Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -148,6 +148,7 @@
 LANGOPT(CharIsSigned      , 1, 1, "signed char")
 LANGOPT(ShortWChar        , 1, 0, "unsigned short wchar_t")
 ENUM_LANGOPT(MSPointerToMemberRepresentationMethod, PragmaMSPointersToMembersKind, 2, PPTMK_BestCase, "member-pointer representation method")
+LANGOPT(LongDouble64      , 1, 0, "64-bit long double")
 
 LANGOPT(ShortEnums        , 1, 0, "short enum types")
 
@@ -227,4 +228,3 @@
 #undef COMPATIBLE_ENUM_LANGOPT
 #undef BENIGN_ENUM_LANGOPT
 #undef VALUE_LANGOPT
-
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -850,6 +850,10 @@
   HelpText<"Force wchar_t to be a short unsigned int">;
 def fno_short_wchar : Flag<["-"], "fno-short-wchar">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Force wchar_t to be an unsigned int">;
+def mlong_double_64 : Flag<["-"], "mlong-double-64">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Force long double to be 64 bit">;
+def mlong_double_default : Flag<["-"], "mlong-double-default">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Force long double to be default width">;
 def fshow_overloads_EQ : Joined<["-"], "fshow-overloads=">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Which overload candidates to show when overload resolution fails: "
            "best|all; defaults to all">;
Index: lib/Basic/TargetInfo.cpp
===================================================================
--- lib/Basic/TargetInfo.cpp
+++ lib/Basic/TargetInfo.cpp
@@ -272,6 +272,11 @@
     UseBitFieldTypeAlignment = false;
   if (Opts.ShortWChar)
     WCharType = UnsignedShort;
+  if (Opts.LongDouble64) {
+    LongDoubleWidth = 64;
+    LongDoubleAlign = 32;
+    LongDoubleFormat = &llvm::APFloat::IEEEdouble;
+  }
 
   if (Opts.OpenCL) {
     // OpenCL C requires specific widths for types, irrespective of
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -4168,6 +4168,14 @@
                                options::OPT_fno_short_wchar))
     A->render(Args, CmdArgs);
 
+  // -mlong-double-default is default, only pass non-default.
+  if (Arg *A = Args.getLastArg(options::OPT_mlong_double_64,
+                               options::OPT_mlong_double_default)) {
+    A->claim();
+    if (!A->getOption().matches(options::OPT_mlong_double_default))
+      A->render(Args, CmdArgs);
+  }
+
   // -fno-pascal-strings is default, only pass non-default.
   if (Args.hasFlag(options::OPT_fpascal_strings,
                    options::OPT_fno_pascal_strings,
@@ -8336,4 +8344,3 @@
 
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs));
 }
-
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1522,6 +1522,7 @@
       Args.getLastArgValue(OPT_fmodule_implementation_of);
   Opts.NativeHalfType = Opts.NativeHalfType;
   Opts.HalfArgsAndReturns = Args.hasArg(OPT_fallow_half_arguments_and_returns);
+  Opts.LongDouble64 = Args.hasArg(OPT_mlong_double_64);
 
   if (!Opts.CurrentModule.empty() && !Opts.ImplementationOfModule.empty() &&
       Opts.CurrentModule != Opts.ImplementationOfModule) {
Index: test/CodeGen/long-double-64-x86.c
===================================================================
--- /dev/null
+++ test/CodeGen/long-double-64-x86.c
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -mlong-double-64 %s -emit-llvm -o - -triple=i686-unknown-unknown | FileCheck %s
+// RUN: %clang_cc1 -mlong-double-64 %s -emit-llvm -o - -triple=x86_64-unknown-unknown | FileCheck %s
+
+long double x = 0;
+// CHECK: double
+// CHECK-NOT: x86_fp80
Index: test/Driver/clang_f_opts.c
===================================================================
--- test/Driver/clang_f_opts.c
+++ test/Driver/clang_f_opts.c
@@ -390,3 +390,8 @@
 // CHECK-WCHAR1-NOT: -fshort-wchar
 // CHECK-WCHAR2: -fshort-wchar
 // CHECK-WCHAR2-NOT: -fno-short-wchar
+
+// RUN: %clang -### -mlong-double-64 -mlong-double-default %s 2>&1 | FileCheck -check-prefix=CHECK-LONG-DOUBLE-64-1 %s
+// RUN: %clang -### -mlong-double-default -mlong-double-64 %s 2>&1 | FileCheck -check-prefix=CHECK-LONG-DOUBLE-64-2 %s
+// CHECK-LONG-DOUBLE-64-1-NOT: -mlong-double-64
+// CHECK-LONG-DOUBLE-64-2: -mlong-double-64
Index: test/Sema/long-double-64.c
===================================================================
--- /dev/null
+++ test/Sema/long-double-64.c
@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 %s -fsyntax-only -mlong-double-64 -verify
+// expected-no-diagnostics
+
+int check_long_double_size[sizeof(long double) == 8 ? 1 : -1];