[MC] Adding code padding for performance stability - infrastructure. NFC.

Infrastructure designed for padding code with nop instructions in key places such that preformance improvement will be achieved.
The infrastructure is implemented such that the padding is done in the Assembler after the layout is done and all IPs and alignments are known.
This patch by itself in a NFC. Future patches will make use of this infrastructure to implement required policies for code padding.

Reviewers:
aaboud
zvi
craig.topper
gadi.haber

Differential revision: https://reviews.llvm.org/D34393

Change-Id: I92110d0c0a757080a8405636914a93ef6f8ad00e
llvm-svn: 316413

6 files changed, 473 insertions, 1 deletions

diff --git a/llvm/lib/MC/CMakeLists.txt b/llvm/lib/MC/CMakeLists.txt
index 562f136a3ce..b9e23d106e2 100644
--- a/llvm/lib/MC/CMakeLists.txt
+++ b/llvm/lib/MC/CMakeLists.txt
@@ -10,6 +10,7 @@ add_llvm_library(LLVMMC
   MCAsmStreamer.cpp
   MCAssembler.cpp
   MCCodeEmitter.cpp
+  MCCodePadder.cpp
   MCCodeView.cpp
   MCContext.cpp
   MCDwarf.cpp
diff --git a/llvm/lib/MC/MCAsmBackend.cpp b/llvm/lib/MC/MCAsmBackend.cpp
index 3642f37aa85..b4a4d0a8996 100644
--- a/llvm/lib/MC/MCAsmBackend.cpp
+++ b/llvm/lib/MC/MCAsmBackend.cpp
@@ -10,6 +10,7 @@
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/MC/MCCodePadder.h"
 #include "llvm/MC/MCFixupKindInfo.h"
 #include <cassert>
 #include <cstddef>
@@ -17,7 +18,10 @@
 
 using namespace llvm;
 
-MCAsmBackend::MCAsmBackend() = default;
+MCAsmBackend::MCAsmBackend() : CodePadder(new MCCodePadder()) {}
+
+MCAsmBackend::MCAsmBackend(std::unique_ptr<MCCodePadder> TargetCodePadder)
+    : CodePadder(std::move(TargetCodePadder)) {}
 
 MCAsmBackend::~MCAsmBackend() = default;
 
@@ -59,3 +63,25 @@ bool MCAsmBackend::fixupNeedsRelaxationAdvanced(
     return true;
   return fixupNeedsRelaxation(Fixup, Value, DF, Layout);
 }
+
+void MCAsmBackend::handleCodePaddingBasicBlockStart(
+    MCObjectStreamer *OS, const MCCodePaddingContext &Context) {
+  CodePadder->handleBasicBlockStart(OS, Context);
+}
+
+void MCAsmBackend::handleCodePaddingBasicBlockEnd(
+    const MCCodePaddingContext &Context) {
+  CodePadder->handleBasicBlockEnd(Context);
+}
+
+void MCAsmBackend::handleCodePaddingInstructionBegin(const MCInst &Inst) {
+  CodePadder->handleInstructionBegin(Inst);
+}
+
+void MCAsmBackend::handleCodePaddingInstructionEnd(const MCInst &Inst) {
+  CodePadder->handleInstructionEnd(Inst);
+}
+
+bool MCAsmBackend::relaxFragment(MCPaddingFragment *PF, MCAsmLayout &Layout) {
+  return CodePadder->relaxFragment(PF, Layout);
+}
\ No newline at end of file
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index eaf6f19326e..29b14414ea2 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -68,6 +68,10 @@ STATISTIC(FragmentLayouts, "Number of fragment layouts");
 STATISTIC(ObjectBytes, "Number of emitted object file bytes");
 STATISTIC(RelaxationSteps, "Number of assembler layout and relaxation steps");
 STATISTIC(RelaxedInstructions, "Number of relaxed instructions");
+STATISTIC(PaddingFragmentsRelaxations,
+          "Number of Padding Fragments relaxations");
+STATISTIC(PaddingFragmentsBytes,
+          "Total size of all padding from adding Fragments");
 
