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circuitpython/py/gc.c
Damien George daab651c5c py, gc: Zero out newly allocated blocks in the GC. 
Also add some more debugging output to gc_dump_alloc_table().

Now that newly allocated heap is always zero'd, maybe we just make this
a policy for the uPy API to keep it simple (ie any new implementation of
memory allocation must zero all allocations).  This follows the D
language philosophy.

Before this patch, a previously used memory block which had pointers in
it may still retain those pointers if the new user of that block does
not actually use the entire block.  Eg, if I want 5 blocks worth of
heap, I actually get 8 (round up to nearest 4).  Then I never use the
last 3, so they keep their old values, which may be pointers pointing to
the heap, hence preventing GC.

In rare (or maybe not that rare) cases, this leads to long, unintentional
"linked lists" within the GC'd heap, filling it up completely.  It's
pretty rare, because you have to reuse exactly that memory which is part
of this "linked list", and reuse it in just the right way.

This should fix issue #522, and might have something to do with
issue #510.
2014-04-25 23:37:55 +01:00

636 lines
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#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include "mpconfig.h"
#include "misc.h"
#include "gc.h"
#include "misc.h"
#include "qstr.h"
#include "obj.h"
#include "runtime.h"
#if MICROPY_ENABLE_GC
#if 0 // print debugging info
#define DEBUG_PRINT (1)
#define DEBUG_printf DEBUG_printf
#else // don't print debugging info
#define DEBUG_printf(...) (void)0
#endif
#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;
#if MICROPY_ENABLE_FINALISER
STATIC byte *gc_finaliser_table_start;
#endif
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;
STATIC machine_uint_t gc_lock_depth;
// 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)
#if MICROPY_ENABLE_FINALISER
// FTB = finaliser table byte
// if set, then the corresponding block may have a finaliser
#define BLOCKS_PER_FTB (8)
#define FTB_GET(block) ((gc_finaliser_table_start[(block) / BLOCKS_PER_FTB] >> ((block) & 7)) & 1)
#define FTB_SET(block) do { gc_finaliser_table_start[(block) / BLOCKS_PER_FTB] |= (1 << ((block) & 7)); } while (0)
#define FTB_CLEAR(block) do { gc_finaliser_table_start[(block) / BLOCKS_PER_FTB] &= (~(1 << ((block) & 7))); } while (0)
#endif
// 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)));
DEBUG_printf("Initializing GC heap: %p..%p = %ld bytes\n", start, end, end - start);
// calculate parameters for GC (T=total, A=alloc table, F=finaliser table, P=pool; all in bytes):
// T = A + F + P
// F = A * BLOCKS_PER_ATB / BLOCKS_PER_FTB
// P = A * BLOCKS_PER_ATB * BYTES_PER_BLOCK
// => T = A * (1 + BLOCKS_PER_ATB / BLOCKS_PER_FTB + BLOCKS_PER_ATB * BYTES_PER_BLOCK)
machine_uint_t total_byte_len = end - start;
#if MICROPY_ENABLE_FINALISER
gc_alloc_table_byte_len = total_byte_len * BITS_PER_BYTE / (BITS_PER_BYTE + BITS_PER_BYTE * BLOCKS_PER_ATB / BLOCKS_PER_FTB + BITS_PER_BYTE * BLOCKS_PER_ATB * BYTES_PER_BLOCK);
#else
gc_alloc_table_byte_len = total_byte_len / (1 + BITS_PER_BYTE / 2 * BYTES_PER_BLOCK);
#endif
gc_alloc_table_start = (byte*)start;
#if MICROPY_ENABLE_FINALISER
machine_uint_t gc_finaliser_table_byte_len = (gc_alloc_table_byte_len * BLOCKS_PER_ATB) / BLOCKS_PER_FTB;
gc_finaliser_table_start = gc_alloc_table_start + gc_alloc_table_byte_len;
#endif
machine_uint_t gc_pool_block_len = gc_alloc_table_byte_len * BLOCKS_PER_ATB;
gc_pool_start = end - gc_pool_block_len * BYTES_PER_BLOCK;
gc_pool_end = end;
// clear ATBs
memset(gc_alloc_table_start, 0, gc_alloc_table_byte_len);
#if MICROPY_ENABLE_FINALISER
// clear FTBs
memset(gc_finaliser_table_start, 0, gc_finaliser_table_byte_len);
#endif
// 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;
}
// unlock the GC
gc_lock_depth = 0;
DEBUG_printf("GC layout:\n");
DEBUG_printf(" alloc table at %p, length " UINT_FMT " bytes, " UINT_FMT " blocks\n", gc_alloc_table_start, gc_alloc_table_byte_len, gc_alloc_table_byte_len * BLOCKS_PER_ATB);
#if MICROPY_ENABLE_FINALISER
DEBUG_printf(" finaliser table at %p, length " UINT_FMT " bytes, " UINT_FMT " blocks\n", gc_finaliser_table_start, gc_finaliser_table_byte_len, gc_finaliser_table_byte_len * BLOCKS_PER_FTB);
