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diff --git a/lib/Target/WebAssembly/WebAssemblyPrepareForLiveIntervals.cpp b/lib/Target/WebAssembly/WebAssemblyPrepareForLiveIntervals.cpp
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index 000000000000..30444ac598a4
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+++ b/lib/Target/WebAssembly/WebAssemblyPrepareForLiveIntervals.cpp
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+//===- WebAssemblyPrepareForLiveIntervals.cpp - Prepare for LiveIntervals -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief Fix up code to meet LiveInterval's requirements.
+///
+/// Some CodeGen passes don't preserve LiveInterval's requirements, because
+/// they run after register allocation and it isn't important. However,
+/// WebAssembly runs LiveIntervals in a late pass. This pass transforms code
+/// to meet LiveIntervals' requirements; primarily, it ensures that all
+/// virtual register uses have definitions (IMPLICIT_DEF definitions if
+/// nothing else).
+///
+//===----------------------------------------------------------------------===//
+
+#include "WebAssembly.h"
+#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
+#include "WebAssemblyMachineFunctionInfo.h"
+#include "WebAssemblySubtarget.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "wasm-prepare-for-live-intervals"
+
+namespace {
+class WebAssemblyPrepareForLiveIntervals final : public MachineFunctionPass {
+public:
+  static char ID; // Pass identification, replacement for typeid
+  WebAssemblyPrepareForLiveIntervals() : MachineFunctionPass(ID) {}
+
+private:
+  const char *getPassName() const override {
+    return "WebAssembly Prepare For LiveIntervals";
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+} // end anonymous namespace
+
+char WebAssemblyPrepareForLiveIntervals::ID = 0;
+FunctionPass *llvm::createWebAssemblyPrepareForLiveIntervals() {
+  return new WebAssemblyPrepareForLiveIntervals();
+}
+
+/// Test whether the given instruction is an ARGUMENT.
+static bool IsArgument(const MachineInstr *MI) {
+  switch (MI->getOpcode()) {
+  case WebAssembly::ARGUMENT_I32:
+  case WebAssembly::ARGUMENT_I64:
+  case WebAssembly::ARGUMENT_F32:
+  case WebAssembly::ARGUMENT_F64:
+    return true;
+  default:
+    return false;
+  }
+}
+
+// Test whether the given register has an ARGUMENT def.
+static bool HasArgumentDef(unsigned Reg, const MachineRegisterInfo &MRI) {
+  for (auto &Def : MRI.def_instructions(Reg))
+    if (IsArgument(&Def))
+      return true;
+  return false;
+}
+
+bool WebAssemblyPrepareForLiveIntervals::runOnMachineFunction(MachineFunction &MF) {
+  DEBUG({
+    dbgs() << "********** Prepare For LiveIntervals **********\n"
+           << "********** Function: " << MF.getName() << '\n';
+  });
+
+  bool Changed = false;
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
+  MachineBasicBlock &Entry = *MF.begin();
+
+  assert(!mustPreserveAnalysisID(LiveIntervalsID) &&
+         "LiveIntervals shouldn't be active yet!");
+
+  // We don't preserve SSA form.
+  MRI.leaveSSA();
+
+  // BranchFolding and perhaps other passes don't preserve IMPLICIT_DEF
+  // instructions. LiveIntervals requires that all paths to virtual register
+  // uses provide a definition. Insert IMPLICIT_DEFs in the entry block to
+  // conservatively satisfy this.
+  //
+  // TODO: This is fairly heavy-handed; find a better approach.
+  //
+  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
+    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
+
+    // Skip unused registers.
+    if (MRI.use_nodbg_empty(Reg))
+      continue;
+
+    // Skip registers that have an ARGUMENT definition.
+    if (HasArgumentDef(Reg, MRI))
+      continue;
+
+    BuildMI(Entry, Entry.begin(), DebugLoc(),
+            TII.get(WebAssembly::IMPLICIT_DEF), Reg);
+    Changed = true;
+  }
+
+  // Move ARGUMENT_* instructions to the top of the entry block, so that their
+  // liveness reflects the fact that these really are live-in values.
+  for (auto MII = Entry.begin(), MIE = Entry.end(); MII != MIE; ) {
+    MachineInstr *MI = &*MII++;
+    if (IsArgument(MI)) {
+      MI->removeFromParent();
+      Entry.insert(Entry.begin(), MI);
+    }
+  }
+
+  // Ok, we're now ready to run LiveIntervalAnalysis again.
+  MF.getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
+
+  return Changed;
+}