diff --git a/src/boot/boot.janet b/src/boot/boot.janet
index 58c46cb5..cbbf06e4 100644
--- a/src/boot/boot.janet
+++ b/src/boot/boot.janet
@@ -390,6 +390,16 @@
          ,;body
          (set ,i (,delta ,i ,step))))))
 
+(defn- for-var-template
+  [i start stop step comparison delta body]
+  (with-syms [s]
+    ~(do
+       (var ,i ,start)
+       (def ,s ,stop)
+       (while (,comparison ,i ,s)
+         ,;body
+         (set ,i (,delta ,i ,step))))))
+
 (defn- check-indexed [x]
   (if (indexed? x)
     x
@@ -484,6 +494,12 @@
       :generate (loop-fiber-template binding object [rest])
       (error (string "unexpected loop verb " verb)))))
 
+(defmacro forv
+  "Do a c style for loop for side effects. The iteration variable i
+  can be mutated in the loop, unlike normal for. Returns nil."
+  [i start stop & body]
+  (for-var-template i start stop 1 < + body))
+
 (defmacro for
   "Do a c style for loop for side effects. Returns nil."
   [i start stop & body]
@@ -556,6 +572,7 @@
 (put _env 'loop1 nil)
 (put _env 'check-indexed nil)
 (put _env 'for-template nil)
+(put _env 'for-var-template nil)
 (put _env 'iterate-template nil)
 (put _env 'each-template nil)
 (put _env 'keys-template nil)
diff --git a/src/core/gc.c b/src/core/gc.c
index 36752b06..e534d9ed 100644
--- a/src/core/gc.c
+++ b/src/core/gc.c
@@ -39,6 +39,7 @@ struct JanetScratch {
 JANET_THREAD_LOCAL void *janet_vm_blocks;
 JANET_THREAD_LOCAL size_t janet_vm_gc_interval;
 JANET_THREAD_LOCAL size_t janet_vm_next_collection;
+JANET_THREAD_LOCAL size_t janet_vm_block_count;
 JANET_THREAD_LOCAL int janet_vm_gc_suspend = 0;
 
 /* Roots */
@@ -327,6 +328,7 @@ void janet_sweep() {
             previous = current;
             current->flags &= ~JANET_MEM_REACHABLE;
         } else {
+            janet_vm_block_count--;
             janet_deinit_block(current);
             if (NULL != previous) {
                 previous->next = next;
@@ -359,6 +361,7 @@ void *janet_gcalloc(enum JanetMemoryType type, size_t size) {
     janet_vm_next_collection += size;
     mem->next = janet_vm_blocks;
     janet_vm_blocks = mem;
+    janet_vm_block_count++;
 
     return (void *)mem;
 }
@@ -388,6 +391,14 @@ void janet_collect(void) {
     uint32_t i;
     if (janet_vm_gc_suspend) return;
     depth = JANET_RECURSION_GUARD;
+    /* Try and prevent many major collections back to back.
+     * A full collection will take O(janet_vm_block_count) time.
+     * If we have a large heap, make sure our interval is not too
+     * small so we won't make many collections over it. This is just a
+     * heuristic for automatically changing the gc interval */
+    if (janet_vm_block_count * 8 > janet_vm_gc_interval) {
+        janet_vm_gc_interval = janet_vm_block_count * sizeof(JanetGCObject);
+    }
     orig_rootcount = janet_vm_root_count;
 #ifdef JANET_NET
     janet_net_markloop();
diff --git a/src/core/state.h b/src/core/state.h
index 7ead6368..c36f1c63 100644
--- a/src/core/state.h
+++ b/src/core/state.h
@@ -71,6 +71,7 @@ extern JANET_THREAD_LOCAL uint32_t janet_vm_cache_deleted;
 extern JANET_THREAD_LOCAL void *janet_vm_blocks;
 extern JANET_THREAD_LOCAL size_t janet_vm_gc_interval;
 extern JANET_THREAD_LOCAL size_t janet_vm_next_collection;
+extern JANET_THREAD_LOCAL size_t janet_vm_block_count;
 extern JANET_THREAD_LOCAL int janet_vm_gc_suspend;
 
 /* GC roots */
diff --git a/src/core/vm.c b/src/core/vm.c
index 6e7a0c3d..f65cfa50 100644
--- a/src/core/vm.c
+++ b/src/core/vm.c
@@ -1395,11 +1395,8 @@ int janet_init(void) {
     /* Garbage collection */
     janet_vm_blocks = NULL;
     janet_vm_next_collection = 0;
-    /* Setting memoryInterval to zero forces
-     * a collection pretty much every cycle, which is
-     * incredibly horrible for performance, but can help ensure
-     * there are no memory bugs during development */
     janet_vm_gc_interval = 0x400000;
+    janet_vm_block_count = 0;
     janet_symcache_init();
     /* Initialize gc roots */
     janet_vm_roots = NULL;