 } // end namespace stats
 } // end anonymous namespace
@@ -283,6 +287,9 @@ uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
   case MCFragment::FT_LEB:
     return cast<MCLEBFragment>(F).getContents().size();
 
+  case MCFragment::FT_Padding:
+    return cast<MCPaddingFragment>(F).getSize();
+
   case MCFragment::FT_SafeSEH:
     return 4;
 
@@ -549,6 +556,13 @@ static void writeFragment(const MCAssembler &Asm, const MCAsmLayout &Layout,
     break;
   }
 
+  case MCFragment::FT_Padding: {
+    if (!Asm.getBackend().writeNopData(FragmentSize, OW))
+      report_fatal_error("unable to write nop sequence of " +
+                         Twine(FragmentSize) + " bytes");
+    break;
+  }
+
   case MCFragment::FT_SafeSEH: {
     const MCSafeSEHFragment &SF = cast<MCSafeSEHFragment>(F);
     OW->write32(SF.getSymbol()->getIndex());
@@ -822,6 +836,19 @@ bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
   return true;
 }
 
+bool MCAssembler::relaxPaddingFragment(MCAsmLayout &Layout,
+                                       MCPaddingFragment &PF) {
+  uint64_t OldSize = PF.getSize();
+  if (!getBackend().relaxFragment(&PF, Layout))
+    return false;
+  uint64_t NewSize = PF.getSize();
+
+  ++stats::PaddingFragmentsRelaxations;
+  stats::PaddingFragmentsBytes += NewSize;
+  stats::PaddingFragmentsBytes -= OldSize;
+  return true;
+}
+
 bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
   uint64_t OldSize = LF.getContents().size();
   int64_t Value;
@@ -916,6 +943,9 @@ bool MCAssembler::layoutSectionOnce(MCAsmLayout &Layout, MCSection &Sec) {
     case MCFragment::FT_LEB:
       RelaxedFrag = relaxLEB(Layout, *cast<MCLEBFragment>(I));
       break;
+    case MCFragment::FT_Padding:
+      RelaxedFrag = relaxPaddingFragment(Layout, *cast<MCPaddingFragment>(I));
+      break;
     case MCFragment::FT_CVInlineLines:
       RelaxedFrag =
           relaxCVInlineLineTable(Layout, *cast<MCCVInlineLineTableFragment>(I));
diff --git a/llvm/lib/MC/MCCodePadder.cpp b/llvm/lib/MC/MCCodePadder.cpp
new file mode 100644
index 00000000000..57547814e59
--- /dev/null
+++ b/llvm/lib/MC/MCCodePadder.cpp
@@ -0,0 +1,371 @@
+//===- MCCodePadder.cpp - Target MC Code Padder ---------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/MC/MCAsmLayout.h"
+#include "llvm/MC/MCCodePadder.h"
+#include "llvm/MC/MCObjectStreamer.h"
+#include <algorithm>
+#include <limits>
+#include <numeric>
+
+using namespace llvm;
+
+//---------------------------------------------------------------------------
+// MCCodePadder
+//
+
+MCCodePadder::~MCCodePadder() {
+  for (auto *Policy : CodePaddingPolicies)
+    delete Policy;
+}
+
+bool MCCodePadder::addPolicy(MCCodePaddingPolicy *Policy) {
+  assert(Policy && "Policy must be valid");
+  return CodePaddingPolicies.insert(Policy).second;
+}
+
+void MCCodePadder::handleBasicBlockStart(MCObjectStreamer *OS,
+                                         const MCCodePaddingContext &Context) {
+  assert(OS != nullptr && "OS must be valid");
+  assert(this->OS == nullptr && "Still handling another basic block");
+  this->OS = OS;
+
+  ArePoliciesActive = usePoliciesForBasicBlock(Context);
+
+  bool InsertionPoint = basicBlockRequiresInsertionPoint(Context);
+  assert((!InsertionPoint ||
+          OS->getCurrentFragment()->getKind() != MCFragment::FT_Align) &&
+         "Cannot insert padding nops right after an alignment fragment as it "
+         "will ruin the alignment");
+
+  uint64_t PoliciesMask = MCPaddingFragment::PFK_None;
+  if (ArePoliciesActive) {
+    PoliciesMask = std::accumulate(
+        CodePaddingPolicies.begin(), CodePaddingPolicies.end(),
+        MCPaddingFragment::PFK_None,
+        [&Context](uint64_t Mask,
+                   const MCCodePaddingPolicy *Policy) -> uint64_t {
+          return Policy->basicBlockRequiresPaddingFragment(Context)
