diff --git a/src/copy-space.h b/src/copy-space.h
index a863ca51a..e51f9f8ee 100644
--- a/src/copy-space.h
+++ b/src/copy-space.h
@@ -10,6 +10,7 @@
 
 #include "assert.h"
 #include "debug.h"
+#include "extents.h"
 #include "gc-align.h"
 #include "gc-attrs.h"
 #include "gc-inline.h"
@@ -100,11 +101,6 @@ copy_space_object_region(struct gc_ref obj) {
   return (gc_ref_value(obj) / COPY_SPACE_REGION_SIZE) & 1;
 }
 
-struct copy_space_extent {
-  uintptr_t low_addr;
-  uintptr_t high_addr;
-};
-
 struct copy_space {
   struct copy_space_block *empty;
   struct copy_space_block *partly_full;
@@ -119,8 +115,7 @@ struct copy_space {
   uint8_t atomic_forward;
   size_t allocated_bytes_at_last_gc;
   size_t fragmentation_at_last_gc;
-  struct copy_space_extent *extents;
-  size_t nextents;
+  struct extents *extents;
   struct copy_space_slab *slabs;
   size_t nslabs;
 };
@@ -542,11 +537,7 @@ copy_space_forward_if_traced(struct copy_space *space, struct gc_edge edge,
 
 static inline int
 copy_space_contains(struct copy_space *space, struct gc_ref ref) {
-  for (size_t i = 0; i < space->nextents; i++)
-    if (space->extents[i].low_addr <= gc_ref_value(ref) &&
-        gc_ref_value(ref) < space->extents[i].high_addr)
-      return 1;
-  return 0;
+  return extents_contain_addr(space->extents, gc_ref_value(ref));
 }
 
 static inline void
@@ -607,10 +598,8 @@ copy_space_init(struct copy_space *space, size_t size, int atomic) {
   space->atomic_forward = atomic;
   space->allocated_bytes_at_last_gc = 0;
   space->fragmentation_at_last_gc = 0;
-  space->extents = calloc(1, sizeof(struct copy_space_extent));
-  space->extents[0].low_addr = (uintptr_t) slabs;
-  space->extents[0].high_addr = space->extents[0].low_addr + reserved;
-  space->nextents = 1;
+  space->extents = extents_allocate(10);
+  extents_adjoin(space->extents, slabs, reserved);
   space->slabs = slabs;
   space->nslabs = nslabs;
   for (size_t slab = 0; slab < nslabs; slab++) {
diff --git a/src/extents.h b/src/extents.h
new file mode 100644
index 000000000..62dba92b9
--- /dev/null
+++ b/src/extents.h
@@ -0,0 +1,88 @@
+#ifndef EXTENTS_H
+#define EXTENTS_H
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "gc-assert.h"
+
+struct extent_range {
+  uintptr_t lo_addr;
+  uintptr_t hi_addr;
+};
+
+struct extents {
+  size_t size;
+  size_t capacity;
+  struct extent_range ranges[];
+};
+
+static inline int
+extents_contain_addr(struct extents *extents, uintptr_t addr) {
+  size_t lo = 0;
+  size_t hi = extents->size;
+  while (lo != hi) {
+    size_t mid = (lo + hi) / 2;
+    struct extent_range range = extents->ranges[mid];
+    if (addr < range.lo_addr) {
+      hi = mid;
+    } else if (addr < range.hi_addr) {
+      return 1;
+    } else {
+      lo = mid + 1;
+    }
+  }
+  return 0;
+}
+
+static struct extents*
+extents_allocate(size_t capacity) {
+  size_t byte_size =
+    sizeof(struct extents) + sizeof(struct extent_range) * capacity;
+  struct extents *ret = malloc(byte_size);
+  if (!ret) __builtin_trap();
+  memset(ret, 0, byte_size);
+  ret->capacity = capacity;
+  return ret;
+}
+
+static struct extents*
+extents_insert(struct extents *old, size_t idx, struct extent_range range) {
+  if (old->size < old->capacity) {
+    size_t bytes_to_move = sizeof(struct extent_range) * (old->size - idx);
+    memmove(&old->ranges[idx + 1], &old->ranges[idx], bytes_to_move);
+    old->ranges[idx] = range;
+    old->size++;
+    return old;
+  } else {
+    struct extents *new_ = extents_allocate(old->capacity * 2 + 1);
+    memcpy(&new_->ranges[0], &old->ranges[0],
+           sizeof(struct extent_range) * idx);
+    memcpy(&new_->ranges[idx + 1], &old->ranges[idx],
+           sizeof(struct extent_range) * (old->size - idx));
+    new_->ranges[idx] = range;
+    new_->size = old->size + 1;
+    free(old);
+    return new_;
+  }
+}
+
+static struct extents*
+extents_adjoin(struct extents *extents, void *lo_addr, size_t size) {
+  size_t i;
+  struct extent_range range = { (uintptr_t)lo_addr, (uintptr_t)lo_addr + size };
+  for (i = 0; i < extents->size; i++) {
+    if (range.hi_addr < extents->ranges[i].lo_addr) {
+      break;
+    } else if (range.hi_addr == extents->ranges[i].lo_addr) {
+      extents->ranges[i].lo_addr = range.lo_addr;
+      return extents;
+    } else if (range.lo_addr == extents->ranges[i].hi_addr) {
+      extents->ranges[i].hi_addr = range.hi_addr;
+      return extents;
+    }
+  }
+  return extents_insert(extents, i, range);
+}
+  
+#endif // EXTENTS_H