#endif
DEBUG_printf(" pool at %p, length " UINT_FMT " bytes, " UINT_FMT " blocks\n", gc_pool_start, gc_pool_block_len * BYTES_PER_BLOCK, gc_pool_block_len);
}
void gc_lock(void) {
gc_lock_depth++;
}
void gc_unlock(void) {
gc_lock_depth--;
}
#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 */ \
)
#define VERIFY_MARK_AND_PUSH(ptr) \
do { \
if (VERIFY_PTR(ptr)) { \
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)
STATIC void gc_drain_stack(void) {
while (gc_sp > gc_stack) {
// pop the next block off the stack
machine_uint_t block = *--gc_sp;
// work out number of consecutive blocks in the chain starting with this one
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);
}
}
}
STATIC void gc_deal_with_stack_overflow(void) {
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();
}
}
}
}
STATIC void gc_sweep(void) {
// 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:
#if MICROPY_ENABLE_FINALISER
if (FTB_GET(block)) {
mp_obj_t obj = (mp_obj_t)PTR_FROM_BLOCK(block);
if (((mp_obj_base_t*)obj)->type != MP_OBJ_NULL) {
// if the object has a type then see if it has a __del__ method
mp_obj_t dest[2];
mp_load_method_maybe(obj, MP_QSTR___del__, dest);
if (dest[0] != MP_OBJ_NULL) {
// load_method returned a method
mp_call_method_n_kw(0, 0, dest);
}
}
// clear finaliser flag
FTB_CLEAR(block);
}
#endif
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;
}
}
}
void gc_collect_start(void) {
gc_lock();
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();
}
}
void gc_collect_end(void) {
gc_deal_with_stack_overflow();
gc_sweep();
gc_unlock();
}
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) {
case AT_FREE:
info->free += 1;
len = 0;
break;
case AT_HEAD:
info->used += 1;
len = 1;
break;
case AT_TAIL:
info->used += 1;
len += 1;
break;
case AT_MARK:
// shouldn't happen
break;
}
}
info->used *= BYTES_PER_BLOCK;
info->free *= BYTES_PER_BLOCK;
}
void *gc_alloc(machine_uint_t n_bytes, bool has_finaliser) {
machine_uint_t n_blocks = ((n_bytes + BYTES_PER_BLOCK - 1) & (~(BYTES_PER_BLOCK - 1))) / BYTES_PER_BLOCK;
DEBUG_printf("gc_alloc(" UINT_FMT " bytes -> " UINT_FMT " blocks)\n", n_bytes, n_blocks);
// check if GC is locked
if (gc_lock_depth > 0) {
return NULL;
}
// 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;
}
DEBUG_printf("gc_alloc(" UINT_FMT "): no free mem, triggering GC\n", n_bytes);
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);
}
// get pointer to first block
void *ret_ptr = (void*)(gc_pool_start + start_block * WORDS_PER_BLOCK);
// zero out the newly allocated blocks
// This is needed because the blocks may have previously held pointers
// to the heap and will not be set to something else if the caller
// doesn't actually use the entire block. As such they will continue
// to point to the heap and may prevent other blocks from being reclaimed.
memset(ret_ptr, 0, (end_block - start_block + 1) * BYTES_PER_BLOCK);
#if MICROPY_ENABLE_FINALISER
if (has_finaliser) {
// clear type pointer in case it is never set (now done above in memset)
//((mp_obj_base_t*)ret_ptr)->type = MP_OBJ_NULL;
// set mp_obj flag only if it has a finaliser
FTB_SET(start_block);
}
#endif
return ret_ptr;
}
/*
void *gc_alloc(machine_uint_t n_bytes) {
return _gc_alloc(n_bytes, false);
}
void *gc_alloc_with_finaliser(machine_uint_t n_bytes) {
return _gc_alloc(n_bytes, true);
}
*/
// force the freeing of a piece of memory
void gc_free(void *ptr_in) {
if (gc_lock_depth > 0) {
// TODO how to deal with this error?
return;
}
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);
}
}
}
machine_uint_t gc_nbytes(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) {
// 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;
}
#if 0
// old, simple realloc that didn't expand memory in place
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 {
// TODO false is incorrect! Should get value from current block!
void *ptr2 = gc_alloc(n_bytes,
#if MICROPY_ENABLE_FINALISER
FTB_GET(BLOCK_FROM_PTR((machine_uint_t)ptr))
#else
false
#endif
);
if (ptr2 == NULL) {
return ptr2;
}
memcpy(ptr2, ptr, n_existing);
gc_free(ptr);
return ptr2;
}
}
#else // Alternative gc_realloc impl
void *gc_realloc(void *ptr_in, machine_uint_t n_bytes) {
if (gc_lock_depth > 0) {
