Add support for subsections to the ELF assembler. Fixes PR8717.

Differential Revision: http://llvm-reviews.chandlerc.com/D598

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179725 91177308-0d34-0410-b5e6-96231b3b80d8

30 files changed, 288 insertions, 118 deletions

diff --git a/include/llvm/MC/MCAssembler.h b/include/llvm/MC/MCAssembler.h
index 32df38b..38a70f0 100644
--- a/include/llvm/MC/MCAssembler.h
+++ b/include/llvm/MC/MCAssembler.h
@@ -82,7 +82,7 @@ private:
   /// @}
 
 protected:
-  MCFragment(FragmentType _Kind, MCSectionData *_Parent);
+  MCFragment(FragmentType _Kind, MCSectionData *_Parent = 0);
 
 public:
   // Only for sentinel.
@@ -92,6 +92,7 @@ public:
   FragmentType getKind() const { return Kind; }
 
   MCSectionData *getParent() const { return Parent; }
+  void setParent(MCSectionData *Value) { Parent = Value; }
 
   MCSymbolData *getAtom() const { return Atom; }
   void setAtom(MCSymbolData *Value) { Atom = Value; }
@@ -132,7 +133,7 @@ class MCEncodedFragment : public MCFragment {
 
   uint8_t BundlePadding;
 public:
-  MCEncodedFragment(MCFragment::FragmentType FType, MCSectionData *SD)
+  MCEncodedFragment(MCFragment::FragmentType FType, MCSectionData *SD = 0)
     : MCFragment(FType, SD), BundlePadding(0)
   {
   }
@@ -347,7 +348,7 @@ class MCAlignFragment : public MCFragment {
 
 public:
   MCAlignFragment(unsigned _Alignment, int64_t _Value, unsigned _ValueSize,
-                  unsigned _MaxBytesToEmit, MCSectionData *SD)
+                  unsigned _MaxBytesToEmit, MCSectionData *SD = 0)
     : MCFragment(FT_Align, SD), Alignment(_Alignment),
       Value(_Value),ValueSize(_ValueSize),
       MaxBytesToEmit(_MaxBytesToEmit), EmitNops(false) {}
@@ -388,7 +389,7 @@ class MCFillFragment : public MCFragment {
 
 public:
   MCFillFragment(int64_t _Value, unsigned _ValueSize, uint64_t _Size,
-                 MCSectionData *SD)
+                 MCSectionData *SD = 0)
     : MCFragment(FT_Fill, SD),
       Value(_Value), ValueSize(_ValueSize), Size(_Size) {
     assert((!ValueSize || (Size % ValueSize) == 0) &&
@@ -421,7 +422,7 @@ class MCOrgFragment : public MCFragment {
   int8_t Value;
 
 public:
-  MCOrgFragment(const MCExpr &_Offset, int8_t _Value, MCSectionData *SD)
+  MCOrgFragment(const MCExpr &_Offset, int8_t _Value, MCSectionData *SD = 0)
     : MCFragment(FT_Org, SD),
       Offset(&_Offset), Value(_Value) {}
 
@@ -450,7 +451,7 @@ class MCLEBFragment : public MCFragment {
 
   SmallString<8> Contents;
 public:
-  MCLEBFragment(const MCExpr &Value_, bool IsSigned_, MCSectionData *SD)
+  MCLEBFragment(const MCExpr &Value_, bool IsSigned_, MCSectionData *SD = 0)
     : MCFragment(FT_LEB, SD),
       Value(&Value_), IsSigned(IsSigned_) { Contents.push_back(0); }
 
@@ -486,7 +487,7 @@ class MCDwarfLineAddrFragment : public MCFragment {
 
 public:
   MCDwarfLineAddrFragment(int64_t _LineDelta, const MCExpr &_AddrDelta,
-                      MCSectionData *SD)
+                      MCSectionData *SD = 0)
     : MCFragment(FT_Dwarf, SD),
       LineDelta(_LineDelta), AddrDelta(&_AddrDelta) { Contents.push_back(0); }
 
@@ -517,7 +518,7 @@ class MCDwarfCallFrameFragment : public MCFragment {
   SmallString<8> Contents;
 
 public:
-  MCDwarfCallFrameFragment(const MCExpr &_AddrDelta,  MCSectionData *SD)
+  MCDwarfCallFrameFragment(const MCExpr &_AddrDelta,  MCSectionData *SD = 0)
     : MCFragment(FT_DwarfFrame, SD),
       AddrDelta(&_AddrDelta) { Contents.push_back(0); }
 
@@ -589,6 +590,10 @@ private:
   /// it.
   unsigned HasInstructions : 1;
 
+  /// Mapping from subsection number to insertion point for subsection numbers
+  /// below that number.
+  SmallVector<std::pair<unsigned, MCFragment *>, 1> SubsectionFragmentMap;
+
   /// @}
 
 public:
@@ -632,6 +637,8 @@ public:
 
   bool empty() const { return Fragments.empty(); }
 
+  iterator getSubsectionInsertionPoint(unsigned Subsection);
+
   bool isBundleLocked() const {
     return BundleLockState != NotBundleLocked;
   }
diff --git a/include/llvm/MC/MCELFStreamer.h b/include/llvm/MC/MCELFStreamer.h
index 6fb2d22..55c05b0 100644
--- a/include/llvm/MC/MCELFStreamer.h
+++ b/include/llvm/MC/MCELFStreamer.h
@@ -50,7 +50,8 @@ public:
 
   virtual void InitSections();
   virtual void InitToTextSection();
-  virtual void ChangeSection(const MCSection *Section);
+  virtual void ChangeSection(const MCSection *Section,
+                             const MCExpr *Subsection);
   virtual void EmitLabel(MCSymbol *Symbol);
   virtual void EmitDebugLabel(MCSymbol *Symbol);
   virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
diff --git a/include/llvm/MC/MCObjectStreamer.h b/include/llvm/MC/MCObjectStreamer.h
index f06c49f..22a2839 100644
--- a/include/llvm/MC/MCObjectStreamer.h
+++ b/include/llvm/MC/MCObjectStreamer.h
@@ -10,6 +10,7 @@
 #ifndef LLVM_MC_MCOBJECTSTREAMER_H
 #define LLVM_MC_MCOBJECTSTREAMER_H
 
