Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -422,19 +422,16 @@
 }
 
 std::error_code resize_file(int FD, uint64_t Size) {
-#if defined(HAVE_POSIX_FALLOCATE)
-  // If we have posix_fallocate use it. Unlike ftruncate it always allocates
-  // space, so we get an error if the disk is full.
-  if (int Err = ::posix_fallocate(FD, 0, Size)) {
-    if (Err != EOPNOTSUPP)
-      return std::error_code(Err, std::generic_category());
-  }
-#endif
-  // Use ftruncate as a fallback. It may or may not allocate space. At least on
-  // OS X with HFS+ it does.
+  // Resize the file using ftruncate. ftruncate may not allocate actual disk
+  // blocks, so even if ftruncate succeeds, subsequent writes may fail due to
+  // "disk full" errors.
+  //
+  // posix_fallocate guarantees that it allocates all disk blocks, but the
+  // function is slow unless your filesystem supports the fallocate system
+  // call. When it falls back, it writes null bytes on each block. So we don't
+  // want to use the function.
   if (::ftruncate(FD, Size) == -1)
     return std::error_code(errno, std::generic_category());
-
   return std::error_code();
 }