diff --git a/llvm/lib/CAS/MappedFileRegionBumpPtr.cpp b/llvm/lib/CAS/MappedFileRegionBumpPtr.cpp
index 3c01331c61b81..e1f73dd9fc048 100644
--- a/llvm/lib/CAS/MappedFileRegionBumpPtr.cpp
+++ b/llvm/lib/CAS/MappedFileRegionBumpPtr.cpp
@@ -53,7 +53,6 @@
 
 #include "llvm/CAS/MappedFileRegionBumpPtr.h"
 #include "OnDiskCommon.h"
-#include "llvm/ADT/StringMap.h"
 #include "llvm/CAS/OnDiskCASLogger.h"
 
 using namespace llvm;
@@ -196,6 +195,8 @@ void MappedFileRegionBumpPtr::destroyImpl() {
       size_t Size = size();
       size_t Capacity = capacity();
       assert(Size < Capacity);
+      // sync to file system to make sure all contents are up-to-date.
+      (void)Region.sync();
       (void)sys::fs::resize_file(*FD, size());
       (void)unlockFileThreadSafe(*SharedLockFD);
 
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index 891aba31d3451..50ef812b60397 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -876,6 +876,12 @@ void mapped_file_region::unmapImpl() {
     ::munmap(Mapping, Size);
 }
 
+std::error_code mapped_file_region::sync() const {
+  if (int Res = ::msync(Mapping, Size, MS_SYNC))
+    return std::error_code(Res, std::generic_category());
+  return std::error_code();
+}
+
 void mapped_file_region::dontNeedImpl() {
   assert(Mode == mapped_file_region::readonly);
   if (!Mapping)
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index 39f8c6925a502..736fd1f58b709 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -1011,6 +1011,12 @@ void mapped_file_region::unmapImpl() {
 
 void mapped_file_region::dontNeedImpl() {}
 
+std::error_code mapped_file_region::sync() const {
+  if (::FlushViewOfFile(Mapping, 0))
+    return std::error_code();
+  return mapWindowsError(::GetLastError());
+}
+
 int mapped_file_region::alignment() {
   SYSTEM_INFO SysInfo;
   ::GetSystemInfo(&SysInfo);