Index: include/llvm/CodeGen/GlobalISel/Combiner.h
===================================================================
--- /dev/null
+++ include/llvm/CodeGen/GlobalISel/Combiner.h
@@ -0,0 +1,43 @@
+//== ----- llvm/CodeGen/GlobalISel/Combiner.h --------------------- == //
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// This contains common code to drive combines. Combiner Passes will need to
+/// setup a CombinerInfo and call combineMachineFunction.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_GLOBALISEL_COMBINER_H
+#define LLVM_CODEGEN_GLOBALISEL_COMBINER_H
+
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+
+namespace llvm {
+class MachineRegisterInfo;
+class CombinerInfo;
+class TargetPassConfig;
+class MachineFunction;
+
+class Combiner {
+public:
+  Combiner(CombinerInfo &CombinerInfo, const TargetPassConfig *TPC);
+
+  bool combineMachineInstrs(MachineFunction &MF);
+
+protected:
+  CombinerInfo &CInfo;
+
+  MachineRegisterInfo *MRI = nullptr;
+  const TargetPassConfig *TPC;
+  MachineIRBuilder Builder;
+};
+
+} // End namespace llvm.
+
+#endif // LLVM_CODEGEN_GLOBALISEL_GICOMBINER_H
Index: include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- /dev/null
+++ include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -0,0 +1,44 @@
+//== llvm/CodeGen/GlobalISel/CombinerHelper.h ---------------- -*- C++ -*-==//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// This contains common combine transformations that may be used in a combine
+/// pass,or by the target elsewhere.
+/// Targets can pick individual opcode transformations from the helper or use
+/// tryCombine which invokes all transformations. All of the transformations return
+/// true if the MachineInstruction changed and false otherwise.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_GLOBALISEL_COMBINER_HELPER_H
+#define LLVM_CODEGEN_GLOBALISEL_COMBINER_HELPER_H
+
+namespace llvm {
+
+class MachineIRBuilder;
+class MachineRegisterInfo;
+class MachineInstr;
+
+class CombinerHelper {
+  MachineIRBuilder &Builder;
+  MachineRegisterInfo &MRI;
+
+public:
+  CombinerHelper(MachineIRBuilder &B);
+
+  /// If \p MI is COPY, try to combine it.
+  /// Returns true if MI changed.
+  bool tryCombineCopy(MachineInstr &MI);
+
+  /// Try to transform \p MI by using all of the above
+  /// combine functions. Returns true if changed.
+  bool tryCombine(MachineInstr &MI);
+};
+} // namespace llvm
+
+#endif
Index: include/llvm/CodeGen/GlobalISel/CombinerInfo.h
===================================================================
--- /dev/null
+++ include/llvm/CodeGen/GlobalISel/CombinerInfo.h
@@ -0,0 +1,47 @@
+//===- llvm/CodeGen/GlobalISel/CombinerInfo.h ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+/// Interface for Targets to specify which operations are combined how and when.
+/// //
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_GLOBALISEL_COMBINER_INFO_H
+#define LLVM_CODEGEN_GLOBALISEL_COMBINER_INFO_H
+
+namespace llvm {
+
+class LegalizerInfo;
+class MachineInstr;
+class MachineIRBuilder;
+class MachineRegisterInfo;
+// Contains information relevant to enabling/disabling various combines for a
+// pass.
+class CombinerInfo {
+public:
+  CombinerInfo(bool AllowIllegalOps, bool ShouldLegalizeIllegal,
+               LegalizerInfo *LInfo)
+      : IllegalOpsAllowed(AllowIllegalOps),
+        LegalizeIllegalOps(ShouldLegalizeIllegal), LInfo(LInfo) {
+    assert(((AllowIllegalOps || !LegalizeIllegalOps) || LInfo) &&
+           "Expecting legalizerInfo when illegalops not allowed");
+  }
+  virtual ~CombinerInfo() = default;
+  /// If \p IllegalOpsAllowed is false, the CombinerHelper will make use of
+  /// the legalizerInfo to check for legality before each transformation.
+  bool IllegalOpsAllowed; // TODO: Make use of this.
+
+  /// If \p LegalizeIllegalOps is true, the Combiner will also legalize the
+  /// illegal ops that are created.
+  bool LegalizeIllegalOps; // TODO: Make use of this.