+                     ? (Mask | Policy->getKindMask())
+                     : Mask;
+        });
+  }
+
+  if (InsertionPoint || PoliciesMask != MCPaddingFragment::PFK_None) {
+    MCPaddingFragment *PaddingFragment = OS->getOrCreatePaddingFragment();
+    if (InsertionPoint)
+      PaddingFragment->setAsInsertionPoint();
+    PaddingFragment->setPaddingPoliciesMask(
+        PaddingFragment->getPaddingPoliciesMask() | PoliciesMask);
+  }
+}
+
+void MCCodePadder::handleBasicBlockEnd(const MCCodePaddingContext &Context) {
+  assert(this->OS != nullptr && "Not handling a basic block");
+  OS = nullptr;
+}
+
+void MCCodePadder::handleInstructionBegin(const MCInst &Inst) {
+  if (!OS)
+    return; // instruction was emitted outside a function
+
+  assert(CurrHandledInstFragment == nullptr && "Can't start handling an "
+                                               "instruction while still "
+                                               "handling another instruction");
+
+  bool InsertionPoint = instructionRequiresInsertionPoint(Inst);
+  assert((!InsertionPoint ||
+          OS->getCurrentFragment()->getKind() != MCFragment::FT_Align) &&
+         "Cannot insert padding nops right after an alignment fragment as it "
+         "will ruin the alignment");
+
+  uint64_t PoliciesMask = MCPaddingFragment::PFK_None;
+  if (ArePoliciesActive) {
+    PoliciesMask = std::accumulate(
+        CodePaddingPolicies.begin(), CodePaddingPolicies.end(),
+        MCPaddingFragment::PFK_None,
+        [&Inst](uint64_t Mask, const MCCodePaddingPolicy *Policy) -> uint64_t {
+          return Policy->instructionRequiresPaddingFragment(Inst)
+                     ? (Mask | Policy->getKindMask())
+                     : Mask;
+        });
+  }
+  MCFragment *CurrFragment = OS->getCurrentFragment();
+  // CurrFragment can be a previously created MCPaddingFragment. If so, let's
+  // update it with the information we have, such as the instruction that it
+  // should point to.
+  bool needToUpdateCurrFragment =
+      CurrFragment != nullptr &&
+      CurrFragment->getKind() == MCFragment::FT_Padding;
+  if (InsertionPoint || PoliciesMask != MCPaddingFragment::PFK_None ||
+      needToUpdateCurrFragment) {
+    // temporarily holding the fragment as CurrHandledInstFragment, to be
+    // updated after the instruction will be written
+    CurrHandledInstFragment = OS->getOrCreatePaddingFragment();
+    if (InsertionPoint)
+      CurrHandledInstFragment->setAsInsertionPoint();
+    CurrHandledInstFragment->setPaddingPoliciesMask(
+        CurrHandledInstFragment->getPaddingPoliciesMask() | PoliciesMask);
+  }
+}
+
+void MCCodePadder::handleInstructionEnd(const MCInst &Inst) {
+  if (!OS)
+    return; // instruction was emitted outside a function
+  if (CurrHandledInstFragment == nullptr)
+    return;
+
+  MCFragment *InstFragment = OS->getCurrentFragment();
+  if (MCDataFragment *InstDataFragment =
+          dyn_cast_or_null<MCDataFragment>(InstFragment))
+    // Inst is a fixed size instruction and was encoded into a MCDataFragment.
+    // Let the fragment hold it and its size. Its size is the current size of
+    // the data fragment, as the padding fragment was inserted right before it
+    // and nothing was written yet except Inst
+    CurrHandledInstFragment->setInstAndInstSize(
+        Inst, InstDataFragment->getContents().size());
+  else if (MCRelaxableFragment *InstRelaxableFragment =
+               dyn_cast_or_null<MCRelaxableFragment>(InstFragment))
+    // Inst may be relaxed and its size may vary.
+    // Let the fragment hold the instruction and the MCRelaxableFragment
+    // that's holding it.
+    CurrHandledInstFragment->setInstAndInstFragment(Inst,
+                                                    InstRelaxableFragment);
+  else
