Index: llvm/include/llvm/AsmParser/LLToken.h
===================================================================
--- llvm/include/llvm/AsmParser/LLToken.h
+++ llvm/include/llvm/AsmParser/LLToken.h
@@ -173,6 +173,7 @@
   kw_amdgpu_kernel,
   kw_amdgpu_gfx,
   kw_tailcc,
+  kw_m68k_rtdcc,
 
   // Attributes:
   kw_attributes,
Index: llvm/include/llvm/IR/CallingConv.h
===================================================================
--- llvm/include/llvm/IR/CallingConv.h
+++ llvm/include/llvm/IR/CallingConv.h
@@ -237,6 +237,9 @@
     /// Preserve X2-X15, X19-X29, SP, Z0-Z31, P0-P15.
     AArch64_SME_ABI_Support_Routines_PreserveMost_From_X2 = 103,
 
+    /// Used for M68k rtd-based CC (similar to X86's stdcall).
+    M68k_RTD = 104,
+
     /// The highest possible ID. Must be some 2^k - 1.
     MaxID = 1023
   };
Index: llvm/lib/AsmParser/LLLexer.cpp
===================================================================
--- llvm/lib/AsmParser/LLLexer.cpp
+++ llvm/lib/AsmParser/LLLexer.cpp
@@ -631,6 +631,7 @@
   KEYWORD(amdgpu_kernel);
   KEYWORD(amdgpu_gfx);
   KEYWORD(tailcc);
+  KEYWORD(m68k_rtdcc);
 
   KEYWORD(cc);
   KEYWORD(c);
Index: llvm/lib/AsmParser/LLParser.cpp
===================================================================
--- llvm/lib/AsmParser/LLParser.cpp
+++ llvm/lib/AsmParser/LLParser.cpp
@@ -2029,6 +2029,7 @@
 ///   ::= 'amdgpu_cs'
 ///   ::= 'amdgpu_kernel'
 ///   ::= 'tailcc'
+///   ::= 'm68k_rtdcc'
 ///   ::= 'cc' UINT
 ///
 bool LLParser::parseOptionalCallingConv(unsigned &CC) {
@@ -2091,6 +2092,7 @@
   case lltok::kw_amdgpu_cs:      CC = CallingConv::AMDGPU_CS; break;
   case lltok::kw_amdgpu_kernel:  CC = CallingConv::AMDGPU_KERNEL; break;
   case lltok::kw_tailcc:         CC = CallingConv::Tail; break;
+  case lltok::kw_m68k_rtdcc:     CC = CallingConv::M68k_RTD; break;
   case lltok::kw_cc: {
       Lex.Lex();
       return parseUInt32(CC);
Index: llvm/lib/IR/AsmWriter.cpp
===================================================================
--- llvm/lib/IR/AsmWriter.cpp
+++ llvm/lib/IR/AsmWriter.cpp
@@ -344,6 +344,7 @@
   case CallingConv::AMDGPU_CS:     Out << "amdgpu_cs"; break;
   case CallingConv::AMDGPU_KERNEL: Out << "amdgpu_kernel"; break;
   case CallingConv::AMDGPU_Gfx:    Out << "amdgpu_gfx"; break;
+  case CallingConv::M68k_RTD:      Out << "m68k_rtdcc"; break;
   }
 }
 
Index: llvm/lib/Target/M68k/M68kExpandPseudo.cpp
===================================================================
--- llvm/lib/Target/M68k/M68kExpandPseudo.cpp
+++ llvm/lib/Target/M68k/M68kExpandPseudo.cpp
@@ -259,32 +259,22 @@
 
     if (StackAdj == 0) {
       MIB = BuildMI(MBB, MBBI, DL, TII->get(M68k::RTS));
-    } else if (isUInt<16>(StackAdj)) {
-
-      if (STI->atLeastM68020()) {
-        llvm_unreachable("RTD is not implemented");
-      } else {
-        // Copy PC from stack to a free address(A0 or A1) register
-        // TODO check if pseudo expand uses free address register
-        BuildMI(MBB, MBBI, DL, TII->get(M68k::MOV32aj), M68k::A1)
-            .addReg(M68k::SP);
+    } else {
+      // Copy return address from stack to a free address(A0 or A1) register
+      // TODO check if pseudo expand uses free address register
+      BuildMI(MBB, MBBI, DL, TII->get(M68k::MOV32aj), M68k::A1)
+          .addReg(M68k::SP);
 