return NULL;
}
// check for pure allocation
if (ptr_in == NULL) {
return gc_alloc(n_bytes, false);
}
machine_uint_t ptr = (machine_uint_t)ptr_in;
// sanity check the ptr
if (!VERIFY_PTR(ptr)) {
return NULL;
}
// get first block
machine_uint_t block = BLOCK_FROM_PTR(ptr);
// sanity check the ptr is pointing to the head of a block
if (ATB_GET_KIND(block) != AT_HEAD) {
return NULL;
}
// compute number of new blocks that are requested
machine_uint_t new_blocks = (n_bytes + BYTES_PER_BLOCK - 1) / BYTES_PER_BLOCK;
// get the number of consecutive tail blocks and
// the number of free blocks after last tail block
// stop if we reach (or are at) end of heap
machine_uint_t n_free = 0;
machine_uint_t n_blocks = 1; // counting HEAD block
machine_uint_t max_block = gc_alloc_table_byte_len * BLOCKS_PER_ATB;
while (block + n_blocks + n_free < max_block) {
if (n_blocks + n_free >= new_blocks) {
// stop as soon as we find enough blocks for n_bytes
break;
}
byte block_type = ATB_GET_KIND(block + n_blocks + n_free);
switch (block_type) {
case AT_FREE: n_free++; continue;
case AT_TAIL: n_blocks++; continue;
case AT_MARK: assert(0);
}
break;
}
// return original ptr if it already has the requested number of blocks
if (new_blocks == n_blocks) {
return ptr_in;
}
// check if we can shrink the allocated area
if (new_blocks < n_blocks) {
// free unneeded tail blocks
for (machine_uint_t bl = block + new_blocks; ATB_GET_KIND(bl) == AT_TAIL; bl++) {
ATB_ANY_TO_FREE(bl);
}
return ptr_in;
}
// check if we can expand in place
if (new_blocks <= n_blocks + n_free) {
// mark few more blocks as used tail
for (machine_uint_t bl = block + n_blocks; bl < block + new_blocks; bl++) {
assert(ATB_GET_KIND(bl) == AT_FREE);
ATB_FREE_TO_TAIL(bl);
}
// zero out the newly allocated blocks (see comment above in gc_alloc)
memset(ptr_in + n_blocks * BYTES_PER_BLOCK, 0, (new_blocks - n_blocks) * BYTES_PER_BLOCK);
return ptr_in;
}
// can't resize inplace; try to find a new contiguous chain
void *ptr_out = gc_alloc(n_bytes,
#if MICROPY_ENABLE_FINALISER
FTB_GET(block)
#else
false
#endif
);
// check that the alloc succeeded
if (ptr_out == NULL) {
return NULL;
}
DEBUG_printf("gc_realloc: allocating new block\n");
memcpy(ptr_out, ptr_in, n_blocks * BYTES_PER_BLOCK);
gc_free(ptr_in);
return ptr_out;
}
#endif // Alternative gc_realloc impl
void gc_dump_info() {
gc_info_t info;
gc_info(&info);
printf("GC: total: " UINT_FMT ", used: " UINT_FMT ", free: " UINT_FMT "\n", info.total, info.used, info.free);
printf(" No. of 1-blocks: " UINT_FMT ", 2-blocks: " UINT_FMT ", max blk sz: " UINT_FMT "\n",
info.num_1block, info.num_2block, info.max_block);
}
void gc_dump_alloc_table(void) {
printf("GC memory layout; from %p:", gc_pool_start);
for (machine_uint_t bl = 0; bl < gc_alloc_table_byte_len * BLOCKS_PER_ATB; bl++) {
if (bl % 64 == 0) {
printf("\n%04x: ", (uint)bl);
}
int c = ' ';
switch (ATB_GET_KIND(bl)) {
case AT_FREE: c = '.'; break;
case AT_HEAD: c = 'h'; break;
/* this prints the uPy object type of the head block
case AT_HEAD: {
machine_uint_t *ptr = gc_pool_start + bl * WORDS_PER_BLOCK;
if (*ptr == (machine_uint_t)&mp_type_tuple) { c = 'T'; }
else if (*ptr == (machine_uint_t)&mp_type_list) { c = 'L'; }
else if (*ptr == (machine_uint_t)&mp_type_dict) { c = 'D'; }
else if (*ptr == (machine_uint_t)&mp_type_float) { c = 'F'; }
else if (*ptr == (machine_uint_t)&mp_type_fun_bc) { c = 'B'; }
else { c = 'h'; }
break;
}
*/
case AT_TAIL: c = 't'; break;
case AT_MARK: c = 'm'; break;
}
printf("%c", c);
}
printf("\n");
}
#if DEBUG_PRINT
void gc_test(void) {
machine_uint_t len = 500;
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, false);
p[0] = gc_alloc(64, false);
p[1] = gc_alloc(1, false);
p[2] = gc_alloc(1, false);
p[3] = gc_alloc(1, false);
machine_uint_t ***p2 = gc_alloc(16, false);
p2[0] = p;
p2[1] = p;
ptrs[0] = p2;
}
for (int i = 0; i < 25; i+=2) {
machine_uint_t *p = gc_alloc(i, false);
printf("p=%p\n", p);
if (i & 3) {
//ptrs[i] = p;
}
}
printf("Before GC:\n");
gc_dump_alloc_table();
printf("Starting GC...\n");
gc_collect_start();
gc_collect_root(ptrs, sizeof(ptrs) / sizeof(void*));
gc_collect_end();
printf("After GC:\n");
gc_dump_alloc_table();
}
#endif
#endif // MICROPY_ENABLE_GC
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