Try to not repeatedly scan through the 'free' bitfield

Seems to help by a couple percent.

src/gc/heap.cpp

@@ -198,16 +198,20 @@ Heap::ThreadBlockCache::~ThreadBlockCache() {
 }
 
 static GCAllocation* allocFromBlock(Block* b) {
-    int i = 0;
     uint64_t mask = 0;
-    for (; i < BITFIELD_ELTS; i++) {
-        mask = b->isfree[i];
+
+    int elts_scanned = 0;
+    for (; b->next_to_check < BITFIELD_ELTS; b->next_to_check++) {
+        elts_scanned++;
+        mask = b->isfree[b->next_to_check];
         if (mask != 0L) {
             break;
         }
     }
+    int i = b->next_to_check;
 
     if (i == BITFIELD_ELTS) {
+        b->next_to_check = 0;
         return NULL;
     }
 

src/gc/heap.h

@@ -69,7 +69,7 @@ static_assert(ATOMS_PER_BLOCK % 64 == 0, "");
 #define BITFIELD_SIZE (ATOMS_PER_BLOCK / 8)
 #define BITFIELD_ELTS (BITFIELD_SIZE / 8)
 
-#define BLOCK_HEADER_SIZE (BITFIELD_SIZE + 2 * sizeof(void*) + sizeof(uint64_t))
+#define BLOCK_HEADER_SIZE (BITFIELD_SIZE + 4 * sizeof(void*))
 #define BLOCK_HEADER_ATOMS ((BLOCK_HEADER_SIZE + ATOM_SIZE - 1) / ATOM_SIZE)
 
 struct Atoms {
@@ -82,6 +82,8 @@ struct Block {
             Block* next, **prev;
             uint64_t size;
             uint64_t isfree[BITFIELD_ELTS];
+            int next_to_check;
+            void* _header_end[0];
         };
         Atoms atoms[ATOMS_PER_BLOCK];
     };
@@ -95,6 +97,8 @@ struct Block {
     static Block* forPointer(void* ptr) { return (Block*)((uintptr_t)ptr & ~(BLOCK_SIZE - 1)); }
 };
 static_assert(sizeof(Block) == BLOCK_SIZE, "bad size");
+static_assert(offsetof(Block, _header_end) >= BLOCK_HEADER_SIZE, "bad header size");
+static_assert(offsetof(Block, _header_end) <= BLOCK_HEADER_SIZE, "bad header size");
 
 constexpr const size_t sizes[] = {
     16,  32,  48,  64,  80,  96,  112, 128,  160,  192,  224,  256,

src/runtime/types.cpp

@@ -252,9 +252,6 @@ extern "C" void listGCHandler(GCVisitor* v, Box* b) {
         v->visit(l->elts);
     if (size)
         v->visitRange((void**)&l->elts->elts[0], (void**)&l->elts->elts[size]);
-
-    static StatCounter sc("gc_listelts_visited");
-    sc.log(size);
 }
 
 extern "C" void setGCHandler(GCVisitor* v, Box* b) {