+#include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCStreamer.h"
 
 namespace llvm {
@@ -32,6 +33,7 @@ class raw_ostream;
 class MCObjectStreamer : public MCStreamer {
   MCAssembler *Assembler;
   MCSectionData *CurSectionData;
+  MCSectionData::iterator CurInsertionPoint;
 
   virtual void EmitInstToData(const MCInst &Inst) = 0;
   virtual void EmitCFIStartProcImpl(MCDwarfFrameInfo &Frame);
@@ -56,6 +58,11 @@ protected:
 
   MCFragment *getCurrentFragment() const;
 
+  void insert(MCFragment *F) const {
+    CurSectionData->getFragmentList().insert(CurInsertionPoint, F);
+    F->setParent(CurSectionData);
+  }
+
   /// Get a data fragment to write into, creating a new one if the current
   /// fragment is not a data fragment.
   MCDataFragment *getOrCreateDataFragment() const;
@@ -76,7 +83,8 @@ public:
   virtual void EmitULEB128Value(const MCExpr *Value);
   virtual void EmitSLEB128Value(const MCExpr *Value);
   virtual void EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol);
-  virtual void ChangeSection(const MCSection *Section);
+  virtual void ChangeSection(const MCSection *Section,
+                             const MCExpr *Subsection);
   virtual void EmitInstruction(const MCInst &Inst);
 
   /// \brief Emit an instruction to a special fragment, because this instruction
diff --git a/include/llvm/MC/MCSection.h b/include/llvm/MC/MCSection.h
index e575424..de2678a 100644
--- a/include/llvm/MC/MCSection.h
+++ b/include/llvm/MC/MCSection.h
@@ -20,6 +20,7 @@
 
 namespace llvm {
   class MCAsmInfo;
+  class MCExpr;
   class raw_ostream;
 
   /// MCSection - Instances of this class represent a uniqued identifier for a
@@ -48,7 +49,8 @@ namespace llvm {
     SectionVariant getVariant() const { return Variant; }
 
     virtual void PrintSwitchToSection(const MCAsmInfo &MAI,
-                                      raw_ostream &OS) const = 0;
+                                      raw_ostream &OS,
+                                      const MCExpr *Subsection) const = 0;
 
     // Convenience routines to get label names for the beginning/end of a
     // section.
diff --git a/include/llvm/MC/MCSectionCOFF.h b/include/llvm/MC/MCSectionCOFF.h
index 07c4714..50e33a5 100644
--- a/include/llvm/MC/MCSectionCOFF.h
+++ b/include/llvm/MC/MCSectionCOFF.h
@@ -60,7 +60,8 @@ namespace llvm {
     int getSelection () const { return Selection; }
 
     virtual void PrintSwitchToSection(const MCAsmInfo &MAI,
-                                      raw_ostream &OS) const;
+                                      raw_ostream &OS,
+                                      const MCExpr *Subsection) const;
     virtual bool UseCodeAlign() const;
     virtual bool isVirtualSection() const;
 
diff --git a/include/llvm/MC/MCSectionELF.h b/include/llvm/MC/MCSectionELF.h
index 4b8b849..5979915 100644
--- a/include/llvm/MC/MCSectionELF.h
+++ b/include/llvm/MC/MCSectionELF.h
@@ -70,7 +70,8 @@ public:
   const MCSymbol *getGroup() const { return Group; }
 
   void PrintSwitchToSection(const MCAsmInfo &MAI,
-                            raw_ostream &OS) const;
+                            raw_ostream &OS,
+                            const MCExpr *Subsection) const;
   virtual bool UseCodeAlign() const;
   virtual bool isVirtualSection() const;
 
diff --git a/include/llvm/MC/MCSectionMachO.h b/include/llvm/MC/MCSectionMachO.h
index 898f571..b68bd85 100644
--- a/include/llvm/MC/MCSectionMachO.h
+++ b/include/llvm/MC/MCSectionMachO.h
@@ -175,7 +175,8 @@ public:
                                            unsigned  &StubSize); // Out.
 
   virtual void PrintSwitchToSection(const MCAsmInfo &MAI,
-                                    raw_ostream &OS) const;
+                                    raw_ostream &OS,
+                                    const MCExpr *Subsection) const;
   virtual bool UseCodeAlign() const;
   virtual bool isVirtualSection() const;
 
diff --git a/include/llvm/MC/MCStreamer.h b/include/llvm/MC/MCStreamer.h
index a069a2b..2cab481 100644
--- a/include/llvm/MC/MCStreamer.h
+++ b/include/llvm/MC/MCStreamer.h
@@ -37,6 +37,8 @@ namespace llvm {
   class raw_ostream;
   class formatted_raw_ostream;
 
+  typedef std::pair<const MCSection *, const MCExpr *> MCSectionSubPair;
+
   /// MCStreamer - Streaming machine code generation interface.  This interface
   /// is intended to provide a programatic interface that is very similar to the
   /// level that an assembler .s file provides.  It has callbacks to emit bytes,
@@ -86,8 +88,7 @@ namespace llvm {
 
     /// SectionStack - This is stack of current and previous section
     /// values saved by PushSection.
-    SmallVector<std::pair<const MCSection *,
-                const MCSection *>, 4> SectionStack;
+    SmallVector<std::pair<MCSectionSubPair, MCSectionSubPair>, 4> SectionStack;
 
     bool AutoInitSections;
 
@@ -174,25 +175,25 @@ namespace llvm {
 
     /// getCurrentSection - Return the current section that the streamer is
     /// emitting code to.
-    const MCSection *getCurrentSection() const {
+    MCSectionSubPair getCurrentSection() const {
       if (!SectionStack.empty())
         return SectionStack.back().first;
-      return NULL;
+      return MCSectionSubPair();
     }
 
     /// getPreviousSection - Return the previous section that the streamer is
     /// emitting code to.
-    const MCSection *getPreviousSection() const {
+    MCSectionSubPair getPreviousSection() const {
       if (!SectionStack.empty())
         return SectionStack.back().second;
-      return NULL;
+      return MCSectionSubPair();
     }
 
     /// ChangeSection - Update streamer for a new active section.
     ///
     /// This is called by PopSection and SwitchSection, if the current
     /// section changes.
-    virtual void ChangeSection(const MCSection *) = 0;
+    virtual void ChangeSection(const MCSection *, const MCExpr *) = 0;
 