-        // Adjust SP
-        FL->emitSPUpdate(MBB, MBBI, StackAdj, /*InEpilogue=*/true);
+      // Adjust SP
+      FL->emitSPUpdate(MBB, MBBI, StackAdj, /*InEpilogue=*/true);
 
-        // Put the return address on stack
-        BuildMI(MBB, MBBI, DL, TII->get(M68k::MOV32ja))
-            .addReg(M68k::SP)
-            .addReg(M68k::A1);
+      // Put the return address on stack
+      BuildMI(MBB, MBBI, DL, TII->get(M68k::MOV32ja))
+          .addReg(M68k::SP)
+          .addReg(M68k::A1);
 
-        // RTS
-        BuildMI(MBB, MBBI, DL, TII->get(M68k::RTS));
-      }
-    } else {
-      // TODO: RTD can only handle immediates as big as 2**16-1.
-      // If we need to pop off bytes before the return address, we
-      // must do it manually.
-      llvm_unreachable("Stack adjustment size not supported");
+      // RTS
+      BuildMI(MBB, MBBI, DL, TII->get(M68k::RTS));
     }
 
     // FIXME: Can rest of the operands be ignored, if there is any?
Index: llvm/lib/Target/M68k/M68kISelLowering.cpp
===================================================================
--- llvm/lib/Target/M68k/M68kISelLowering.cpp
+++ llvm/lib/Target/M68k/M68kISelLowering.cpp
@@ -2950,9 +2950,8 @@
 
 /// Determines whether the callee is required to pop its own arguments.
 /// Callee pop is necessary to support tail calls.
-bool M68k::isCalleePop(CallingConv::ID CallingConv, bool IsVarArg,
-                       bool GuaranteeTCO) {
-  return false;
+bool M68k::isCalleePop(CallingConv::ID CC, bool IsVarArg, bool GuaranteeTCO) {
+  return CC == CallingConv::M68k_RTD;
 }
 
 // Return true if it is OK for this CMOV pseudo-opcode to be cascaded
Index: llvm/test/CodeGen/M68k/CConv/rtd-call.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/M68k/CConv/rtd-call.ll
@@ -0,0 +1,20 @@
+; RUN: llc -mtriple=m68k %s -stop-after=finalize-isel -o - | FileCheck %s
+
+; We want to make sure caller doesn't pop the stack for callees using
+; the M68k_RTD CC. However, we've implemented some frame optimization
+; techniques to eliminate as many as frame setup/destroy instructions.
+; Therefore, to make test case small and concise, we check the MIR generated
+; after ISel instead.
+
+declare dso_local m68k_rtdcc void @callee(i32 noundef)
+
+; CHECK-LABEL: caller
+define dso_local i32 @caller(ptr noundef %y) {
+entry:
+  %0 = load i32, ptr %y, align 4
+; CHECK: ADJCALLSTACKDOWN
+  call m68k_rtdcc void @callee(i32 noundef %0)
+; CHECK: ADJCALLSTACKUP 4, 4
+  ret i32 %0
+}
+
Index: llvm/test/CodeGen/M68k/CConv/rtd-ret.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/M68k/CConv/rtd-ret.ll
@@ -0,0 +1,20 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
+; RUN: llc -mtriple=m68k < %s | FileCheck %s
+
+define dso_local m68k_rtdcc i32 @ret(i32 noundef %a, i32 noundef %b, i32 noundef %c) {
+; CHECK-LABEL: ret:
+; CHECK:         .cfi_startproc
+; CHECK-NEXT:  ; %bb.0: ; %entry
+; CHECK-NEXT:    move.l (8,%sp), %d0
+; CHECK-NEXT:    add.l (4,%sp), %d0
+; CHECK-NEXT:    add.l (12,%sp), %d0
+; CHECK-NEXT:    move.l (%sp), %a1
+; CHECK-NEXT:    adda.l #12, %sp
+; CHECK-NEXT:    move.l %a1, (%sp)
+; CHECK-NEXT:    rts
+entry:
+  %add = add nsw i32 %b, %a
+  %add1 = add nsw i32 %add, %c
+  ret i32 %add1
+}
+