Add heuristics to choose when to compact or mark in place

We can choose to compact (evacuate) or mark in place.  What we choose
has some effects on how we mark.

whippet.h

@@ -288,6 +288,11 @@ struct mark_space {
   uintptr_t fragmentation_granules_since_last_collection; // atomically
 };
 
+enum gc_kind {
+  GC_KIND_MARK_IN_PLACE,
+  GC_KIND_COMPACT
+};
+
 struct heap {
   struct mark_space mark_space;
   struct large_object_space large_object_space;
@@ -297,6 +302,7 @@ struct heap {
   pthread_cond_t mutator_cond;
   size_t size;
   int collecting;
+  enum gc_kind gc_kind;
   int multithreaded;
   size_t active_mutator_count;
   size_t mutator_count;
@@ -605,7 +611,7 @@ static void enqueue_mutator_for_tracing(struct mutator *mut) {
 }
 
 static int heap_should_mark_while_stopping(struct heap *heap) {
-  return 1;
+  return atomic_load(&heap->gc_kind) == GC_KIND_MARK_IN_PLACE;
 }
 
 static int mutator_should_mark_while_stopping(struct mutator *mut) {
@@ -805,6 +811,42 @@ static double heap_fragmentation(struct heap *heap) {
 
 static void determine_collection_kind(struct heap *heap,
                                       enum gc_reason reason) {
+  switch (reason) {
+    case GC_REASON_LARGE_ALLOCATION:
+      // We are collecting because a large allocation could not find
+      // enough free blocks, and we decided not to expand the heap.
+      // Let's evacuate to maximize the free block yield.
+      heap->gc_kind = GC_KIND_COMPACT;
+      break;
+    case GC_REASON_SMALL_ALLOCATION: {
+      // We are making a small allocation and ran out of blocks.
+      // Evacuate if the heap is "too fragmented", where fragmentation
+      // is measured as a percentage of granules that couldn't be used
+      // for allocations in the last cycle.
+      double fragmentation = heap_fragmentation(heap);
+      if (atomic_load(&heap->gc_kind) == GC_KIND_COMPACT) {
+        // For some reason, we already decided to compact in the past.
+        // Keep going until we measure that wasted space due to
+        // fragmentation is below a low-water-mark.
+        if (fragmentation < heap->fragmentation_low_threshold) {
+          DEBUG("returning to in-place collection, fragmentation %.2f%% < %.2f%%\n",
+                fragmentation * 100.,
+                heap->fragmentation_low_threshold * 100.);
+          atomic_store(&heap->gc_kind, GC_KIND_MARK_IN_PLACE);
+        }
+      } else {
+        // Otherwise switch to evacuation mode if the heap is too
+        // fragmented.
+        if (fragmentation > heap->fragmentation_high_threshold) {
+          DEBUG("triggering compaction due to fragmentation %.2f%% > %.2f%%\n",
+                fragmentation * 100.,
+                heap->fragmentation_high_threshold * 100.);
+          atomic_store(&heap->gc_kind, GC_KIND_COMPACT);
+        }
+      }
+      break;
+    }
+  }
 }
 
 static void collect(struct mutator *mut, enum gc_reason reason) {