Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1949,7 +1949,7 @@
 def fapple_link_rtlib : Flag<["-"], "fapple-link-rtlib">, Group<f_Group>,
   HelpText<"Force linking the clang builtins runtime library">;
 def flto_EQ : Joined<["-"], "flto=">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
-  HelpText<"Set LTO mode to either 'full' or 'thin'">, Values<"thin,full">;
+  HelpText<"Set LTO mode to either 'full' or 'thin'">, Values<"thin,full,auto,jobserver">;
 def flto : Flag<["-"], "flto">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Enable LTO in 'full' mode">;
 def fno_lto : Flag<["-"], "fno-lto">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
@@ -4244,8 +4244,6 @@
 defm branch_count_reg : BooleanFFlag<"branch-count-reg">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm default_inline : BooleanFFlag<"default-inline">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm fat_lto_objects : BooleanFFlag<"fat-lto-objects">, Group<clang_ignored_gcc_optimization_f_Group>;
-def : Flag<["-"], "flto=auto">, Group<clang_ignored_gcc_optimization_f_Group>;
-def : Flag<["-"], "flto=jobserver">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm float_store : BooleanFFlag<"float-store">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm friend_injection : BooleanFFlag<"friend-injection">, Group<clang_ignored_f_Group>;
 defm function_attribute_list : BooleanFFlag<"function-attribute-list">, Group<clang_ignored_f_Group>;
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -609,6 +609,8 @@
 
   LTOMode = llvm::StringSwitch<LTOKind>(LTOName)
                 .Case("full", LTOK_Full)
+                .Case("auto", LTOK_Full)      // GCC compatibility
+                .Case("jobserver", LTOK_Full) // GCC compatibility
                 .Case("thin", LTOK_Thin)
                 .Default(LTOK_Unknown);
 
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -1707,8 +1707,7 @@
     StringRef S = A->getValue();
     if (S == "thin")
       Opts.PrepareForThinLTO = true;
-    else if (S != "full")
-      Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << S;
+    // -flto=full and invalid values are handled elsewhere.
   }
   if (Arg *A = Args.getLastArg(OPT_fthinlto_index_EQ)) {
     if (IK.getLanguage() != Language::LLVM_IR)
Index: clang/test/CodeGen/thinlto-backend-option.ll
===================================================================
--- clang/test/CodeGen/thinlto-backend-option.ll
+++ clang/test/CodeGen/thinlto-backend-option.ll
@@ -12,6 +12,3 @@
 ; RUN: llvm-lto -thinlto -o %t %t.o
 ; RUN: not %clang_cc1 -x ir %t.o -fthinlto-index=%t.thinlto.bc -mllvm -nonexistent -emit-obj -o /dev/null 2>&1 | FileCheck %s -check-prefix=UNKNOWN
 ; UNKNOWN: clang (LLVM option parsing): Unknown command line argument '-nonexistent'
-
-; RUN: not %clang_cc1 -flto=thinfoo 2>&1 | FileCheck %s -check-prefix=INVALID
-; INVALID: error: invalid value 'thinfoo' in '-flto=thinfoo'
Index: clang/test/Driver/autocomplete.c
===================================================================
--- clang/test/Driver/autocomplete.c
+++ clang/test/Driver/autocomplete.c
@@ -72,7 +72,9 @@
 // FFPALL-NEXT: off
 // FFPALL-NEXT: on
 // RUN: %clang --autocomplete=-flto= | FileCheck %s -check-prefix=FLTOALL
-// FLTOALL: full
+// FLTOALL: auto
+// FLTOALL-NEXT: full
+// FLTOALL-NEXT: jobserver
 // FLTOALL-NEXT: thin
 // RUN: %clang --autocomplete=-fveclib= | FileCheck %s -check-prefix=FVECLIBALL
 // FVECLIBALL: Accelerate