Index: cmake/modules/TableGen.cmake
===================================================================
--- cmake/modules/TableGen.cmake
+++ cmake/modules/TableGen.cmake
@@ -25,7 +25,7 @@
     file(RELATIVE_PATH ofn_rel
       ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_BINARY_DIR}/${ofn})
     set(additional_cmdline
-      -o ${ofn_rel}.tmp
+      -o ${ofn_rel}
       -d ${ofn_rel}.d
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
       DEPFILE ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.d
@@ -36,7 +36,7 @@
     file(GLOB local_tds "*.td")
     file(GLOB_RECURSE global_tds "${LLVM_MAIN_INCLUDE_DIR}/llvm/*.td")
     set(additional_cmdline
-      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
+      -o ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
       )
   endif()
 
@@ -69,8 +69,7 @@
   # dependency twice in the result file when
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
-  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-    # Generate tablegen output in a temporary file.
+  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
@@ -83,20 +82,9 @@
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     COMMENT "Building ${ofn}..."
     )
-  add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
-    # Only update the real output file if there are any differences.
-    # This prevents recompilation of all the files depending on it if there
-    # aren't any.
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different
-        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-        ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
-    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${ofn}.tmp
-    COMMENT "Updating ${ofn}..."
-    )
 
   # `make clean' must remove all those generated files:
-  set_property(DIRECTORY APPEND
-    PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn}.tmp ${ofn})
+  set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${ofn})
 
   set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT} ${CMAKE_CURRENT_BINARY_DIR}/${ofn} PARENT_SCOPE)
   set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/${ofn} PROPERTIES
Index: lib/TableGen/Main.cpp
===================================================================
--- lib/TableGen/Main.cpp
+++ lib/TableGen/Main.cpp
@@ -100,23 +100,39 @@
   if (Parser.ParseFile())
     return 1;
 
-  std::error_code EC;
-  ToolOutputFile Out(OutputFilename, EC, sys::fs::F_Text);
-  if (EC)
-    return reportError(argv0, "error opening " + OutputFilename + ":" +
-                                  EC.message() + "\n");
+  // Write output to memory.
+  std::string OutString;
+  raw_string_ostream Out(OutString);
+  if (MainFn(Out, Records))
+    return 1;
+
+  // Always write the depfile, even if the main output hasn't changed.
+  // If it's missing, Ninja considers the output dirty.  If this was below
+  // the early exit below and someone deleted the .inc.d file but not the .inc
+  // file, tablegen would never write the depfile.
   if (!DependFilename.empty()) {
     if (int Ret = createDependencyFile(Parser, argv0))
       return Ret;
   }
 
-  if (MainFn(Out.os(), Records))
-    return 1;
+  // Only updates the real output file if there are any differences.
+  // This prevents recompilation of all the files depending on it if there
+  // aren't any.
+  if (auto ExistingOrErr = MemoryBuffer::getFile(OutputFilename))
+    if (std::move(ExistingOrErr.get())->getBuffer() == Out.str())
+      return 0;
+
+  std::error_code EC;
+  ToolOutputFile OutFile(OutputFilename, EC, sys::fs::F_Text);
+  if (EC)
+    return reportError(argv0, "error opening " + OutputFilename + ":" +
+                                  EC.message() + "\n");
+  OutFile.os() << Out.str();
 
   if (ErrorsPrinted > 0)
     return reportError(argv0, Twine(ErrorsPrinted) + " errors.\n");
 
   // Declare success.
-  Out.keep();
+  OutFile.keep();
   return 0;
 }
Index: utils/gn/secondary/llvm/utils/TableGen/tablegen.gni
===================================================================
--- utils/gn/secondary/llvm/utils/TableGen/tablegen.gni
+++ utils/gn/secondary/llvm/utils/TableGen/tablegen.gni
@@ -60,11 +60,6 @@
     depfile = "$gen_output.d"
     td_file = rebase_path(td_file, root_build_dir)
 
-    # FIXME: The cmake build lets tablegen write to a temp file and then copies
-    # it over the final output only if it has changed, for ninja's restat
-    # optimization. Instead of doing that in cmake, llvm-tblgen should do this
-    # itself. r330742 tried this, but it caused problems. Fix those and reland,
-    # so that the gn build has the optimization too.
     args = [
              rebase_path(tblgen_executable, root_build_dir),
              "-I",