+  const LegalizerInfo *LInfo;
+  virtual bool combine(MachineInstr &MI, MachineIRBuilder &B) const = 0;
+};
+} // namespace llvm
+
+#endif
Index: include/llvm/CodeGen/GlobalISel/PreLegalizeCombiner.h
===================================================================
--- /dev/null
+++ include/llvm/CodeGen/GlobalISel/PreLegalizeCombiner.h
@@ -0,0 +1,50 @@
+//==  llvm/include/CodeGen/GlobalISel/PreLegalizeCombiner.h -----------------//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass runs before legalization.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_GLOBALISEL_PRELEGALIZE_GICOMBINER_H
+#define LLVM_CODEGEN_GLOBALISEL_PRELEGALIZE_GICOMBINER_H
+
+#include "llvm/CodeGen/GlobalISel/Combiner.h"
+#include "llvm/CodeGen/GlobalISel/CombinerInfo.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+
+namespace llvm {
+class MachineRegisterInfo;
+
+class PreLegalizeCombinerInfo : public CombinerInfo {
+public:
+  PreLegalizeCombinerInfo()
+      : CombinerInfo(/*AllowIllegalOps*/ true, /*ShouldLegalizeIllegal*/ false,
+                     /*LegalizerInfo*/ nullptr) {}
+  virtual bool combine(MachineInstr &MI, MachineIRBuilder &B) const override;
+};
+
+class PreLegalizeCombiner : public MachineFunctionPass {
+public:
+  static char ID;
+
+  PreLegalizeCombiner() : MachineFunctionPass(ID) {}
+
+  StringRef getPassName() const override { return "PreLegalizeCombiner"; }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override;
+};
+
+} // End namespace llvm.
+
+#endif // LLVM_CODEGEN_GLOBALISEL_PRELEGALIZE_GICOMBINER_H
Index: include/llvm/InitializePasses.h
===================================================================
--- include/llvm/InitializePasses.h
+++ include/llvm/InitializePasses.h
@@ -151,6 +151,7 @@
 void initializeGVNHoistLegacyPassPass(PassRegistry&);
 void initializeGVNLegacyPassPass(PassRegistry&);
 void initializeGVNSinkLegacyPassPass(PassRegistry&);
+void initializePreLegalizeCombinerPass(PassRegistry &);
 void initializeGlobalDCELegacyPassPass(PassRegistry&);
 void initializeGlobalMergePass(PassRegistry&);
 void initializeGlobalOptLegacyPassPass(PassRegistry&);
Index: lib/CodeGen/GlobalISel/CMakeLists.txt
===================================================================
--- lib/CodeGen/GlobalISel/CMakeLists.txt
+++ lib/CodeGen/GlobalISel/CMakeLists.txt
@@ -1,6 +1,9 @@
 add_llvm_library(LLVMGlobalISel
         CallLowering.cpp
         GlobalISel.cpp
+        Combiner.cpp
+        CombinerHelper.cpp
+        PreLegalizeCombiner.cpp
         IRTranslator.cpp
         InstructionSelect.cpp
         InstructionSelector.cpp
Index: lib/CodeGen/GlobalISel/Combiner.cpp
===================================================================
--- /dev/null
+++ lib/CodeGen/GlobalISel/Combiner.cpp
@@ -0,0 +1,81 @@
+//===-- lib/CodeGen/GlobalISel/GICombiner.cpp -----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file constains common code to combine machine functions at generic
+// level. //
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/GlobalISel/Combiner.h"
+#include "llvm/CodeGen/GlobalISel/CombinerInfo.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
+#include "llvm/CodeGen/GlobalISel/GISelWorkList.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "gi-combiner"
+
+using namespace llvm;
+
+Combiner::Combiner(CombinerInfo &Info, const TargetPassConfig *TPC)
+    : CInfo(Info), TPC(TPC) {
+  (void)this->TPC; // FIXME: Remove when used.
+}
+
+bool Combiner::combineMachineInstrs(MachineFunction &MF) {
+  // If the ISel pipeline failed, do not bother running this pass.
+  // FIXME: Should this be here or in individual combiner passes.
+  if (MF.getProperties().hasProperty(
+          MachineFunctionProperties::Property::FailedISel))
+    return false;
+
+  MRI = &MF.getRegInfo();
+  Builder.setMF(MF);
+
+  DEBUG(dbgs() << "Generic MI Combiner for: " << MF.getName() << '\n');
+
+  MachineOptimizationRemarkEmitter MORE(MF, /*MBFI=*/nullptr);
+
+  bool MFChanged = false;
+  bool Changed;
+
+  do {
+    // Collect all instructions. Do a post order traversal for basic blocks and
+    // insert with list bottom up, so while we pop_back_val, we'll traverse top
+    // down RPOT.
+    Changed = false;
+    GISelWorkList<512> WorkList;
+    for (MachineBasicBlock *MBB : post_order(&MF)) {
+      if (MBB->empty())
+        continue;
+      for (auto MII = MBB->rbegin(), MIE = MBB->rend(); MII != MIE;) {
+        MachineInstr *CurMI = &*MII;
+        ++MII;
+        // Erase dead insts before even adding to the list.
+        if (isTriviallyDead(*CurMI, *MRI)) {
+          DEBUG(dbgs() << *CurMI << "Is dead; erasing.\n");
+          CurMI->eraseFromParentAndMarkDBGValuesForRemoval();
+          continue;
+        }
+        WorkList.insert(CurMI);
+      }
+    }
+    // Main Loop. Process the instructions here.
