remove the larger buckets, and hoist some math out of loops.

For some reason the larger bucket sizes are causing a large perf hit
in spectral_norm.  It's unclear exactly why this is happening, but
theories are legion.  More investigation is warranted, but this gets us
back from the perf regression.

Also hoist the atom_idx calculation out of a couple of loops that were
iterating over object indices.

src/gc/heap.cpp

@@ -359,8 +359,8 @@ SmallArena::Block** SmallArena::_freeChain(Block** head) {
         int first_obj = b->minObjIndex();
         int atoms_per_obj = b->atomsPerObj();
 
-        for (int obj_idx = first_obj; obj_idx < num_objects; obj_idx++) {
-            int atom_idx = obj_idx * atoms_per_obj;
+        for (int atom_idx = first_obj * atoms_per_obj; atom_idx < num_objects * atoms_per_obj;
+             atom_idx += atoms_per_obj) {
 
             if (b->isfree.isSet(atom_idx))
                 continue;
@@ -507,8 +507,8 @@ void SmallArena::_getChainStatistics(HeapStatistics* stats, Block** head) {
         int first_obj = b->minObjIndex();
         int atoms_per_obj = b->atomsPerObj();
 
-        for (int obj_idx = first_obj; obj_idx < num_objects; obj_idx++) {
-            int atom_idx = obj_idx * atoms_per_obj;
+        for (int atom_idx = first_obj * atoms_per_obj; atom_idx < num_objects * atoms_per_obj;
+             atom_idx += atoms_per_obj) {
 
             if (b->isfree.isSet(atom_idx))
                 continue;

src/gc/heap.h

@@ -102,8 +102,8 @@ constexpr uintptr_t HUGE_ARENA_START = 0x3270000000L;
 // bitmap for objects of a given size (constant for the block)
 //
 static const size_t sizes[] = {
-    16,  32,  48,  64,  80,  96,   112,  128,  160,  192,  224,  256,  320,  384,
-    448, 512, 640, 768, 896, 1024, 1280, 1536, 1792, 2048, 2560, 3072, 3584, // 4096,
+    16,  32,  48,  64,  80,  96,  112, 128, 160, 192,
+    224, 256, 320, 384, 448, 512, 640, 768, 896, 1024 //, 1280, 1536, 1792, 2048, 2560, 3072, 3584, // 4096,
 };
 static constexpr size_t NUM_BUCKETS = sizeof(sizes) / sizeof(sizes[0]);