+    llvm_unreachable("After encoding an instruction current fragment must be "
+                     "either a MCDataFragment or a MCRelaxableFragment");
+
+  CurrHandledInstFragment = nullptr;
+}
+
+MCPFRange &MCCodePadder::getJurisdiction(MCPaddingFragment *Fragment,
+                                         MCAsmLayout &Layout) {
+  auto JurisdictionLocation = FragmentToJurisdiction.find(Fragment);
+  if (JurisdictionLocation != FragmentToJurisdiction.end())
+    return JurisdictionLocation->second;
+
+  MCPFRange Jurisdiction;
+
+  // Forward scanning the fragments in this section, starting from the given
+  // fragments, and adding relevant MCPaddingFragments to the Jurisdiction
+  for (MCFragment *CurrFragment = Fragment; CurrFragment != nullptr;
+       CurrFragment = CurrFragment->getNextNode()) {
+
+    MCPaddingFragment *CurrPaddingFragment =
+        dyn_cast<MCPaddingFragment>(CurrFragment);
+    if (CurrPaddingFragment == nullptr)
+      continue;
+
+    if (CurrPaddingFragment != Fragment &&
+        CurrPaddingFragment->isInsertionPoint())
+      // Found next insertion point Fragment. From now on it's its jurisdiction.
+      break;
+    for (const auto *Policy : CodePaddingPolicies) {
+      if (CurrPaddingFragment->hasPaddingPolicy(Policy->getKindMask())) {
+        Jurisdiction.push_back(CurrPaddingFragment);
+        break;
+      }
+    }
+  }
+
+  auto InsertionResult =
+      FragmentToJurisdiction.insert(std::make_pair(Fragment, Jurisdiction));
+  assert(InsertionResult.second &&
+         "Insertion to FragmentToJurisdiction failed");
+  return InsertionResult.first->second;
+}
+
+uint64_t MCCodePadder::getMaxWindowSize(MCPaddingFragment *Fragment,
+                                        MCAsmLayout &Layout) {
+  auto MaxFragmentSizeLocation = FragmentToMaxWindowSize.find(Fragment);
+  if (MaxFragmentSizeLocation != FragmentToMaxWindowSize.end())
+    return MaxFragmentSizeLocation->second;
+
+  MCPFRange &Jurisdiction = getJurisdiction(Fragment, Layout);
+  uint64_t JurisdictionMask = MCPaddingFragment::PFK_None;
+  for (const auto *Protege : Jurisdiction)
+    JurisdictionMask |= Protege->getPaddingPoliciesMask();
+
+  uint64_t MaxFragmentSize = UINT64_C(0);
+  for (const auto *Policy : CodePaddingPolicies)
+    if ((JurisdictionMask & Policy->getKindMask()) !=
+        MCPaddingFragment::PFK_None)
+      MaxFragmentSize = std::max(MaxFragmentSize, Policy->getWindowSize());
+
+  auto InsertionResult =
+      FragmentToMaxWindowSize.insert(std::make_pair(Fragment, MaxFragmentSize));
+  assert(InsertionResult.second &&
+         "Insertion to FragmentToMaxWindowSize failed");
+  return InsertionResult.first->second;
+}
+
+bool MCCodePadder::relaxFragment(MCPaddingFragment *Fragment,
+                                 MCAsmLayout &Layout) {
+  if (!Fragment->isInsertionPoint())
+    return false;
+  uint64_t OldSize = Fragment->getSize();
+
+  uint64_t MaxWindowSize = getMaxWindowSize(Fragment, Layout);
+  if (MaxWindowSize == UINT64_C(0))
+    return false;
+  assert(isPowerOf2_64(MaxWindowSize) &&
+         "MaxWindowSize must be an integer power of 2");
+  uint64_t SectionAlignment = Fragment->getParent()->getAlignment();
+  assert(isPowerOf2_64(SectionAlignment) &&
+         "SectionAlignment must be an integer power of 2");
+
+  MCPFRange &Jurisdiction = getJurisdiction(Fragment, Layout);
+  uint64_t OptimalSize = UINT64_C(0);
+  double OptimalWeight = std::numeric_limits<double>::max();
+  uint64_t MaxFragmentSize = MaxWindowSize - UINT16_C(1);
+  for (uint64_t Size = UINT64_C(0); Size <= MaxFragmentSize; ++Size) {
+    Fragment->setSize(Size);
+    Layout.invalidateFragmentsFrom(Fragment);
+    double SizeWeight = 0.0;
+    // The section is guaranteed to be aligned to SectionAlignment, but that
+    // doesn't guarantee the exact section offset w.r.t. the policies window