     /// pushSection - Save the current and previous section on the
     /// section stack.
@@ -208,11 +209,19 @@ namespace llvm {
     bool PopSection() {
       if (SectionStack.size() <= 1)
         return false;
-      const MCSection *oldSection = SectionStack.pop_back_val().first;
-      const MCSection *curSection = SectionStack.back().first;
+      MCSectionSubPair oldSection = SectionStack.pop_back_val().first;
+      MCSectionSubPair curSection = SectionStack.back().first;
 
       if (oldSection != curSection)
-        ChangeSection(curSection);
+        ChangeSection(curSection.first, curSection.second);
+      return true;
+    }
+
+    bool SubSection(const MCExpr *Subsection) {
+      if (SectionStack.empty())
+        return false;
+
+      SwitchSection(SectionStack.back().first.first, Subsection);
       return true;
     }
 
@@ -220,25 +229,26 @@ namespace llvm {
     /// @p Section.  This is required to update CurSection.
     ///
     /// This corresponds to assembler directives like .section, .text, etc.
-    void SwitchSection(const MCSection *Section) {
+    void SwitchSection(const MCSection *Section, const MCExpr *Subsection = 0) {
       assert(Section && "Cannot switch to a null section!");
-      const MCSection *curSection = SectionStack.back().first;
+      MCSectionSubPair curSection = SectionStack.back().first;
       SectionStack.back().second = curSection;
-      if (Section != curSection) {
-        SectionStack.back().first = Section;
-        ChangeSection(Section);
+      if (MCSectionSubPair(Section, Subsection) != curSection) {
+        SectionStack.back().first = MCSectionSubPair(Section, Subsection);
+        ChangeSection(Section, Subsection);
       }
     }
 
     /// SwitchSectionNoChange - Set the current section where code is being
     /// emitted to @p Section.  This is required to update CurSection. This
     /// version does not call ChangeSection.
-    void SwitchSectionNoChange(const MCSection *Section) {
+    void SwitchSectionNoChange(const MCSection *Section,
+                               const MCExpr *Subsection = 0) {
       assert(Section && "Cannot switch to a null section!");
-      const MCSection *curSection = SectionStack.back().first;
+      MCSectionSubPair curSection = SectionStack.back().first;
       SectionStack.back().second = curSection;
-      if (Section != curSection)
-        SectionStack.back().first = Section;
+      if (MCSectionSubPair(Section, Subsection) != curSection)
+        SectionStack.back().first = MCSectionSubPair(Section, Subsection);
     }
 
     /// Initialize the streamer.
diff --git a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index d4a745d..2d13db2 100644
--- a/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -135,7 +135,7 @@ const DataLayout &AsmPrinter::getDataLayout() const {
 
 /// getCurrentSection() - Return the current section we are emitting to.
 const MCSection *AsmPrinter::getCurrentSection() const {
-  return OutStreamer.getCurrentSection();
+  return OutStreamer.getCurrentSection().first;
 }
 
 
diff --git a/lib/MC/MCAsmStreamer.cpp b/lib/MC/MCAsmStreamer.cpp
index b35a9de..9e86785 100644
--- a/lib/MC/MCAsmStreamer.cpp
+++ b/lib/MC/MCAsmStreamer.cpp
@@ -124,7 +124,8 @@ public:
   /// @name MCStreamer Interface
   /// @{
 
-  virtual void ChangeSection(const MCSection *Section);
+  virtual void ChangeSection(const MCSection *Section,
+                             const MCExpr *Subsection);
 
   virtual void InitSections() {
     InitToTextSection();
@@ -328,9 +329,10 @@ static inline int64_t truncateToSize(int64_t Value, unsigned Bytes) {
   return Value & ((uint64_t) (int64_t) -1 >> (64 - Bytes * 8));
 }
 
-void MCAsmStreamer::ChangeSection(const MCSection *Section) {
+void MCAsmStreamer::ChangeSection(const MCSection *Section,
+                                  const MCExpr *Subsection) {
   assert(Section && "Cannot switch to a null section!");
-  Section->PrintSwitchToSection(MAI, OS);
+  Section->PrintSwitchToSection(MAI, OS, Subsection);
 }
 
 void MCAsmStreamer::EmitEHSymAttributes(const MCSymbol *Symbol,
@@ -637,7 +639,8 @@ static void PrintQuotedString(StringRef Data, raw_ostream &OS) {
 
 
 void MCAsmStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
-  assert(getCurrentSection() && "Cannot emit contents before setting section!");
+  assert(getCurrentSection().first &&
+         "Cannot emit contents before setting section!");
   if (Data.empty()) return;
 
   if (Data.size() == 1) {
@@ -668,7 +671,8 @@ void MCAsmStreamer::EmitIntValue(uint64_t Value, unsigned Size,
 
 void MCAsmStreamer::EmitValueImpl(const MCExpr *Value, unsigned Size,
                                   unsigned AddrSpace) {
-  assert(getCurrentSection() && "Cannot emit contents before setting section!");
+  assert(getCurrentSection().first &&
+         "Cannot emit contents before setting section!");
   const char *Directive = 0;
   switch (Size) {
   default: break;
@@ -1363,7 +1367,8 @@ void MCAsmStreamer::EmitTCEntry(const MCSymbol &S) {
 }
 
 void MCAsmStreamer::EmitInstruction(const MCInst &Inst) {
-  assert(getCurrentSection() && "Cannot emit contents before setting section!");
+  assert(getCurrentSection().first &&
+         "Cannot emit contents before setting section!");
 
   // Show the encoding in a comment if we have a code emitter.
   if (Emitter)
diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 8970e18..fb5ab28 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -214,7 +214,8 @@ MCFragment::~MCFragment() {
 MCFragment::MCFragment(FragmentType _Kind, MCSectionData *_Parent)
   : Kind(_Kind), Parent(_Parent), Atom(0), Offset(~UINT64_C(0))
 {
-  Parent->getFragmentList().push_back(this);
+  if (Parent)
+    Parent->getFragmentList().push_back(this);
 }
 
 /* *** */
@@ -242,6 +243,36 @@ MCSectionData::MCSectionData(const MCSection &_Section, MCAssembler *A)
     A->getSectionList().push_back(this);
 }
 
