Index: llvm/lib/LTO/Caching.cpp
===================================================================
--- llvm/lib/LTO/Caching.cpp
+++ llvm/lib/LTO/Caching.cpp
@@ -17,6 +17,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
@@ -72,7 +73,20 @@
                              MBOrErr.getError().message() + "\n");
 
         // This is atomic on POSIX systems.
-        if (auto EC = sys::fs::rename(TempFilename, EntryPath))
+        // On Windows, it can fail with permission denied if the destination
+        // file already exists. Since the existing file should be semantically
+        // equivalent to the one we are trying to write, we count that as
+        // a success if we can open the destination file for reading.
+        auto EC = sys::fs::rename(TempFilename, EntryPath);
+        if (EC == errc::permission_denied) {
+          int FD;
+          EC = sys::fs::openFileForRead(EntryPath, FD);
+          if (!EC) {
+            sys::Process::SafelyCloseFileDescriptor(FD);
+            sys::fs::remove(TempFilename);
+          }
+        }
+        if (EC)
           report_fatal_error(Twine("Failed to rename temporary file ") +
                              TempFilename + " to " + EntryPath + ": " +
                              EC.message() + "\n");