+    // size.
+    // As a concrete example, the section could be aligned to 16B, but a
+    // policy's window size can be 32B. That means that the section actual start
+    // address can either be 0mod32 or 16mod32. The said policy will act
+    // differently for each case, so we need to take both into consideration.
+    for (uint64_t Offset = UINT64_C(0); Offset < MaxWindowSize;
+         Offset += SectionAlignment) {
+      double OffsetWeight = std::accumulate(
+          CodePaddingPolicies.begin(), CodePaddingPolicies.end(), 0.0,
+          [&Jurisdiction, &Offset, &Layout](
+              double Weight, const MCCodePaddingPolicy *Policy) -> double {
+            double PolicyWeight =
+                Policy->computeRangePenaltyWeight(Jurisdiction, Offset, Layout);
+            assert(PolicyWeight >= 0.0 && "A penalty weight must be positive");
+            return Weight + PolicyWeight;
+          });
+      SizeWeight = std::max(SizeWeight, OffsetWeight);
+    }
+    if (SizeWeight < OptimalWeight) {
+      OptimalWeight = SizeWeight;
+      OptimalSize = Size;
+    }
+    if (OptimalWeight == 0.0)
+      break;
+  }
+
+  Fragment->setSize(OptimalSize);
+  Layout.invalidateFragmentsFrom(Fragment);
+  return OldSize != OptimalSize;
+}
+
+//---------------------------------------------------------------------------
+// MCCodePaddingPolicy
+//
+
+uint64_t MCCodePaddingPolicy::getNextFragmentOffset(const MCFragment *Fragment,
+                                                    const MCAsmLayout &Layout) {
+  assert(Fragment != nullptr && "Fragment cannot be null");
+  MCFragment const *NextFragment = Fragment->getNextNode();
+  return NextFragment == nullptr
+             ? Layout.getSectionAddressSize(Fragment->getParent())
+             : Layout.getFragmentOffset(NextFragment);
+}
+
+uint64_t
+MCCodePaddingPolicy::getFragmentInstByte(const MCPaddingFragment *Fragment,
+                                         MCAsmLayout &Layout) const {
+  uint64_t InstByte = getNextFragmentOffset(Fragment, Layout);
+  if (InstByteIsLastByte)
+    InstByte += Fragment->getInstSize() - UINT64_C(1);
+  return InstByte;
+}
+
+uint64_t
+MCCodePaddingPolicy::computeWindowEndAddress(const MCPaddingFragment *Fragment,
+                                             uint64_t Offset,
+                                             MCAsmLayout &Layout) const {
+  uint64_t InstByte = getFragmentInstByte(Fragment, Layout);
+  return alignTo(InstByte + UINT64_C(1) + Offset, WindowSize) - Offset;
+}
+
+double MCCodePaddingPolicy::computeRangePenaltyWeight(
+    const MCPFRange &Range, uint64_t Offset, MCAsmLayout &Layout) const {
+
+  SmallVector<MCPFRange, 8> Windows;
+  SmallVector<MCPFRange, 8>::iterator CurrWindowLocation = Windows.end();
+  for (const MCPaddingFragment *Fragment : Range) {
+    if (!Fragment->hasPaddingPolicy(getKindMask()))
+      continue;
+    uint64_t FragmentWindowEndAddress =
+        computeWindowEndAddress(Fragment, Offset, Layout);
+    if (CurrWindowLocation == Windows.end() ||
+        FragmentWindowEndAddress !=
+            computeWindowEndAddress(*CurrWindowLocation->begin(), Offset,
+                                    Layout)) {
+      // next window is starting
+      Windows.push_back(MCPFRange());
+      CurrWindowLocation = Windows.end() - 1;
+    }
+    CurrWindowLocation->push_back(Fragment);
+  }
+
+  if (Windows.empty())
+    return 0.0;
+
+  double RangeWeight = 0.0;
+  SmallVector<MCPFRange, 8>::iterator I = Windows.begin();
+  RangeWeight += computeFirstWindowPenaltyWeight(*I, Offset, Layout);
+  ++I;
+  RangeWeight += std::accumulate(
+      I, Windows.end(), 0.0,
+      [this, &Layout, &Offset](double Weight, MCPFRange &Window) -> double {
+        return Weight += computeWindowPenaltyWeight(Window, Offset, Layout);
+      });