+MCSectionData::iterator
+MCSectionData::getSubsectionInsertionPoint(unsigned Subsection) {
+  if (Subsection == 0 && SubsectionFragmentMap.empty())
+    return end();
+
+  SmallVectorImpl<std::pair<unsigned, MCFragment *> >::iterator MI =
+    std::lower_bound(SubsectionFragmentMap.begin(), SubsectionFragmentMap.end(),
+                     std::make_pair(Subsection, (MCFragment *)0));
+  bool ExactMatch = false;
+  if (MI != SubsectionFragmentMap.end()) {
+    ExactMatch = MI->first == Subsection;
+    if (ExactMatch)
+      ++MI;
+  }
+  iterator IP;
+  if (MI == SubsectionFragmentMap.end())
+    IP = end();
+  else
+    IP = MI->second;
+  if (!ExactMatch && Subsection != 0) {
+    // The GNU as documentation claims that subsections have an alignment of 4,
+    // although this appears not to be the case.
+    MCFragment *F = new MCDataFragment();
+    SubsectionFragmentMap.insert(MI, std::make_pair(Subsection, F));
+    getFragmentList().insert(IP, F);
+    F->setParent(this);
+  }
+  return IP;
+}
+
 /* *** */
 
 MCSymbolData::MCSymbolData() : Symbol(0) {}
diff --git a/lib/MC/MCDwarf.cpp b/lib/MC/MCDwarf.cpp
index 549ab7f..1dff71a 100644
--- a/lib/MC/MCDwarf.cpp
+++ b/lib/MC/MCDwarf.cpp
@@ -197,6 +197,8 @@ static inline void EmitDwarfLineTable(MCStreamer *MCOS,
   // actually a DW_LNE_end_sequence.
 
   // Switch to the section to be able to create a symbol at its end.
+  // TODO: keep track of the last subsection so that this symbol appears in the
+  // correct place.
   MCOS->SwitchSection(Section);
 
   MCContext &context = MCOS->getContext();
@@ -787,7 +789,7 @@ void MCGenDwarfLabelEntry::Make(MCSymbol *Symbol, MCStreamer *MCOS,
   if (Symbol->isTemporary())
     return;
   MCContext &context = MCOS->getContext();
-  if (context.getGenDwarfSection() != MCOS->getCurrentSection())
+  if (context.getGenDwarfSection() != MCOS->getCurrentSection().first)
     return;
 
   // The dwarf label's name does not have the symbol name's leading
diff --git a/lib/MC/MCELFStreamer.cpp b/lib/MC/MCELFStreamer.cpp
index 7faa6f4..116f86f 100644
--- a/lib/MC/MCELFStreamer.cpp
+++ b/lib/MC/MCELFStreamer.cpp
@@ -109,14 +109,15 @@ void MCELFStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
   llvm_unreachable("invalid assembler flag!");
 }
 
-void MCELFStreamer::ChangeSection(const MCSection *Section) {
+void MCELFStreamer::ChangeSection(const MCSection *Section,
+                                  const MCExpr *Subsection) {
   MCSectionData *CurSection = getCurrentSectionData();
   if (CurSection && CurSection->isBundleLocked())
     report_fatal_error("Unterminated .bundle_lock when changing a section");
   const MCSymbol *Grp = static_cast<const MCSectionELF *>(Section)->getGroup();
   if (Grp)
     getAssembler().getOrCreateSymbolData(*Grp);
-  this->MCObjectStreamer::ChangeSection(Section);
+  this->MCObjectStreamer::ChangeSection(Section, Subsection);
 }
 
 void MCELFStreamer::EmitWeakReference(MCSymbol *Alias, const MCSymbol *Symbol) {
@@ -318,7 +319,7 @@ void MCELFStreamer::EmitValueToAlignment(unsigned ByteAlignment,
 // entry in the module's symbol table (the first being the null symbol).
 void MCELFStreamer::EmitFileDirective(StringRef Filename) {
   MCSymbol *Symbol = getAssembler().getContext().GetOrCreateSymbol(Filename);
-  Symbol->setSection(*getCurrentSection());
+  Symbol->setSection(*getCurrentSection().first);
   Symbol->setAbsolute();
 
   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
@@ -434,11 +435,13 @@ void MCELFStreamer::EmitInstToData(const MCInst &Inst) {
       // Optimize memory usage by emitting the instruction to a
       // MCCompactEncodedInstFragment when not in a bundle-locked group and
       // there are no fixups registered.
-      MCCompactEncodedInstFragment *CEIF = new MCCompactEncodedInstFragment(SD);
+      MCCompactEncodedInstFragment *CEIF = new MCCompactEncodedInstFragment();
+      insert(CEIF);
       CEIF->getContents().append(Code.begin(), Code.end());
       return;
     } else {
-      DF = new MCDataFragment(SD);
+      DF = new MCDataFragment();
+      insert(DF);
       if (SD->getBundleLockState() == MCSectionData::BundleLockedAlignToEnd) {
         // If this is a new fragment created for a bundle-locked group, and the
         // group was marked as "align_to_end", set a flag in the fragment.
diff --git a/lib/MC/MCMachOStreamer.cpp b/lib/MC/MCMachOStreamer.cpp
index 7d08d0e..e08b01b 100644
--- a/lib/MC/MCMachOStreamer.cpp
+++ b/lib/MC/MCMachOStreamer.cpp
@@ -122,11 +122,11 @@ void MCMachOStreamer::EmitLabel(MCSymbol *Symbol) {
   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
 
   // isSymbolLinkerVisible uses the section.
-  Symbol->setSection(*getCurrentSection());
+  Symbol->setSection(*getCurrentSection().first);
   // We have to create a new fragment if this is an atom defining symbol,
   // fragments cannot span atoms.
   if (getAssembler().isSymbolLinkerVisible(*Symbol))
-    new MCDataFragment(getCurrentSectionData());
+    insert(new MCDataFragment());
 
   MCObjectStreamer::EmitLabel(Symbol);
 
diff --git a/lib/MC/MCNullStreamer.cpp b/lib/MC/MCNullStreamer.cpp
index c872b22..659706a 100644
--- a/lib/MC/MCNullStreamer.cpp
+++ b/lib/MC/MCNullStreamer.cpp
@@ -30,13 +30,14 @@ namespace {
     virtual void InitSections() {
     }
 
