[hack] Added a check to decrease latest_heap_size

Summary:
Because in some cases the heap can naturally decrease without garbage collection, latest_heap_size can be set to an abnormally high value, preventing GC from happening again.
To fix this, we maintain a new invariant that the latest_heap_size must be at most the current used_heap_size. Thus, if the heap usage decreases after an oldify, latest_heap_size will also decrease, allowing garbage collection to occur again.

Reviewed By: dabek

Differential Revision: D3673343

fbshipit-source-id: 0948e95fb4b54da4ef6374e7323d68cf0afbec41

hack/heap/hh_shared.c

@@ -1337,8 +1337,16 @@ void hh_collect(value aggressive_val) {
   size_t mem_size = 0;
 
   float space_overhead = aggressive ? 1.2 : 2.0;
-  if(used_heap_size() < (size_t)(space_overhead * latest_heap_size)) {
+  size_t used = used_heap_size();
+  if(used < (size_t)(space_overhead * latest_heap_size)) {
     // We have not grown past twice the size since we last gc'd
+
+    /* We maintain the invariant that the latest_heap_size
+      is at most the current heap size, to make sure the GC
+      always collects */
+    if (used < latest_heap_size) {
+      latest_heap_size = used;
+    }
     return;
   }