+  return RangeWeight;
+}
+
+double MCCodePaddingPolicy::computeFirstWindowPenaltyWeight(
+    const MCPFRange &Window, uint64_t Offset, MCAsmLayout &Layout) const {
+  if (Window.empty())
+    return 0.0;
+  uint64_t WindowEndAddress =
+      computeWindowEndAddress(*Window.begin(), Offset, Layout);
+
+  MCPFRange FullWindowFirstPart; // will hold all the fragments that are in the
+								 // same window as the fragments in the given
+								 // window but their penalty weight should not
+								 // be added
+  for (const MCFragment *Fragment = (*Window.begin())->getPrevNode();
+       Fragment != nullptr; Fragment = Fragment->getPrevNode()) {
+    const MCPaddingFragment *PaddingNopFragment =
+        dyn_cast<MCPaddingFragment>(Fragment);
+    if (PaddingNopFragment == nullptr ||
+        !PaddingNopFragment->hasPaddingPolicy(getKindMask()))
+      continue;
+    if (WindowEndAddress !=
+        computeWindowEndAddress(PaddingNopFragment, Offset, Layout))
+      break;
+
+    FullWindowFirstPart.push_back(PaddingNopFragment);
+  }
+
+  std::reverse(FullWindowFirstPart.begin(), FullWindowFirstPart.end());
+  double FullWindowFirstPartWeight =
+      computeWindowPenaltyWeight(FullWindowFirstPart, Offset, Layout);
+
+  MCPFRange FullWindow(
+      FullWindowFirstPart); // will hold all the fragments that are in the
+                            // same window as the fragments in the given
+                            // window, whether their weight should be added
+                            // or not
+  FullWindow.append(Window.begin(), Window.end());
+  double FullWindowWeight =
+      computeWindowPenaltyWeight(FullWindow, Offset, Layout);
+
+  assert(FullWindowWeight >= FullWindowFirstPartWeight &&
+         "More fragments necessarily means bigger weight");
+  return FullWindowWeight - FullWindowFirstPartWeight;
+}
diff --git a/llvm/lib/MC/MCFragment.cpp b/llvm/lib/MC/MCFragment.cpp
index 31acca01bed..94839de14f8 100644
--- a/llvm/lib/MC/MCFragment.cpp
+++ b/llvm/lib/MC/MCFragment.cpp
@@ -278,6 +278,9 @@ void MCFragment::destroy() {
     case FT_LEB:
       delete cast<MCLEBFragment>(this);
       return;
+    case FT_Padding:
+      delete cast<MCPaddingFragment>(this);
+      return;
     case FT_SafeSEH:
       delete cast<MCSafeSEHFragment>(this);
       return;
@@ -322,6 +325,7 @@ LLVM_DUMP_METHOD void MCFragment::dump() const {
   case MCFragment::FT_Dwarf: OS << "MCDwarfFragment"; break;
   case MCFragment::FT_DwarfFrame: OS << "MCDwarfCallFrameFragment"; break;
   case MCFragment::FT_LEB:   OS << "MCLEBFragment"; break;
+  case MCFragment::FT_Padding: OS << "MCPaddingFragment"; break;
   case MCFragment::FT_SafeSEH:    OS << "MCSafeSEHFragment"; break;
   case MCFragment::FT_CVInlineLines: OS << "MCCVInlineLineTableFragment"; break;
   case MCFragment::FT_CVDefRange: OS << "MCCVDefRangeTableFragment"; break;
@@ -419,6 +423,19 @@ LLVM_DUMP_METHOD void MCFragment::dump() const {
     OS << " Value:" << LF->getValue() << " Signed:" << LF->isSigned();
     break;
   }
+  case MCFragment::FT_Padding: {
+    const MCPaddingFragment *F = cast<MCPaddingFragment>(this);
+    OS << "\n       ";
+    OS << " PaddingPoliciesMask:" << F->getPaddingPoliciesMask()
+       << " IsInsertionPoint:" << F->isInsertionPoint()
+       << " Size:" << F->getSize();
+    OS << "\n       ";
+    OS << " Inst:";
+    F->getInst().dump_pretty(OS);
+    OS << " InstSize:" << F->getInstSize();
+    OS << "\n       ";
+    break;
+  }
   case MCFragment::FT_SafeSEH: {
     const MCSafeSEHFragment *F = cast<MCSafeSEHFragment>(this);
     OS << "\n       ";
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 2fd969de296..f226c2f0a30 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -108,6 +108,16 @@ MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() {
   return F;
 }
 
