Index: llvm/lib/LTO/Caching.cpp
===================================================================
--- llvm/lib/LTO/Caching.cpp
+++ llvm/lib/LTO/Caching.cpp
@@ -71,13 +71,14 @@
                              TempFilename + ": " +
                              MBOrErr.getError().message() + "\n");
 
-        // This is atomic on POSIX systems.
-        // 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 give AddBuffer
-        // a copy of the bytes we wrote in that case. We do this instead of
-        // just using the existing file, because the pruner might delete the
-        // file before we get a chance to use it.
+        // On POSIX systems, rename will atomically replace the destination if
+        // it already exists. We try to emulate this on Windows, but this may
+        // fail with a permission denied error (for example, if the destination
+        // is currently mmapped). Since the existing file should be semantically
+        // equivalent to the one we are trying to write, we give AddBuffer a
+        // copy of the bytes we wrote in that case. We do this instead of just
+        // using the existing file, because the pruner might delete the file
+        // before we get a chance to use it.
         auto EC = sys::fs::rename(TempFilename, EntryPath);
         if (EC == errc::permission_denied) {
           auto MBCopy = MemoryBuffer::getMemBufferCopy(