py: Fix GC realloc issue, where memory chunks were never shrunk.

Previously, a realloc to a smaller memory chunk size would not free the unused blocks in the tail of the chunk.

Author: dpgeorge

py/gc.c

@@ -567,22 +567,28 @@ void *gc_realloc(void *ptr_in, mp_uint_t n_bytes) {
  // 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
+ // Get the total number of consecutive blocks that are already allocated to
+ // this chunk of memory, and then count the number of free blocks following
+ // it. Stop if we reach the end of the heap, or if we find enough extra
+ // free blocks to satisfy the realloc. Note that we need to compute the
+ // total size of the existing memory chunk so we can correctly and
+ // efficiently shrink it (see below for shrinking code).
  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;
+ for (mp_uint_t bl = block + n_blocks; bl < max_block; bl++) {
+ byte block_type = ATB_GET_KIND(bl);
+ if (block_type == AT_TAIL) {
+ n_blocks++;
+ continue;
  }
- 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);
+ if (block_type == AT_FREE) {
+ n_free++;
+ if (n_blocks + n_free >= new_blocks) {
+ // stop as soon as we find enough blocks for n_bytes
+ break;
+ }
+ continue;
  }
  break;
  }
@@ -595,7 +601,7 @@ void *gc_realloc(void *ptr_in, mp_uint_t n_bytes) {
  // 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++) {
+ for (mp_uint_t bl = block + new_blocks, count = n_blocks - new_blocks; count > 0; bl++, count--) {
  ATB_ANY_TO_FREE(bl);
  }