-    virtual void ChangeSection(const MCSection *Section) {
+    virtual void ChangeSection(const MCSection *Section,
+                               const MCExpr *Subsection) {
     }
 
     virtual void EmitLabel(MCSymbol *Symbol) {
       assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
-      assert(getCurrentSection() && "Cannot emit before setting section!");
-      Symbol->setSection(*getCurrentSection());
+      assert(getCurrentSection().first &&"Cannot emit before setting section!");
+      Symbol->setSection(*getCurrentSection().first);
     }
     virtual void EmitDebugLabel(MCSymbol *Symbol) {
       EmitLabel(Symbol);
diff --git a/lib/MC/MCObjectStreamer.cpp b/lib/MC/MCObjectStreamer.cpp
index 0d2ce83..d21ce8d 100644
--- a/lib/MC/MCObjectStreamer.cpp
+++ b/lib/MC/MCObjectStreamer.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/MC/MCObjectStreamer.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCAssembler.h"
@@ -45,14 +46,15 @@ void MCObjectStreamer::reset() {
   if (Assembler)
     Assembler->reset();
   CurSectionData = 0;
+  CurInsertionPoint = MCSectionData::iterator();
   MCStreamer::reset();
 }
 
 MCFragment *MCObjectStreamer::getCurrentFragment() const {
   assert(getCurrentSectionData() && "No current section!");
 
-  if (!getCurrentSectionData()->empty())
-    return &getCurrentSectionData()->getFragmentList().back();
+  if (CurInsertionPoint != getCurrentSectionData()->getFragmentList().begin())
+    return prior(CurInsertionPoint);
 
   return 0;
 }
@@ -61,8 +63,10 @@ MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() const {
   MCDataFragment *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
   // When bundling is enabled, we don't want to add data to a fragment that
   // already has instructions (see MCELFStreamer::EmitInstToData for details)
-  if (!F || (Assembler->isBundlingEnabled() && F->hasInstructions()))
-    F = new MCDataFragment(getCurrentSectionData());
+  if (!F || (Assembler->isBundlingEnabled() && F->hasInstructions())) {
+    F = new MCDataFragment();
+    insert(F);
+  }
   return F;
 }
 
@@ -145,7 +149,7 @@ void MCObjectStreamer::EmitULEB128Value(const MCExpr *Value) {
     return;
   }
   Value = ForceExpAbs(Value);
-  new MCLEBFragment(*Value, false, getCurrentSectionData());
+  insert(new MCLEBFragment(*Value, false));
 }
 
 void MCObjectStreamer::EmitSLEB128Value(const MCExpr *Value) {
@@ -155,7 +159,7 @@ void MCObjectStreamer::EmitSLEB128Value(const MCExpr *Value) {
     return;
   }
   Value = ForceExpAbs(Value);
-  new MCLEBFragment(*Value, true, getCurrentSectionData());
+  insert(new MCLEBFragment(*Value, true));
 }
 
 void MCObjectStreamer::EmitWeakReference(MCSymbol *Alias,
@@ -163,10 +167,20 @@ void MCObjectStreamer::EmitWeakReference(MCSymbol *Alias,
   report_fatal_error("This file format doesn't support weak aliases.");
 }
 
-void MCObjectStreamer::ChangeSection(const MCSection *Section) {
+void MCObjectStreamer::ChangeSection(const MCSection *Section,
+                                     const MCExpr *Subsection) {
   assert(Section && "Cannot switch to a null section!");
 
   CurSectionData = &getAssembler().getOrCreateSectionData(*Section);
+
+  int64_t IntSubsection = 0;
+  if (Subsection &&
+      !Subsection->EvaluateAsAbsolute(IntSubsection, getAssembler()))
+    report_fatal_error("Cannot evaluate subsection number");
+  if (IntSubsection < 0 || IntSubsection > 8192)
+    report_fatal_error("Subsection number out of range");
+  CurInsertionPoint =
+    CurSectionData->getSubsectionInsertionPoint(unsigned(IntSubsection));
 }
 
 void MCObjectStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
@@ -185,7 +199,7 @@ void MCObjectStreamer::EmitInstruction(const MCInst &Inst) {
 
   // Now that a machine instruction has been assembled into this section, make
   // a line entry for any .loc directive that has been seen.
-  MCLineEntry::Make(this, getCurrentSection());
+  MCLineEntry::Make(this, getCurrentSection().first);
 
   // If this instruction doesn't need relaxation, just emit it as data.
   MCAssembler &Assembler = getAssembler();
@@ -216,8 +230,8 @@ void MCObjectStreamer::EmitInstruction(const MCInst &Inst) {
 void MCObjectStreamer::EmitInstToFragment(const MCInst &Inst) {
   // Always create a new, separate fragment here, because its size can change
   // during relaxation.
-  MCRelaxableFragment *IF =
-    new MCRelaxableFragment(Inst, getCurrentSectionData());
+  MCRelaxableFragment *IF = new MCRelaxableFragment(Inst);
+  insert(IF);
 
   SmallString<128> Code;
   raw_svector_ostream VecOS(Code);
@@ -258,7 +272,7 @@ void MCObjectStreamer::EmitDwarfAdvanceLineAddr(int64_t LineDelta,
     return;
   }
   AddrDelta = ForceExpAbs(AddrDelta);
-  new MCDwarfLineAddrFragment(LineDelta, *AddrDelta, getCurrentSectionData());
+  insert(new MCDwarfLineAddrFragment(LineDelta, *AddrDelta));
 }
 
 void MCObjectStreamer::EmitDwarfAdvanceFrameAddr(const MCSymbol *LastLabel,
@@ -270,7 +284,7 @@ void MCObjectStreamer::EmitDwarfAdvanceFrameAddr(const MCSymbol *LastLabel,
     return;
   }
   AddrDelta = ForceExpAbs(AddrDelta);
-  new MCDwarfCallFrameFragment(*AddrDelta, getCurrentSectionData());
+  insert(new MCDwarfCallFrameFragment(*AddrDelta));
 }
 
 void MCObjectStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
@@ -284,8 +298,7 @@ void MCObjectStreamer::EmitValueToAlignment(unsigned ByteAlignment,
                                             unsigned MaxBytesToEmit) {
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
-  new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
-                      getCurrentSectionData());
+  insert(new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit));
 
