Index: clang/lib/Basic/Targets/M68k.h
===================================================================
--- clang/lib/Basic/Targets/M68k.h
+++ clang/lib/Basic/Targets/M68k.h
@@ -54,6 +54,7 @@
   std::string_view getClobbers() const override;
   BuiltinVaListKind getBuiltinVaListKind() const override;
   bool setCPU(const std::string &Name) override;
+  CallingConvCheckResult checkCallingConvention(CallingConv CC) const override;
 };
 
 } // namespace targets
Index: clang/lib/Basic/Targets/M68k.cpp
===================================================================
--- clang/lib/Basic/Targets/M68k.cpp
+++ clang/lib/Basic/Targets/M68k.cpp
@@ -250,5 +250,16 @@
   return TargetInfo::VoidPtrBuiltinVaList;
 }
 
+TargetInfo::CallingConvCheckResult
+M68kTargetInfo::checkCallingConvention(CallingConv CC) const {
+  switch (CC) {
+  case CC_C:
+  // The M68k_RTD calling convention.
+  case CC_X86StdCall:
+    return CCCR_OK;
+  default:
+    return TargetInfo::checkCallingConvention(CC);
+  }
+}
 } // namespace targets
 } // namespace clang
Index: clang/lib/CodeGen/CGCall.cpp
===================================================================
--- clang/lib/CodeGen/CGCall.cpp
+++ clang/lib/CodeGen/CGCall.cpp
@@ -48,7 +48,10 @@
 unsigned CodeGenTypes::ClangCallConvToLLVMCallConv(CallingConv CC) {
   switch (CC) {
   default: return llvm::CallingConv::C;
-  case CC_X86StdCall: return llvm::CallingConv::X86_StdCall;
+  case CC_X86StdCall:
+    return CGM.getTriple().getArch() == llvm::Triple::m68k
+               ? llvm::CallingConv::M68k_RTD
+               : llvm::CallingConv::X86_StdCall;
   case CC_X86FastCall: return llvm::CallingConv::X86_FastCall;
   case CC_X86RegCall: return llvm::CallingConv::X86_RegCall;
   case CC_X86ThisCall: return llvm::CallingConv::X86_ThisCall;
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -553,9 +553,10 @@
   if (Arg *A = Args.getLastArg(OPT_fdefault_calling_conv_EQ)) {
     auto DefaultCC = LangOpts.getDefaultCallingConv();
 
-    bool emitError = (DefaultCC == LangOptions::DCC_FastCall ||
-                      DefaultCC == LangOptions::DCC_StdCall) &&
-                     Arch != llvm::Triple::x86;
+    bool emitError =
+        DefaultCC == LangOptions::DCC_FastCall && Arch != llvm::Triple::x86;
+    emitError |= DefaultCC == LangOptions::DCC_StdCall &&
+                 !(Arch == llvm::Triple::m68k || Arch == llvm::Triple::x86);
     emitError |= (DefaultCC == LangOptions::DCC_VectorCall ||
                   DefaultCC == LangOptions::DCC_RegCall) &&
                  !T.isX86();
@@ -3747,7 +3748,7 @@
       Diags.Report(diag::err_drv_argument_not_allowed_with)
           << A->getSpelling() << "-fdefault-calling-conv";
     else {
-      if (T.getArch() != llvm::Triple::x86)
+      if (T.getArch() != llvm::Triple::x86 && T.getArch() != llvm::Triple::m68k)
         Diags.Report(diag::err_drv_argument_not_allowed_with)
             << A->getSpelling() << T.getTriple();
       else
Index: clang/test/CodeGen/mrtd.c
===================================================================
--- clang/test/CodeGen/mrtd.c
+++ clang/test/CodeGen/mrtd.c
@@ -1,20 +1,26 @@
-// RUN: %clang_cc1 -mrtd -triple i386-unknown-unknown -std=c89 -emit-llvm -o - %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -mrtd -triple i386-unknown-unknown -std=c89 -emit-llvm -o - %s 2>&1 | FileCheck --check-prefixes=CHECK,X86 %s
+// RUN: %clang_cc1 -mrtd -triple m68k-unknown-unknown -std=c89 -emit-llvm -o - %s 2>&1 | FileCheck --check-prefixes=CHECK,M68k %s
 
-// CHECK: mrtd.c:10:3: warning: function with no prototype cannot use the stdcall calling convention
+// CHECK: mrtd.c:13:3: warning: function with no prototype cannot use the stdcall calling convention
 
 void baz(int arg);
 
-// CHECK: define{{.*}} x86_stdcallcc void @foo(i32 noundef %arg) [[NUW:#[0-9]+]]
+// X86: define{{.*}} x86_stdcallcc void @foo(i32 noundef %arg) [[NUW:#[0-9]+]]
+// M68k: define{{.*}} cc104 void @foo(i32 noundef %arg)
 void foo(int arg) {
-// CHECK: call x86_stdcallcc i32 @bar(
+// X86: call x86_stdcallcc i32 @bar(
+// M68k: call cc104 i32 @bar(
   bar(arg);
-// CHECK: call x86_stdcallcc void @baz(i32
+// X86: call x86_stdcallcc void @baz(i32
+// M68k: call cc104 void @baz(i32
   baz(arg);
 }
 
-// CHECK: declare x86_stdcallcc i32 @bar(...)
+// X86: declare x86_stdcallcc i32 @bar(...)
+// M68k: declare cc104 i32 @bar(...)
 
-// CHECK: declare x86_stdcallcc void @baz(i32 noundef)
+// X86: declare x86_stdcallcc void @baz(i32 noundef)
+// M68k: declare cc104 void @baz(i32 noundef)
 
 void qux(int arg, ...) { }
 // CHECK: define{{.*}} void @qux(i32 noundef %arg, ...)
@@ -22,7 +28,8 @@
 void quux(int a1, int a2, int a3) {
   qux(a1, a2, a3);
 }
-// CHECK-LABEL: define{{.*}} x86_stdcallcc void @quux
+// X86-LABEL: define{{.*}} x86_stdcallcc void @quux
+// M68k-LABEL: define{{.*}} cc104 void @quux
 // CHECK: call void (i32, ...) @qux
 
-// CHECK: attributes [[NUW]] = { noinline nounwind{{.*}} }
+// X86: attributes [[NUW]] = { noinline nounwind{{.*}} }
Index: clang/test/Sema/mrtd.c
===================================================================
--- clang/test/Sema/mrtd.c
+++ clang/test/Sema/mrtd.c
@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -DMRTD -mrtd -triple i386-unknown-unknown -verify %s
+// RUN: %clang_cc1 -DMRTD -mrtd -triple m68k-unknown-unknown -verify %s
 // RUN: %clang_cc1 -triple i386-unknown-unknown -verify %s
+// RUN: %clang_cc1 -triple m68k-unknown-unknown -verify %s
 
 #ifndef MRTD
 // expected-note@+5 {{previous declaration is here}}