+MCPaddingFragment *MCObjectStreamer::getOrCreatePaddingFragment() {
+  MCPaddingFragment *F =
+      dyn_cast_or_null<MCPaddingFragment>(getCurrentFragment());
+  if (!F) {
+    F = new MCPaddingFragment();
+    insert(F);
+  }
+  return F;
+}
+
 void MCObjectStreamer::visitUsedSymbol(const MCSymbol &Sym) {
   Assembler->registerSymbol(Sym);
 }
@@ -247,6 +257,13 @@ bool MCObjectStreamer::mayHaveInstructions(MCSection &Sec) const {
 
 void MCObjectStreamer::EmitInstruction(const MCInst &Inst,
                                        const MCSubtargetInfo &STI, bool) {
+  getAssembler().getBackend().handleCodePaddingInstructionBegin(Inst);
+  EmitInstructionImpl(Inst, STI);
+  getAssembler().getBackend().handleCodePaddingInstructionEnd(Inst);
+}
+
+void MCObjectStreamer::EmitInstructionImpl(const MCInst &Inst,
+                                           const MCSubtargetInfo &STI) {
   MCStreamer::EmitInstruction(Inst, STI);
 
   MCSection *Sec = getCurrentSectionOnly();
@@ -467,6 +484,16 @@ void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset,
   insert(new MCOrgFragment(*Offset, Value, Loc));
 }
 
+void MCObjectStreamer::EmitCodePaddingBasicBlockStart(
+    const MCCodePaddingContext &Context) {
+  getAssembler().getBackend().handleCodePaddingBasicBlockStart(this, Context);
+}
+
+void MCObjectStreamer::EmitCodePaddingBasicBlockEnd(
+    const MCCodePaddingContext &Context) {
+  getAssembler().getBackend().handleCodePaddingBasicBlockEnd(Context);
+}
+
 // Associate DTPRel32 fixup with data and resize data area
 void MCObjectStreamer::EmitDTPRel32Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();