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_WINDOWS
+    /* 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) {
+      MmapFixedOrDie(region_beg, region->mapped_user,
+                                      "SizeClassAllocator: region data");
+      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
@@ -334,8 +334,12 @@
 }
 
 void ReleaseMemoryPagesToOS(uptr beg, uptr end) {
-  // This is almost useless on 32-bits.
-  // FIXME: add madvise-analog when we move to 64-bits.
+  uptr beg_aligned = RoundDownTo(beg, GetPageSizeCached()),
+       end_aligned = RoundDownTo(end, GetPageSizeCached());
+  CHECK(beg < end);                // make sure the region is sane
+  if (beg_aligned == end_aligned)  // make sure we're freeing at least 1 page;
+    return;
+  UnmapOrDie((void *)beg, end_aligned - beg_aligned);
 }
 
 void SetShadowRegionHugePageMode(uptr addr, uptr size) {
Index: compiler-rt/test/asan/TestCases/Windows/sanitizer_purge.cpp
===================================================================
--- /dev/null
+++ compiler-rt/test/asan/TestCases/Windows/sanitizer_purge.cpp
@@ -0,0 +1,31 @@
+#include <Windows.h>
+#include <stdio.h>
+#include <sanitizer/allocator_interface.h>
+#include <psapi.h>
+
+// RUN: %clang_cl_asan -Od %s -Fe%t 
+// RUN: %t
+// REQUIRES: asan-64-bits
+
+size_t GetRSS() {
+  PROCESS_MEMORY_COUNTERS counters;
+  if (!GetProcessMemoryInfo(GetCurrentProcess(), &counters, sizeof(counters)))
+    return 0;
+  return counters.WorkingSetSize;
+}
+
+int main(){
+    for (int i = 0; i < 1000; i++) {
+        void* a = malloc(1000);
+        free(a);
+    }
+    size_t rss_pre  = GetRSS();
+    __sanitizer_purge_allocator();
+    size_t rss_post = GetRSS();
+
+    if (rss_pre <= rss_post){
+        return -1;
+    }
+
+    return 0;
+}
\ No newline at end of file