   // Update the maximum alignment on the current section if necessary.
   if (ByteAlignment > getCurrentSectionData()->getAlignment())
@@ -302,7 +315,7 @@ bool MCObjectStreamer::EmitValueToOffset(const MCExpr *Offset,
                                          unsigned char Value) {
   int64_t Res;
   if (Offset->EvaluateAsAbsolute(Res, getAssembler())) {
-    new MCOrgFragment(*Offset, Value, getCurrentSectionData());
+    insert(new MCOrgFragment(*Offset, Value));
     return false;
   }
 
diff --git a/lib/MC/MCParser/AsmParser.cpp b/lib/MC/MCParser/AsmParser.cpp
index 3e39a9c..8783bc0 100644
--- a/lib/MC/MCParser/AsmParser.cpp
+++ b/lib/MC/MCParser/AsmParser.cpp
@@ -602,7 +602,7 @@ bool AsmParser::Run(bool NoInitialTextSection, bool NoFinalize) {
   // If we are generating dwarf for assembly source files save the initial text
   // section and generate a .file directive.
   if (getContext().getGenDwarfForAssembly()) {
-    getContext().setGenDwarfSection(getStreamer().getCurrentSection());
+    getContext().setGenDwarfSection(getStreamer().getCurrentSection().first);
     MCSymbol *SectionStartSym = getContext().CreateTempSymbol();
     getStreamer().EmitLabel(SectionStartSym);
     getContext().setGenDwarfSectionStartSym(SectionStartSym);
@@ -667,7 +667,7 @@ bool AsmParser::Run(bool NoInitialTextSection, bool NoFinalize) {
 }
 
 void AsmParser::checkForValidSection() {
-  if (!ParsingInlineAsm && !getStreamer().getCurrentSection()) {
+  if (!ParsingInlineAsm && !getStreamer().getCurrentSection().first) {
     TokError("expected section directive before assembly directive");
     Out.InitToTextSection();
   }
@@ -1493,7 +1493,8 @@ bool AsmParser::ParseStatement(ParseStatementInfo &Info) {
   // section is the initial text section then generate a .loc directive for
   // the instruction.
   if (!HadError && getContext().getGenDwarfForAssembly() &&
-      getContext().getGenDwarfSection() == getStreamer().getCurrentSection()) {
+      getContext().getGenDwarfSection() ==
+      getStreamer().getCurrentSection().first) {
 
     unsigned Line = SrcMgr.FindLineNumber(IDLoc, CurBuffer);
 
@@ -2484,7 +2485,7 @@ bool AsmParser::ParseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
 
   // Check whether we should use optimal code alignment for this .align
   // directive.
-  bool UseCodeAlign = getStreamer().getCurrentSection()->UseCodeAlign();
+  bool UseCodeAlign = getStreamer().getCurrentSection().first->UseCodeAlign();
   if ((!HasFillExpr || Lexer.getMAI().getTextAlignFillValue() == FillExpr) &&
       ValueSize == 1 && UseCodeAlign) {
     getStreamer().EmitCodeAlignment(Alignment, MaxBytesToFill);
diff --git a/lib/MC/MCParser/DarwinAsmParser.cpp b/lib/MC/MCParser/DarwinAsmParser.cpp
index 6d6409f..7eb8b74 100644
--- a/lib/MC/MCParser/DarwinAsmParser.cpp
+++ b/lib/MC/MCParser/DarwinAsmParser.cpp
@@ -566,10 +566,10 @@ bool DarwinAsmParser::ParseDirectivePopSection(StringRef, SMLoc) {
 /// ParseDirectivePrevious:
 ///   ::= .previous
 bool DarwinAsmParser::ParseDirectivePrevious(StringRef DirName, SMLoc) {
-  const MCSection *PreviousSection = getStreamer().getPreviousSection();
-  if (PreviousSection == NULL)
+  MCSectionSubPair PreviousSection = getStreamer().getPreviousSection();
+  if (PreviousSection.first == NULL)
       return TokError(".previous without corresponding .section");
-  getStreamer().SwitchSection(PreviousSection);
+  getStreamer().SwitchSection(PreviousSection.first, PreviousSection.second);
   return false;
 }
 
diff --git a/lib/MC/MCParser/ELFAsmParser.cpp b/lib/MC/MCParser/ELFAsmParser.cpp
index 4c45e08..3134fc3 100644
--- a/lib/MC/MCParser/ELFAsmParser.cpp
+++ b/lib/MC/MCParser/ELFAsmParser.cpp
@@ -76,6 +76,7 @@ public:
       &ELFAsmParser::ParseDirectiveSymbolAttribute>(".internal");
     addDirectiveHandler<
       &ELFAsmParser::ParseDirectiveSymbolAttribute>(".hidden");
+    addDirectiveHandler<&ELFAsmParser::ParseDirectiveSubsection>(".subsection");
   }
 
   // FIXME: Part of this logic is duplicated in the MCELFStreamer. What is
@@ -147,9 +148,11 @@ public:
   bool ParseDirectiveVersion(StringRef, SMLoc);
   bool ParseDirectiveWeakref(StringRef, SMLoc);
   bool ParseDirectiveSymbolAttribute(StringRef, SMLoc);
+  bool ParseDirectiveSubsection(StringRef, SMLoc);
 
 private:
   bool ParseSectionName(StringRef &SectionName);
+  bool ParseSectionArguments(bool IsPush);
 };
 
 }
@@ -191,12 +194,15 @@ bool ELFAsmParser::ParseDirectiveSymbolAttribute(StringRef Directive, SMLoc) {
 
 bool ELFAsmParser::ParseSectionSwitch(StringRef Section, unsigned Type,
                                       unsigned Flags, SectionKind Kind) {
-  if (getLexer().isNot(AsmToken::EndOfStatement))
-    return TokError("unexpected token in section switching directive");
-  Lex();
+  const MCExpr *Subsection = 0;
+  if (getLexer().isNot(AsmToken::EndOfStatement)) {
+    if (getParser().parseExpression(Subsection))
+      return true;
+  }
 
   getStreamer().SwitchSection(getContext().getELFSection(
-                                Section, Type, Flags, Kind));
+                                Section, Type, Flags, Kind),
+                              Subsection);
 
   return false;
 }
@@ -316,7 +322,7 @@ static int parseSectionFlags(StringRef flagsStr) {
 bool ELFAsmParser::ParseDirectivePushSection(StringRef s, SMLoc loc) {
   getStreamer().PushSection();
 
