Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1034,7 +1034,7 @@
   HelpText<"Don't use __cxa_atexit for calling destructors">;
 def fno_use_init_array : Flag<["-"], "fno-use-init-array">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Don't use .init_array instead of .ctors">;
-def fno_unit_at_a_time : Flag<["-"], "fno-unit-at-a-time">, Group<f_Group>;
+def fno_unit_at_a_time : Flag<["-"], "fno-unit-at-a-time">, Group<f_Group>, Flags<[CC1Option]>;
 def fno_unwind_tables : Flag<["-"], "fno-unwind-tables">, Group<f_Group>;
 def fno_verbose_asm : Flag<["-"], "fno-verbose-asm">, Group<f_Group>;
 def fno_working_directory : Flag<["-"], "fno-working-directory">, Group<f_Group>;
@@ -1242,7 +1242,7 @@
 def ftrapv_handler : Separate<["-"], "ftrapv-handler">, Group<f_Group>, Flags<[CC1Option]>;
 def ftrap_function_EQ : Joined<["-"], "ftrap-function=">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Issue call to specified function rather than a trap instruction">;
-def funit_at_a_time : Flag<["-"], "funit-at-a-time">, Group<f_Group>;
+def funit_at_a_time : Flag<["-"], "funit-at-a-time">, Group<f_Group>, Flags<[CC1Option]>;
 def funroll_loops : Flag<["-"], "funroll-loops">, Group<f_Group>,
   HelpText<"Turn on loop unroller">, Flags<[CC1Option]>;
 def fno_unroll_loops : Flag<["-"], "fno-unroll-loops">, Group<f_Group>,
Index: include/clang/Frontend/CodeGenOptions.def
===================================================================
--- include/clang/Frontend/CodeGenOptions.def
+++ include/clang/Frontend/CodeGenOptions.def
@@ -211,6 +211,8 @@
 CODEGENOPT(WholeProgramVTables, 1, 0) ///< Whether to apply whole-program
                                       ///  vtable optimization.
 
+CODEGENOPT(UnitAtTime, 1, 1) ///< Whether to use IPO.
+
 /// Whether to use public LTO visibility for entities in std and stdext
 /// namespaces. This is enabled by clang-cl's /MT and /MTd flags.
 CODEGENOPT(LTOVisibilityPublicStd, 1, 0)
Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -332,6 +332,8 @@
   PMBuilder.PrepareForLTO = CodeGenOpts.PrepareForLTO;
   PMBuilder.RerollLoops = CodeGenOpts.RerollLoops;
 
+  PMBuilder.DisableUnitAtATime = !CodeGenOpts.UnitAtTime;
+
   // Add target-specific passes that need to run as early as possible.
   if (TM)
     PMBuilder.addExtension(
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -6193,13 +6193,8 @@
       CmdArgs.push_back("-fno-dollars-in-identifiers");
   }
 
-  // -funit-at-a-time is default, and we don't support -fno-unit-at-a-time for
-  // practical purposes.
-  if (Arg *A = Args.getLastArg(options::OPT_funit_at_a_time,
-                               options::OPT_fno_unit_at_a_time)) {
-    if (A->getOption().matches(options::OPT_fno_unit_at_a_time))
-      D.Diag(diag::warn_drv_clang_unsupported) << A->getAsString(Args);
-  }
+  Args.AddLastArg(CmdArgs, options::OPT_funit_at_a_time,
+                  options::OPT_fno_unit_at_a_time);
 
   if (Args.hasFlag(options::OPT_fapple_pragma_pack,
                    options::OPT_fno_apple_pragma_pack, false))
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -497,6 +497,8 @@
   Opts.DebugColumnInfo = Args.hasArg(OPT_dwarf_column_info);
   Opts.EmitCodeView = Args.hasArg(OPT_gcodeview);
   Opts.WholeProgramVTables = Args.hasArg(OPT_fwhole_program_vtables);
+  Opts.UnitAtTime = Args.hasFlag(OPT_funit_at_a_time,
+                                 OPT_fno_unit_at_a_time, true);
   Opts.LTOVisibilityPublicStd = Args.hasArg(OPT_flto_visibility_public_std);
   Opts.SplitDwarfFile = Args.getLastArgValue(OPT_split_dwarf_file);
   Opts.SplitDwarfInlining = !Args.hasArg(OPT_fno_split_dwarf_inlining);
Index: test/Driver/clang_f_opts.c
===================================================================
--- test/Driver/clang_f_opts.c
+++ test/Driver/clang_f_opts.c
@@ -286,6 +286,8 @@
 // RUN:     -malign-functions=100                                             \
 // RUN:     -malign-loops=100                                                 \
 // RUN:     -malign-jumps=100                                                 \
+// RUN:     -funit-at-a-time                                                  \
+// RUN:     -fno-unit-at-a-time                                               \
 // RUN:     %s 2>&1 | FileCheck --check-prefix=IGNORE %s
 // IGNORE-NOT: error: unknown argument