+    while (!WorkList.empty()) {
+      MachineInstr *CurrInst = WorkList.pop_back_val();
+      DEBUG(dbgs() << "Try combining " << *CurrInst << "\n";);
+      Changed |= CInfo.combine(*CurrInst, Builder);
+    }
+    MFChanged |= Changed;
+  } while (Changed);
+
+  return MFChanged;
+}
Index: lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- /dev/null
+++ lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -0,0 +1,42 @@
+//== ---lib/CodeGen/GlobalISel/GICombinerHelper.cpp --------------------- == //
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+
+#define DEBUG_TYPE "gi-combine"
+
+using namespace llvm;
+
+CombinerHelper::CombinerHelper(MachineIRBuilder &B) :
+  Builder(B), MRI(Builder.getMF().getRegInfo()) {}
+
+bool CombinerHelper::tryCombineCopy(MachineInstr &MI) {
+  if (MI.getOpcode() != TargetOpcode::COPY)
+    return false;
+  unsigned DstReg = MI.getOperand(0).getReg();
+  unsigned SrcReg = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(DstReg);
+  LLT SrcTy = MRI.getType(SrcReg);
+  // Simple Copy Propagation.
+  // a(sx) = COPY b(sx) -> Replace all uses of a with b.
+  if (DstTy.isValid() && SrcTy.isValid() && DstTy == SrcTy) {
+    MI.eraseFromParent();
+    MRI.replaceRegWith(DstReg, SrcReg);
+    return true;
+  }
+  return false;
+}
+
+bool CombinerHelper::tryCombine(MachineInstr &MI) {
+  // Right now just do the above.
+  return tryCombineCopy(MI);
+}
Index: lib/CodeGen/GlobalISel/GlobalISel.cpp
===================================================================
--- lib/CodeGen/GlobalISel/GlobalISel.cpp
+++ lib/CodeGen/GlobalISel/GlobalISel.cpp
@@ -19,6 +19,7 @@
 void llvm::initializeGlobalISel(PassRegistry &Registry) {
   initializeIRTranslatorPass(Registry);
   initializeLegalizerPass(Registry);
+  initializePreLegalizeCombinerPass(Registry);
   initializeLocalizerPass(Registry);
   initializeRegBankSelectPass(Registry);
   initializeInstructionSelectPass(Registry);
Index: lib/CodeGen/GlobalISel/PreLegalizeCombiner.cpp
===================================================================
--- /dev/null
+++ lib/CodeGen/GlobalISel/PreLegalizeCombiner.cpp
@@ -0,0 +1,60 @@
+//=== lib/CodeGen/GlobalISel/PreLegalizeCombiner.cpp -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass does combining of machine instructions at the generic MI level,
+// before the legalizer.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/GlobalISel/PreLegalizeCombiner.h"
+#include "llvm/CodeGen/GlobalISel/Combiner.h"
+#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
+#include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "prelegalize-combiner"
+
+using namespace llvm;
+
+static cl::opt<bool> EnablePreLegalizeCombiner(
+    "enable-prelegalize-gi-combiner", cl::Hidden,
+    cl::desc("Enable the prelegalize machine instruction combiner"),
+    cl::init(true));
+
+char PreLegalizeCombiner::ID = 0;
+INITIALIZE_PASS_BEGIN(PreLegalizeCombiner, DEBUG_TYPE,
+                      "Combine generic instructions before legalization", false,
+                      false)
+INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
+INITIALIZE_PASS_END(PreLegalizeCombiner, DEBUG_TYPE,
+                    "Combine generic instructions before legalization", false,
+                    false)
+
+void PreLegalizeCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<TargetPassConfig>();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+bool PreLegalizeCombiner::runOnMachineFunction(MachineFunction &MF) {
+  if (!EnablePreLegalizeCombiner) {
+    DEBUG(dbgs() << "PreLegalizeCombiner is disabled\n");
+    return false;
+  }
+  const auto *TPC = &getAnalysis<TargetPassConfig>();
+  PreLegalizeCombinerInfo PCInfo;
+  Combiner Combiner(PCInfo, TPC);
+  return Combiner.combineMachineInstrs(MF);
+}
+
+bool PreLegalizeCombinerInfo::combine(MachineInstr &MI, MachineIRBuilder &B) const {
+  CombinerHelper Helper(B);
+  // For now try all combines in the combiner helper.
+  return Helper.tryCombine(MI);
+}
Index: test/CodeGen/AArch64/GlobalISel/combine-copy.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/GlobalISel/combine-copy.mir
@@ -0,0 +1,27 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -O0 -run-pass=prelegalize-combiner -global-isel %s -o - | FileCheck %s
+
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64--"
+  define void @test_copy_propagation() {
+  entry:
+    ret void
+  }
+...
+
+---
+name:            test_copy_propagation
+body: |
+  bb.0.entry:
+    liveins: %x0, %x1, %x2, %x3
+
+    ; CHECK-LABEL: name: test_copy_propagation
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY %x0
+    ; CHECK: %x0 = COPY [[COPY]](s64)
+    %0:_(s64) = COPY %x0
+    %1:_(s64) = COPY %0
+    %2:_(s64) = COPY %1
+    %x0 = COPY %2
+...
+