Index: llvm/trunk/lib/Target/AVR/AVR.h
===================================================================
--- llvm/trunk/lib/Target/AVR/AVR.h
+++ llvm/trunk/lib/Target/AVR/AVR.h
@@ -27,10 +27,12 @@
                                CodeGenOpt::Level OptLevel);
 FunctionPass *createAVRExpandPseudoPass();
 FunctionPass *createAVRFrameAnalyzerPass();
+FunctionPass *createAVRRelaxMemPass();
 FunctionPass *createAVRDynAllocaSRPass();
 FunctionPass *createAVRBranchSelectionPass();
 
 void initializeAVRExpandPseudoPass(PassRegistry&);
+void initializeAVRRelaxMemPass(PassRegistry&);
 
 /// Contains the AVR backend.
 namespace AVR {
Index: llvm/trunk/lib/Target/AVR/AVRFrameLowering.cpp
===================================================================
--- llvm/trunk/lib/Target/AVR/AVRFrameLowering.cpp
+++ llvm/trunk/lib/Target/AVR/AVRFrameLowering.cpp
@@ -338,7 +338,6 @@
     // pointer since it is guaranteed to contain a copy of SP.
     unsigned STOpc =
         (Opcode == AVR::STDWSPQRr) ? AVR::STDWPtrQRr : AVR::STDPtrQRr;
-    assert(isUInt<6>(MI.getOperand(1).getImm()) && "Offset is out of range");
 
     MI.setDesc(TII.get(STOpc));
     MI.getOperand(0).setReg(AVR::R29R28);
Index: llvm/trunk/lib/Target/AVR/AVRRelaxMemOperations.cpp
===================================================================
--- llvm/trunk/lib/Target/AVR/AVRRelaxMemOperations.cpp
+++ llvm/trunk/lib/Target/AVR/AVRRelaxMemOperations.cpp
@@ -0,0 +1,149 @@
+//===-- AVRRelaxMemOperations.cpp - Relax out of range loads/stores -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a pass which relaxes out of range memory operations into
+// equivalent operations which handle bigger addresses.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AVR.h"
+#include "AVRInstrInfo.h"
+#include "AVRTargetMachine.h"
+#include "MCTargetDesc/AVRMCTargetDesc.h"
+
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+
+using namespace llvm;
+
+#define AVR_RELAX_MEM_OPS_NAME "AVR memory operation relaxation pass"
+
+namespace {
+
+class AVRRelaxMem : public MachineFunctionPass {
+public:
+  static char ID;
+
+  AVRRelaxMem() : MachineFunctionPass(ID) {
+    initializeAVRRelaxMemPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  StringRef getPassName() const override { return AVR_RELAX_MEM_OPS_NAME; }
+
+private:
+  typedef MachineBasicBlock Block;
+  typedef Block::iterator BlockIt;
+
+  const TargetInstrInfo *TII;
+
+  template <unsigned OP> bool relax(Block &MBB, BlockIt MBBI);
+
+  bool runOnBasicBlock(Block &MBB);
+  bool runOnInstruction(Block &MBB, BlockIt MBBI);
+
+  MachineInstrBuilder buildMI(Block &MBB, BlockIt MBBI, unsigned Opcode) {
+    return BuildMI(MBB, MBBI, MBBI->getDebugLoc(), TII->get(Opcode));
+  }
+};
+
+char AVRRelaxMem::ID = 0;
+
+bool AVRRelaxMem::runOnMachineFunction(MachineFunction &MF) {
+  bool Modified = false;
+
+  const AVRSubtarget &STI = MF.getSubtarget<AVRSubtarget>();
+  TII = STI.getInstrInfo();
+
+  for (Block &MBB : MF) {
+    bool BlockModified = runOnBasicBlock(MBB);
+    Modified |= BlockModified;
+  }
+
+  return Modified;
+}
+
+bool AVRRelaxMem::runOnBasicBlock(Block &MBB) {
+  bool Modified = false;
+
+  BlockIt MBBI = MBB.begin(), E = MBB.end();
+  while (MBBI != E) {
+    BlockIt NMBBI = std::next(MBBI);
+    Modified |= runOnInstruction(MBB, MBBI);
+    MBBI = NMBBI;
+  }
+
+  return Modified;
+}
+
+template <>
+bool AVRRelaxMem::relax<AVR::STDWPtrQRr>(Block &MBB, BlockIt MBBI) {
+  MachineInstr &MI = *MBBI;
+
+  MachineOperand &Ptr = MI.getOperand(0);
+  MachineOperand &Src = MI.getOperand(2);
+  int64_t Imm = MI.getOperand(1).getImm();
+
+  // We can definitely optimise this better.
+  if (Imm > 63) {
+    // Push the previous state of the pointer register.
+    // This instruction must preserve the value.
+    buildMI(MBB, MBBI, AVR::PUSHWRr)
+      .addReg(Ptr.getReg());
+
+    // Add the immediate to the pointer register.
+    buildMI(MBB, MBBI, AVR::SBCIWRdK)
+      .addReg(Ptr.getReg(), RegState::Define)
+      .addReg(Ptr.getReg())
+      .addImm(-Imm);
+
+    // Store the value in the source register to the address
+    // pointed to by the pointer register.
+    buildMI(MBB, MBBI, AVR::STWPtrRr)
+      .addReg(Ptr.getReg())
+      .addReg(Src.getReg(), getKillRegState(Src.isKill()));
+
+    // Pop the original state of the pointer register.
+    buildMI(MBB, MBBI, AVR::POPWRd)
+      .addReg(Ptr.getReg(), getKillRegState(Ptr.isKill()));
+
+    MI.removeFromParent();
+  }
+
+  return false;
+}
+
+bool AVRRelaxMem::runOnInstruction(Block &MBB, BlockIt MBBI) {
+  MachineInstr &MI = *MBBI;
+  int Opcode = MBBI->getOpcode();
+
+#define RELAX(Op)                \
+  case Op:                       \
+    return relax<Op>(MBB, MI)
+
+  switch (Opcode) {
+    RELAX(AVR::STDWPtrQRr);
+  }
+#undef RELAX
+  return false;
+}
+
+} // end of anonymous namespace
+
+INITIALIZE_PASS(AVRRelaxMem, "avr-relax-mem",
+                AVR_RELAX_MEM_OPS_NAME, false, false)
+
+namespace llvm {
+
+FunctionPass *createAVRRelaxMemPass() { return new AVRRelaxMem(); }
+
+} // end of namespace llvm
Index: llvm/trunk/lib/Target/AVR/AVRTargetMachine.cpp
===================================================================
--- llvm/trunk/lib/Target/AVR/AVRTargetMachine.cpp
+++ llvm/trunk/lib/Target/AVR/AVRTargetMachine.cpp
@@ -80,6 +80,7 @@
 
