Damien's avatar
Damien committed
232
233
234
235
                break;
        }
    }

236
237
    info->used *= BYTES_PER_BLOCK;
    info->free *= BYTES_PER_BLOCK;
Damien's avatar
Damien committed
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
}

void *gc_alloc(machine_uint_t n_bytes) {
    machine_uint_t n_blocks = ((n_bytes + BYTES_PER_BLOCK - 1) & (~(BYTES_PER_BLOCK - 1))) / BYTES_PER_BLOCK;
    //printf("gc_alloc(%u bytes -> %u blocks)\n", n_bytes, n_blocks);

    // check for 0 allocation
    if (n_blocks == 0) {
        return NULL;
    }

    machine_uint_t i;
    machine_uint_t end_block;
    machine_uint_t start_block;
    machine_uint_t n_free = 0;
    int collected = 0;
    for (;;) {

        // look for a run of n_blocks available blocks
        for (i = 0; i < gc_alloc_table_byte_len; i++) {
            byte a = gc_alloc_table_start[i];
            if (ATB_0_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 0; goto found; } } else { n_free = 0; }
            if (ATB_1_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 1; goto found; } } else { n_free = 0; }
            if (ATB_2_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 2; goto found; } } else { n_free = 0; }
            if (ATB_3_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 3; goto found; } } else { n_free = 0; }
        }

        // nothing found!
        if (collected) {
            return NULL;
        }
        gc_collect();
        collected = 1;
    }

    // found, ending at block i inclusive
found:
    // get starting and end blocks, both inclusive
    end_block = i;
    start_block = i - n_free + 1;

    // mark first block as used head
    ATB_FREE_TO_HEAD(start_block);

    // mark rest of blocks as used tail
    // TODO for a run of many blocks can make this more efficient
    for (machine_uint_t bl = start_block + 1; bl <= end_block; bl++) {
        ATB_FREE_TO_TAIL(bl);
    }

    // return pointer to first block
    return (void*)(gc_pool_start + start_block * WORDS_PER_BLOCK);
}

292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
// force the freeing of a piece of memory
void gc_free(void *ptr_in) {
    machine_uint_t ptr = (machine_uint_t)ptr_in;

    if (VERIFY_PTR(ptr)) {
        machine_uint_t block = BLOCK_FROM_PTR(ptr);
        if (ATB_GET_KIND(block) == AT_HEAD) {
            // free head and all of its tail blocks
            do {
                ATB_ANY_TO_FREE(block);
                block += 1;
            } while (ATB_GET_KIND(block) == AT_TAIL);
        }
    }
}

Damien's avatar
Damien committed
308
309
310
machine_uint_t gc_nbytes(void *ptr_in) {
    machine_uint_t ptr = (machine_uint_t)ptr_in;

311
    if (VERIFY_PTR(ptr)) {
Damien's avatar
Damien committed
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
        machine_uint_t block = BLOCK_FROM_PTR(ptr);
        if (ATB_GET_KIND(block) == AT_HEAD) {
            // work out number of consecutive blocks in the chain starting with this on
            machine_uint_t n_blocks = 0;
            do {
                n_blocks += 1;
            } while (ATB_GET_KIND(block + n_blocks) == AT_TAIL);
            return n_blocks * BYTES_PER_BLOCK;
        }
    }

    // invalid pointer
    return 0;
}

void *gc_realloc(void *ptr, machine_uint_t n_bytes) {
    machine_uint_t n_existing = gc_nbytes(ptr);
    if (n_bytes <= n_existing) {
        return ptr;
    } else {
332
        // TODO check if we can grow inplace
Damien's avatar
Damien committed
333
334
        void *ptr2 = gc_alloc(n_bytes);
        memcpy(ptr2, ptr, n_existing);
335
        gc_free(ptr);
Damien's avatar
Damien committed
336
337
338
339
340
        return ptr2;
    }
}

/*
341
static void gc_dump_at(void) {
342
343
344
345
346
347
348
349
350
351
352
353
    for (machine_uint_t bl = 0; bl < gc_alloc_table_byte_len * BLOCKS_PER_ATB; bl++) {
        printf("block % 6u ", bl);
        switch (ATB_GET_KIND(bl)) {
            case AT_FREE: printf("FREE"); break;
            case AT_HEAD: printf("HEAD"); break;
            case AT_TAIL: printf("TAIL"); break;
            default: printf("MARK"); break;
        }
        printf("\n");
    }
}

354
int main(void) {
Damien's avatar
Damien committed
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
    machine_uint_t len = 1000;
    machine_uint_t *heap = malloc(len);
    gc_init(heap, heap + len / sizeof(machine_uint_t));
    void *ptrs[100];
    {
        machine_uint_t *p = gc_alloc(16);
        p[0] = gc_alloc(64);
        p[1] = gc_alloc(1);
        p[2] = gc_alloc(1);
        p[3] = gc_alloc(1);
        machine_uint_t *p2 = gc_alloc(16);
        p2[0] = p;
        p2[1] = p;
        ptrs[0] = p2;
    }
    for (int i = 0; i < 50; i+=2) {
        machine_uint_t *p = gc_alloc(i);
        printf("p=%p\n", p);
        if (i & 3) {
            //ptrs[i] = p;
        }
    }

    gc_dump_at();
    gc_collect(ptrs, sizeof(ptrs) / sizeof(void*));
    gc_dump_at();
}
*/