Use doubly linked list for O(1) free()

src/small_allocator.c

@@ -11,12 +11,14 @@
 typedef struct BlockHeader {
     size_t size_and_flags;       // size | free_bit
     struct BlockHeader* next;
+    struct BlockHeader* prev;
+    // Padding to ensure we always return 16 byte aligned ponters
+    // remove padding & set ALIGNMENT to 8 to get 8 byte alignmet
+    // (for 64 bit systems, 32 bit systems might need other adjustments)
+    uint8_t pad[32 - (sizeof(size_t) + 2*sizeof(void*))];
 } BlockHeader;
 
 
-//Note:
-// On 64‑bit systems, allocated pointers are 16‑byte aligned.
-// On 32‑bit systems, allocated pointers are 8‑byte aligned.
 _Static_assert(sizeof(BlockHeader) % ALIGNMENT == 0,
                "BlockHeader size must be a multiple of ALIGNMENT");
 
@@ -58,17 +60,17 @@ void small_allocator_init(SmallAllocator* a, void* backing_store, size_t size) {
     a->size = size;
 
     BlockHeader* first = (BlockHeader*)a->base;
-    size_t usable = size - sizeof(BlockHeader);
-    usable = ALIGN(usable);
+    size_t usable = ALIGN(size - sizeof(BlockHeader));
 
     first->size_and_flags = usable | BLOCK_FREE;
     first->next = NULL;
+    first->prev = NULL;
 
     a->heap_start = first;
     a->last_alloc = first;
 }
 
-/* --- Block splitting --- */
+/* Splitting */
 
 static void split_block(BlockHeader* block, size_t size) {
     size = ALIGN(size);
@@ -81,35 +83,40 @@ static void split_block(BlockHeader* block, size_t size) {
     BlockHeader* new_block =
         (BlockHeader*)(base + sizeof(BlockHeader) + size);
 
-    size_t new_size = bsize - size - sizeof(BlockHeader);
-    new_size = ALIGN(new_size);
+    size_t new_size = ALIGN(bsize - size - sizeof(BlockHeader));
 
     block_set_size(block, size);
     block_set_used(block);
 
     new_block->size_and_flags = new_size | BLOCK_FREE;
     new_block->next = block->next;
+    new_block->prev = block;
+
+    if (block->next)
+        block->next->prev = new_block;
 
     block->next = new_block;
 }
 
-/* --- Adjacent-only coalescing --- */
+/* Coalescing */
 
 static void merge_with_next(BlockHeader* block) {
     BlockHeader* next = block->next;
     if (!next || !block_is_free(next))
         return;
 
-    size_t new_size = block_size(block) +
-                      sizeof(BlockHeader) +
-                      block_size(next);
+    size_t new_size = ALIGN(block_size(block) +
+                            sizeof(BlockHeader) +
+                            block_size(next));
 
-    new_size = ALIGN(new_size);
     block_set_size(block, new_size);
     block->next = next->next;
+
+    if (next->next)
+        next->next->prev = block;
 }
 
-/* --- Allocation (next-fit) --- */
+/* Allocation (next-fit) */
 
 void* small_alloc(SmallAllocator* a, size_t size) {
     if (size == 0)
@@ -133,7 +140,7 @@ void* small_alloc(SmallAllocator* a, size_t size) {
     return NULL;
 }
 
-/* --- Free --- */
+/* Free */
 
 void small_free(SmallAllocator* a, void* ptr) {
     if (!ptr)
@@ -146,16 +153,10 @@ void small_free(SmallAllocator* a, void* ptr) {
 
     merge_with_next(block);
 
-    BlockHeader* prev = NULL;
-    BlockHeader* curr = a->heap_start;
-
-    while (curr && curr != block) {
-        prev = curr;
-        curr = curr->next;
-    }
-
-    if (prev && block_is_free(prev)) {
+    if (block->prev && block_is_free(block->prev)) {
+        BlockHeader* prev = block->prev;
         merge_with_next(prev);
+
         if (a->last_alloc == block)
             a->last_alloc = prev;
     }