py/gc.c

/*
 * This file is part of the Micro Python project, http://micropython.org/
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2013, 2014 Damien P. George
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <stdbool.h>

#include "mpconfig.h"
#include "misc.h"
#include "gc.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 mp_uint_t gc_alloc_table_byte_len;
#if MICROPY_ENABLE_FINALISER
STATIC byte *gc_finaliser_table_start;
#endif
STATIC mp_uint_t *gc_pool_start;
STATIC mp_uint_t *gc_pool_end;

STATIC int gc_stack_overflow;
STATIC mp_uint_t gc_stack[STACK_SIZE];
STATIC mp_uint_t *gc_sp;
STATIC mp_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) - (mp_uint_t)gc_pool_start) / BYTES_PER_BLOCK)
#define PTR_FROM_BLOCK(block) (((block) * BYTES_PER_BLOCK + (mp_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*)((mp_uint_t)end & (~(BYTES_PER_BLOCK - 1)));
    DEBUG_printf("Initializing GC heap: %p..%p = " UINT_FMT " bytes\n", start, end, (byte*)end - (byte*)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)
    mp_uint_t total_byte_len = (byte*)end - (byte*)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
    mp_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

    mp_uint_t gc_pool_block_len = gc_alloc_table_byte_len * BLOCKS_PER_ATB;
    gc_pool_start = (mp_uint_t*)((byte*)end - gc_pool_block_len * BYTES_PER_BLOCK);
    gc_pool_end = (mp_uint_t*)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--;
}

bool gc_is_locked(void) {
    return gc_lock_depth != 0;
}

#define VERIFY_PTR(ptr) ( \
        (ptr & (BYTES_PER_BLOCK - 1)) == 0          /* must be aligned on a block */ \
        && ptr >= (mp_uint_t)gc_pool_start     /* must be above start of pool */ \
        && ptr < (mp_uint_t)gc_pool_end        /* must be below end of pool */ \
    )

#define VERIFY_MARK_AND_PUSH(ptr) \
    do { \
        if (VERIFY_PTR(ptr)) { \
            mp_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
        mp_uint_t block = *--gc_sp;

        // work out number of consecutive blocks in the chain starting with this one
        mp_uint_t n_blocks = 0;
        do {
            n_blocks += 1;
        } while (ATB_GET_KIND(block + n_blocks) == AT_TAIL);

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

#if MICROPY_PY_GC_COLLECT_RETVAL
uint gc_collected;
#endif

STATIC void gc_sweep(void) {
    #if MICROPY_PY_GC_COLLECT_RETVAL
    gc_collected = 0;
    #endif
    // free unmarked heads and their tails
    int free_tail = 0;
    for (mp_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;
                #if MICROPY_PY_GC_COLLECT_RETVAL
                gc_collected++;
                #endif
                // fall through to free the head

            case AT_TAIL:
                if (free_tail) {
                    DEBUG_printf("gc_sweep(%p)\n",PTR_FROM_BLOCK(block));
                    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, mp_uint_t len) {
    for (mp_uint_t i = 0; i < len; i++) {
        mp_uint_t ptr = (mp_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(mp_uint_t);
    info->used = 0;
    info->free = 0;
    info->num_1block = 0;
    info->num_2block = 0;
    info->max_block = 0;
    for (mp_uint_t block = 0, len = 0; block < gc_alloc_table_byte_len * BLOCKS_PER_ATB; block++) {
        mp_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(mp_uint_t n_bytes, bool has_finaliser) {
    mp_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;
    }

    mp_uint_t i;
    mp_uint_t end_block;
    mp_uint_t start_block;
    mp_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 (mp_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);
    DEBUG_printf("gc_alloc(%p)\n", ret_ptr);

    // zero out the additional bytes of 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((byte*)ret_ptr + n_bytes, 0, (end_block - start_block + 1) * BYTES_PER_BLOCK - n_bytes);

#if MICROPY_ENABLE_FINALISER
    if (has_finaliser) {
        // clear type pointer in case it is never set
        ((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(mp_uint_t n_bytes) {
    return _gc_alloc(n_bytes, false);
}

void *gc_alloc_with_finaliser(mp_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;
    }

    mp_uint_t ptr = (mp_uint_t)ptr_in;
    DEBUG_printf("gc_free(%p)\n", ptr);

    if (VERIFY_PTR(ptr)) {
        mp_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);
        }
    }
}

mp_uint_t gc_nbytes(void *ptr_in) {
    mp_uint_t ptr = (mp_uint_t)ptr_in;

    if (VERIFY_PTR(ptr)) {
        mp_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
            mp_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, mp_uint_t n_bytes) {
    mp_uint_t n_existing = gc_nbytes(ptr);
    if (n_bytes <= n_existing) {
        return ptr;
    } else {
        bool has_finaliser;
        if (ptr == NULL) {
            has_finaliser = false;
        } else {
#if MICROPY_ENABLE_FINALISER
            has_finaliser = FTB_GET(BLOCK_FROM_PTR((mp_uint_t)ptr));
#else
            has_finaliser = false;
#endif
        }
        void *ptr2 = gc_alloc(n_bytes, has_finaliser);
        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, mp_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);
    }

    mp_uint_t ptr = (mp_uint_t)ptr_in;

    // sanity check the ptr
    if (!VERIFY_PTR(ptr)) {
        return NULL;
    }

    // get first block
    mp_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
    mp_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
    mp_uint_t n_free   = 0;
    mp_uint_t n_blocks = 1; // counting HEAD block
    mp_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 (mp_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 (mp_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 additional bytes of the newly allocated blocks (see comment above in gc_alloc)
        memset((byte*)ptr_in + n_bytes, 0, new_blocks * BYTES_PER_BLOCK - n_bytes);

        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(%p -> %p)\n", ptr_in, ptr_out);
    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 (mp_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: {
                mp_uint_t *ptr = gc_pool_start + bl * WORDS_PER_BLOCK;
                if (*ptr == (mp_uint_t)&mp_type_tuple) { c = 'T'; }
                else if (*ptr == (mp_uint_t)&mp_type_list) { c = 'L'; }
                else if (*ptr == (mp_uint_t)&mp_type_dict) { c = 'D'; }
                else if (*ptr == (mp_uint_t)&mp_type_float) { c = 'F'; }
                else if (*ptr == (mp_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) {
    mp_uint_t len = 500;
    mp_uint_t *heap = malloc(len);
    gc_init(heap, heap + len / sizeof(mp_uint_t));
    void *ptrs[100];
    {
        mp_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);
        mp_uint_t ***p2 = gc_alloc(16, false);
        p2[0] = p;
        p2[1] = p;
        ptrs[0] = p2;
    }
    for (int i = 0; i < 25; i+=2) {
        mp_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