   auto &PR = *PassRegistry::getPassRegistry();
   initializeAVRExpandPseudoPass(PR);
+  initializeAVRRelaxMemPass(PR);
 }
 
 const AVRSubtarget *AVRTargetMachine::getSubtargetImpl() const {
@@ -108,6 +109,9 @@
   addPass(createAVRDynAllocaSRPass());
 }
 
-void AVRPassConfig::addPreSched2() { addPass(createAVRExpandPseudoPass()); }
+void AVRPassConfig::addPreSched2() {
+  addPass(createAVRRelaxMemPass());
+  addPass(createAVRExpandPseudoPass());
+}
 
 } // end of namespace llvm
Index: llvm/trunk/lib/Target/AVR/CMakeLists.txt
===================================================================
--- llvm/trunk/lib/Target/AVR/CMakeLists.txt
+++ llvm/trunk/lib/Target/AVR/CMakeLists.txt
@@ -24,6 +24,7 @@
   AVRISelDAGToDAG.cpp
   AVRISelLowering.cpp
   AVRMCInstLower.cpp
+  AVRRelaxMemOperations.cpp
   AVRRegisterInfo.cpp
   AVRSubtarget.cpp
   AVRTargetMachine.cpp
Index: llvm/trunk/test/CodeGen/AVR/relax-mem/STDWPtrQRr.mir
===================================================================
--- llvm/trunk/test/CodeGen/AVR/relax-mem/STDWPtrQRr.mir
+++ llvm/trunk/test/CodeGen/AVR/relax-mem/STDWPtrQRr.mir
@@ -0,0 +1,31 @@
+# RUN: llc -O0 -run-pass=avr-relax-mem %s -o - 2>&1 | FileCheck %s
+
+--- |
+  target triple = "avr--"
+  define void @test() {
+  entry:
+    ret void
+  }
+...
+
+---
+name:            test
+body: |
+  bb.0.entry:
+
+    ; CHECK-LABEL: test
+
+    ; We shouldn't expand things which already have 6-bit imms.
+    ; CHECK: STDWPtrQRr %r29r28, 63, %r1r0
+    STDWPtrQRr %r29r28, 63, %r1r0
+
+    ; We shouldn't expand things which already have 6-bit imms.
+    ; CHECK-NEXT: STDWPtrQRr %r29r28, 0, %r1r0
+    STDWPtrQRr %r29r28, 0, %r1r0
+
+    ; CHECK-NEXT: PUSHWRr %r29r28, implicit-def %sp, implicit %sp
+    ; CHECK-NEXT: %r29r28 = SBCIWRdK %r29r28, -64, implicit-def %sreg, implicit %sreg
+    ; CHECK-NEXT: STWPtrRr %r29r28, %r1r0
+    ; CHECK-NEXT: POPWRd %r29r28, implicit-def %sp, implicit %sp
+    STDWPtrQRr %r29r28, 64, %r1r0
+...