py/gc.c - lite/micropython - Linaro Git Browser

 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>

 #include "mpconfig.h"
 #include "gc.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

 typedef unsigned char byte;

 #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)

 // 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\n", start, end);

     // 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;
     }

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

 #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:
                 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_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();
 }

 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) {
     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 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);
     }

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

 // 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);
         }
     }
 }

 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;
 }

 // use this realloc for now, one below is broken
 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 check if we can grow inplace
         void *ptr2 = gc_alloc(n_bytes);
         if (ptr2 == NULL) {
             return ptr2;
         }
         memcpy(ptr2, ptr, n_existing);
         gc_free(ptr);
         return ptr2;
     }
 }

 #if 0
 void *gc_realloc(void *ptr_in, machine_uint_t n_bytes) {
     void *ptr_out = NULL;
     machine_uint_t block = 0;
     machine_uint_t ptr = (machine_uint_t)ptr_in;

     if (ptr_in == NULL) {
         return gc_alloc(n_bytes);
     }

     if (VERIFY_PTR(ptr)                         /* verify pointer */
         && (block = BLOCK_FROM_PTR(ptr))        /* get first block */
         && ATB_GET_KIND(block) == AT_HEAD) {    /* make sure it's a HEAD block */

         byte block_type;
         machine_uint_t n_free   = 0;
         machine_uint_t n_blocks = 1; /* counting HEAD block */

         /* get the number of consecutive tail blocks and
            the number of free blocks after last tail block */
         // XXX make sure we stop if we get to end of heap
         do {
             block_type = ATB_GET_KIND(block + n_blocks + n_free);
             switch (block_type) {
                 case AT_FREE: n_free++; break;
                 case AT_TAIL: n_blocks++; break;
                 default: break;
             }
         /* stop as soon as we find enough blocks for n_bytes */
         // XXX check for n_bytes is wrong since we don't include n_free
         } while (block_type != AT_HEAD && (n_bytes > (n_blocks * BYTES_PER_BLOCK)));

         /* number of allocated bytes */
         machine_uint_t n_existing = n_blocks * BYTES_PER_BLOCK;

         /* check if realloc'ing to a smaller size */
         if (n_bytes <= n_existing) {
             ptr_out = ptr_in;
             /* free unneeded tail blocks */
             for (machine_uint_t bl = block + n_blocks; ATB_GET_KIND(bl) == AT_TAIL; bl++) {
                 ATB_ANY_TO_FREE(bl);
             }

         /* check if we can expand in place */
         // XXX disabled for now
         } else if (0 && n_bytes <= (n_existing + (n_free * BYTES_PER_BLOCK))) {
             /* number of blocks needed to expand +1 if there's a remainder */
             // XXX this has a bug, but don't know why; try: l=[i for i in range(1000)]; for i in l: print(i/3)
             machine_uint_t n_diff = ( n_bytes - n_existing)/BYTES_PER_BLOCK+
                                     ((n_bytes - n_existing)%BYTES_PER_BLOCK!=0);

             DEBUG_printf("gc_realloc: expanding " UINT_FMT " blocks (" UINT_FMT " bytes) to " UINT_FMT " blocks (" UINT_FMT " bytes)\n",
                     n_existing/BYTES_PER_BLOCK, n_existing, n_existing/BYTES_PER_BLOCK+n_diff, n_existing + n_diff*BYTES_PER_BLOCK);

             /* mark rest of blocks as used tail */
             for (machine_uint_t bl = block + n_blocks; bl < (block + n_blocks + n_diff); bl++) {
                 ATB_FREE_TO_TAIL(bl);
             }
             ptr_out = ptr_in;

         /* try to find a new contiguous chain */
         } else if ((ptr_out = gc_alloc(n_bytes)) != NULL) {
             DEBUG_printf("gc_realloc: allocated new block \n");
             memcpy(ptr_out, ptr_in, n_existing);
             gc_free(ptr_in);
         }
     }

     return ptr_out;
 }
 #endif

 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:");
     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;
             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);
         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 < 25; i+=2) {
         machine_uint_t *p = gc_alloc(i);
         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
	#include <stdio.h>
	#include <stdlib.h>
	#include <stdint.h>
	#include <string.h>