-  if (ParseDirectiveSection(s, loc)) {
+  if (ParseSectionArguments(/*IsPush=*/true)) {
     getStreamer().PopSection();
     return true;
   }
@@ -332,6 +338,10 @@ bool ELFAsmParser::ParseDirectivePopSection(StringRef, SMLoc) {
 
 // FIXME: This is a work in progress.
 bool ELFAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
+  return ParseSectionArguments(/*IsPush=*/false);
+}
+
+bool ELFAsmParser::ParseSectionArguments(bool IsPush) {
   StringRef SectionName;
 
   if (ParseSectionName(SectionName))
@@ -341,6 +351,7 @@ bool ELFAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
   int64_t Size = 0;
   StringRef GroupName;
   unsigned Flags = 0;
+  const MCExpr *Subsection = 0;
 
   // Set the defaults first.
   if (SectionName == ".fini" || SectionName == ".init" ||
@@ -352,6 +363,14 @@ bool ELFAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
   if (getLexer().is(AsmToken::Comma)) {
     Lex();
 
+    if (IsPush && getLexer().isNot(AsmToken::String)) {
+      if (getParser().parseExpression(Subsection))
+        return true;
+      if (getLexer().isNot(AsmToken::Comma))
+        goto EndStmt;
+      Lex();
+    }
+   
     if (getLexer().isNot(AsmToken::String))
       return TokError("expected string in directive");
 
@@ -408,6 +427,7 @@ bool ELFAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
     }
   }
 
+EndStmt:
   if (getLexer().isNot(AsmToken::EndOfStatement))
     return TokError("unexpected token in directive");
 
@@ -444,15 +464,16 @@ bool ELFAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
   SectionKind Kind = computeSectionKind(Flags);
   getStreamer().SwitchSection(getContext().getELFSection(SectionName, Type,
                                                          Flags, Kind, Size,
-                                                         GroupName));
+                                                         GroupName),
+                              Subsection);
   return false;
 }
 
 bool ELFAsmParser::ParseDirectivePrevious(StringRef DirName, SMLoc) {
-  const MCSection *PreviousSection = getStreamer().getPreviousSection();
-  if (PreviousSection == NULL)
+  MCSectionSubPair PreviousSection = getStreamer().getPreviousSection();
+  if (PreviousSection.first == NULL)
       return TokError(".previous without corresponding .section");
-  getStreamer().SwitchSection(PreviousSection);
+  getStreamer().SwitchSection(PreviousSection.first, PreviousSection.second);
 
   return false;
 }
@@ -613,6 +634,20 @@ bool ELFAsmParser::ParseDirectiveWeakref(StringRef, SMLoc) {
   return false;
 }
 
+bool ELFAsmParser::ParseDirectiveSubsection(StringRef, SMLoc) {
+  const MCExpr *Subsection = 0;
+  if (getLexer().isNot(AsmToken::EndOfStatement)) {
+    if (getParser().parseExpression(Subsection))
+     return true;
+  }
+
+  if (getLexer().isNot(AsmToken::EndOfStatement))
+    return TokError("unexpected token in directive");
+
+  getStreamer().SubSection(Subsection);
+  return false;
+}
+
 namespace llvm {
 
 MCAsmParserExtension *createELFAsmParser() {
diff --git a/lib/MC/MCPureStreamer.cpp b/lib/MC/MCPureStreamer.cpp
index c9779e0..8ae724f 100644
--- a/lib/MC/MCPureStreamer.cpp
+++ b/lib/MC/MCPureStreamer.cpp
@@ -118,16 +118,16 @@ void MCPureStreamer::InitToTextSection() {
 void MCPureStreamer::EmitLabel(MCSymbol *Symbol) {
   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
   assert(!Symbol->isVariable() && "Cannot emit a variable symbol!");
-  assert(getCurrentSection() && "Cannot emit before setting section!");
+  assert(getCurrentSection().first && "Cannot emit before setting section!");
 
-  Symbol->setSection(*getCurrentSection());
+  Symbol->setSection(*getCurrentSection().first);
 
   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
 
   // We have to create a new fragment if this is an atom defining symbol,
   // fragments cannot span atoms.
   if (getAssembler().isSymbolLinkerVisible(SD.getSymbol()))
-    new MCDataFragment(getCurrentSectionData());
+    insert(new MCDataFragment());
 
   // FIXME: This is wasteful, we don't necessarily need to create a data
   // fragment. Instead, we should mark the symbol as pointing into the data
@@ -162,8 +162,7 @@ void MCPureStreamer::EmitValueToAlignment(unsigned ByteAlignment,
   // MCObjectStreamer.
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
-  new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
-                      getCurrentSectionData());
+  insert(new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit));
 
   // Update the maximum alignment on the current section if necessary.
   if (ByteAlignment > getCurrentSectionData()->getAlignment())
@@ -176,8 +175,8 @@ void MCPureStreamer::EmitCodeAlignment(unsigned ByteAlignment,
   // MCObjectStreamer.
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
-  MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit,
-                                           getCurrentSectionData());
+  MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit);
+  insert(F);
   F->setEmitNops(true);
 
   // Update the maximum alignment on the current section if necessary.
@@ -187,13 +186,13 @@ void MCPureStreamer::EmitCodeAlignment(unsigned ByteAlignment,
 
 bool MCPureStreamer::EmitValueToOffset(const MCExpr *Offset,
                                        unsigned char Value) {
-  new MCOrgFragment(*Offset, Value, getCurrentSectionData());
+  insert(new MCOrgFragment(*Offset, Value));
   return false;
 }
 
 void MCPureStreamer::EmitInstToFragment(const MCInst &Inst) {
-  MCRelaxableFragment *IF =
-    new MCRelaxableFragment(Inst, getCurrentSectionData());
+  MCRelaxableFragment *IF = new MCRelaxableFragment(Inst);
+  insert(IF);
 
