Index: compiler-rt/lib/sanitizer_common/sanitizer_allocator_primary64.h
===================================================================
--- compiler-rt/lib/sanitizer_common/sanitizer_allocator_primary64.h
+++ compiler-rt/lib/sanitizer_common/sanitizer_allocator_primary64.h
@@ -144,6 +144,17 @@
     CompactPtrT *free_array = GetFreeArray(region_beg);
 
     BlockingMutexLock l(&region->mutex);
+#if SANITIZER_WINDOWS64
+    /* On Windows unmapping of memory during __sanitizer_purge_allocator is
+    explicit and immediate, so unmapped regions must be explicitly mapped back
+    in when they are accessed again. */
+    if (region->rtoi.last_released_bytes > 0) {
+      if (!MmapFixedOrDie(region_beg, region->mapped_user, "SizeClassAllocator: region data") )
+        return false;
+      region->rtoi.n_freed_at_last_release = 0;
+      region->rtoi.last_released_bytes = 0;
+    }
+#endif
     if (UNLIKELY(region->num_freed_chunks < n_chunks)) {
       if (UNLIKELY(!PopulateFreeArray(stat, class_id, region,
                                       n_chunks - region->num_freed_chunks)))
Index: compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
===================================================================
--- compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
+++ compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
@@ -334,10 +334,17 @@
 }
 
 void ReleaseMemoryPagesToOS(uptr beg, uptr end) {
-  // This is almost useless on 32-bits.
-  // FIXME: add madvise-analog when we move to 64-bits.
+#if SANITIZER_WINDOWS64
+  uptr beg_aligned = (beg & ~0xfff), end_aligned = (end & ~0xFFF);
+  CHECK(beg < end);  // make sure the region is sane
+  if (beg_aligned == end_aligned)  // make sure we're freeing at least 1 page;
+    return;
+  size_t size = end_aligned - beg_aligned;
+  UnmapOrDie((void *)beg, size);
+#endif
 }
 
+
 void SetShadowRegionHugePageMode(uptr addr, uptr size) {
   // FIXME: probably similar to ReleaseMemoryToOS.
 }