	#include "mpconfig.h"
	#include "gc.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

	typedef unsigned char byte;

	#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)

	// 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\n", start, end);

	// 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;
	}

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

	#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:
	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_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();
	}

	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) {
	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 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);
	}

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

	// 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);
	}
	}
	}

	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;
	}

	// use this realloc for now, one below is broken
	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 check if we can grow inplace
	void *ptr2 = gc_alloc(n_bytes);
	if (ptr2 == NULL) {
	return ptr2;
	}
	memcpy(ptr2, ptr, n_existing);
	gc_free(ptr);
	return ptr2;
	}
	}

	#if 0
	void gc_realloc(void ptr_in, machine_uint_t n_bytes) {
	void *ptr_out = NULL;
	machine_uint_t block = 0;
	machine_uint_t ptr = (machine_uint_t)ptr_in;

	if (ptr_in == NULL) {
	return gc_alloc(n_bytes);
	}

	if (VERIFY_PTR(ptr) /* verify pointer */
	&& (block = BLOCK_FROM_PTR(ptr)) /* get first block */
	&& ATB_GET_KIND(block) == AT_HEAD) { /* make sure it's a HEAD block */

	byte block_type;
	machine_uint_t n_free = 0;
	machine_uint_t n_blocks = 1; /* counting HEAD block */

	/* get the number of consecutive tail blocks and
	the number of free blocks after last tail block */
	// XXX make sure we stop if we get to end of heap
	do {
	block_type = ATB_GET_KIND(block + n_blocks + n_free);
	switch (block_type) {
	case AT_FREE: n_free++; break;
	case AT_TAIL: n_blocks++; break;
	default: break;
	}
	/* stop as soon as we find enough blocks for n_bytes */
	// XXX check for n_bytes is wrong since we don't include n_free
	} while (block_type != AT_HEAD && (n_bytes > (n_blocks * BYTES_PER_BLOCK)));

	/* number of allocated bytes */
	machine_uint_t n_existing = n_blocks * BYTES_PER_BLOCK;

	/* check if realloc'ing to a smaller size */
	if (n_bytes <= n_existing) {
	ptr_out = ptr_in;
	/* free unneeded tail blocks */
	for (machine_uint_t bl = block + n_blocks; ATB_GET_KIND(bl) == AT_TAIL; bl++) {
	ATB_ANY_TO_FREE(bl);
	}

	/* check if we can expand in place */
	// XXX disabled for now
	} else if (0 && n_bytes <= (n_existing + (n_free * BYTES_PER_BLOCK))) {
	/* number of blocks needed to expand +1 if there's a remainder */
	// XXX this has a bug, but don't know why; try: l=[i for i in range(1000)]; for i in l: print(i/3)
	machine_uint_t n_diff = ( n_bytes - n_existing)/BYTES_PER_BLOCK+
	((n_bytes - n_existing)%BYTES_PER_BLOCK!=0);

	DEBUG_printf("gc_realloc: expanding " UINT_FMT " blocks (" UINT_FMT " bytes) to " UINT_FMT " blocks (" UINT_FMT " bytes)\n",
	n_existing/BYTES_PER_BLOCK, n_existing, n_existing/BYTES_PER_BLOCK+n_diff, n_existing + n_diff*BYTES_PER_BLOCK);

	/* mark rest of blocks as used tail */
	for (machine_uint_t bl = block + n_blocks; bl < (block + n_blocks + n_diff); bl++) {
	ATB_FREE_TO_TAIL(bl);
	}
	ptr_out = ptr_in;

	/* try to find a new contiguous chain */
	} else if ((ptr_out = gc_alloc(n_bytes)) != NULL) {
	DEBUG_printf("gc_realloc: allocated new block \n");
	memcpy(ptr_out, ptr_in, n_existing);
	gc_free(ptr_in);
	}
	}

	return ptr_out;
	}
	#endif

	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:");
	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;
	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);
	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 < 25; i+=2) {
	machine_uint_t *p = gc_alloc(i);
	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