gc: "new" gc_realloc: Rewrite in plain C, fixing bunch of bugs.

There were typos, various rounding errors trying to do concurrent counting
in bytes vs blocks, complex conditional paths, superfluous variables, etc.,
etc., all leading to obscure segfaults.

py/gc.c

+#include <assert.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdbool.h>
@@ -459,16 +460,18 @@ 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, false);
     }
 
-    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
+    machine_uint_t new_blocks = (n_bytes + BYTES_PER_BLOCK) / BYTES_PER_BLOCK;
+    // get first block
+    machine_uint_t block = BLOCK_FROM_PTR(ptr);
+
+    // Sabity checks
+    if (VERIFY_PTR(ptr) && ATB_GET_KIND(block) == AT_HEAD) {
 
         byte block_type;
         machine_uint_t n_free   = 0;
@@ -478,42 +481,39 @@ void *gc_realloc(void *ptr_in, machine_uint_t n_bytes) {
         // 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
-        while ((block + n_blocks + n_free) < max_block
+        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
-                && (n_bytes > ((n_blocks+n_free) * BYTES_PER_BLOCK))
-                // stop if block is HEAD
-                && (block_type = ATB_GET_KIND(block + n_blocks + n_free)) != AT_HEAD) {
+                break;
+            }
+            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;
+                case AT_FREE: n_free++; continue;
+                case AT_TAIL: n_blocks++; continue;
+                case AT_MARK: assert(0);
             }
+            break;
         }
-        // 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;
+        if (new_blocks == n_blocks) {
+            return ptr_in;
+        }
+
+        if (new_blocks < n_blocks) {
             // free unneeded tail blocks
-            for (machine_uint_t bl = block + n_blocks; ATB_GET_KIND(bl) == AT_TAIL; bl++) {
+            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
-        } else if (n_bytes <= (n_existing + (n_free * BYTES_PER_BLOCK))) {
-            // number of blocks needed to expand +1 if there's a remainder
-            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++) {
+        } else 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);
             }
-            ptr_out = ptr_in;
+            return ptr_in;
 
         // try to find a new contiguous chain
         } else if ((ptr_out = gc_alloc(n_bytes,
@@ -524,12 +524,13 @@ void *gc_realloc(void *ptr_in, machine_uint_t n_bytes) {
 #endif
                         )) != NULL) {
             DEBUG_printf("gc_realloc: allocating new block\n");
-            memcpy(ptr_out, ptr_in, n_existing);
+            memcpy(ptr_out, ptr_in, n_blocks * BYTES_PER_BLOCK);
             gc_free(ptr_in);
+            return ptr_out;
         }
     }
 
-    return ptr_out;
+    return NULL;
 }
 #endif // Alternative gc_realloc impl