   // Add the fixups and data.
   //
diff --git a/lib/MC/MCSectionCOFF.cpp b/lib/MC/MCSectionCOFF.cpp
index aac9377..6cedf06 100644
--- a/lib/MC/MCSectionCOFF.cpp
+++ b/lib/MC/MCSectionCOFF.cpp
@@ -29,7 +29,8 @@ bool MCSectionCOFF::ShouldOmitSectionDirective(StringRef Name,
 }
 
 void MCSectionCOFF::PrintSwitchToSection(const MCAsmInfo &MAI,
-                                         raw_ostream &OS) const {
+                                         raw_ostream &OS,
+                                         const MCExpr *Subsection) const {
 
   // standard sections don't require the '.section'
   if (ShouldOmitSectionDirective(SectionName, MAI)) {
diff --git a/lib/MC/MCSectionELF.cpp b/lib/MC/MCSectionELF.cpp
index 0775cfa..bf1a984 100644
--- a/lib/MC/MCSectionELF.cpp
+++ b/lib/MC/MCSectionELF.cpp
@@ -10,6 +10,7 @@
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/raw_ostream.h"
@@ -32,10 +33,14 @@ bool MCSectionELF::ShouldOmitSectionDirective(StringRef Name,
 }
 
 void MCSectionELF::PrintSwitchToSection(const MCAsmInfo &MAI,
-                                        raw_ostream &OS) const {
+                                        raw_ostream &OS,
+                                        const MCExpr *Subsection) const {
 
   if (ShouldOmitSectionDirective(SectionName, MAI)) {
-    OS << '\t' << getSectionName() << '\n';
+    OS << '\t' << getSectionName();
+    if (Subsection)
+      OS << '\t' << *Subsection;
+    OS << '\n';
     return;
   }
 
@@ -129,6 +134,9 @@ void MCSectionELF::PrintSwitchToSection(const MCAsmInfo &MAI,
   if (Flags & ELF::SHF_GROUP)
     OS << "," << Group->getName() << ",comdat";
   OS << '\n';
+
+  if (Subsection)
+    OS << "\t.subsection\t" << *Subsection << '\n';
 }
 
 bool MCSectionELF::UseCodeAlign() const {
diff --git a/lib/MC/MCSectionMachO.cpp b/lib/MC/MCSectionMachO.cpp
index fc32315..8704513 100644
--- a/lib/MC/MCSectionMachO.cpp
+++ b/lib/MC/MCSectionMachO.cpp
@@ -91,7 +91,8 @@ MCSectionMachO::MCSectionMachO(StringRef Segment, StringRef Section,
 }
 
 void MCSectionMachO::PrintSwitchToSection(const MCAsmInfo &MAI,
-                                          raw_ostream &OS) const {
+                                          raw_ostream &OS,
+                                          const MCExpr *Subsection) const {
   OS << "\t.section\t" << getSegmentName() << ',' << getSectionName();
 
   // Get the section type and attributes.
diff --git a/lib/MC/MCStreamer.cpp b/lib/MC/MCStreamer.cpp
index d02e553..4839c34 100644
--- a/lib/MC/MCStreamer.cpp
+++ b/lib/MC/MCStreamer.cpp
@@ -24,8 +24,7 @@ using namespace llvm;
 MCStreamer::MCStreamer(StreamerKind Kind, MCContext &Ctx)
     : Kind(Kind), Context(Ctx), EmitEHFrame(true), EmitDebugFrame(false),
       CurrentW64UnwindInfo(0), LastSymbol(0), AutoInitSections(false) {
-  const MCSection *section = 0;
-  SectionStack.push_back(std::make_pair(section, section));
+  SectionStack.push_back(std::pair<MCSectionSubPair, MCSectionSubPair>());
 }
 
 MCStreamer::~MCStreamer() {
@@ -40,9 +39,8 @@ void MCStreamer::reset() {
   EmitDebugFrame = false;
   CurrentW64UnwindInfo = 0;
   LastSymbol = 0;
-  const MCSection *section = 0;
   SectionStack.clear();
-  SectionStack.push_back(std::make_pair(section, section));
+  SectionStack.push_back(std::pair<MCSectionSubPair, MCSectionSubPair>());
 }
 
 const MCExpr *MCStreamer::BuildSymbolDiff(MCContext &Context,
@@ -188,15 +186,15 @@ void MCStreamer::EmitEHSymAttributes(const MCSymbol *Symbol,
 
 void MCStreamer::EmitLabel(MCSymbol *Symbol) {
   assert(!Symbol->isVariable() && "Cannot emit a variable symbol!");
-  assert(getCurrentSection() && "Cannot emit before setting section!");
-  Symbol->setSection(*getCurrentSection());
+  assert(getCurrentSection().first && "Cannot emit before setting section!");
+  Symbol->setSection(*getCurrentSection().first);
   LastSymbol = Symbol;
 }
 
 void MCStreamer::EmitDebugLabel(MCSymbol *Symbol) {
   assert(!Symbol->isVariable() && "Cannot emit a variable symbol!");
-  assert(getCurrentSection() && "Cannot emit before setting section!");
-  Symbol->setSection(*getCurrentSection());
+  assert(getCurrentSection().first && "Cannot emit before setting section!");
+  Symbol->setSection(*getCurrentSection().first);
   LastSymbol = Symbol;
 }
 
diff --git a/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp b/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
index b83577a..3b811df 100644
--- a/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
+++ b/lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp
@@ -63,14 +63,15 @@ public:
 
   ~AArch64ELFStreamer() {}
 
-  virtual void ChangeSection(const MCSection *Section) {
+  virtual void ChangeSection(const MCSection *Section,
+                             const MCExpr *Subsection) {
     // We have to keep track of the mapping symbol state of any sections we
     // use. Each one should start off as EMS_None, which is provided as the
     // default constructor by DenseMap::lookup.
-    LastMappingSymbols[getPreviousSection()] = LastEMS;
+    LastMappingSymbols[getPreviousSection().first] = LastEMS;
     LastEMS = LastMappingSymbols.lookup(Section);
 
-    MCELFStreamer::ChangeSection(Section);
+    MCELFStreamer::ChangeSection(Section, Subsection);
   }
 
   /// This function is the one used to emit instruction data into the ELF
@@ -129,7 +130,7 @@ private:
     MCELF::SetType(SD, ELF::STT_NOTYPE);
     MCELF::SetBinding(SD, ELF::STB_LOCAL);
     SD.setExternal(false);
-    Symbol->setSection(*getCurrentSection());
+    Symbol->setSection(*getCurrentSection().first);
 
     const MCExpr *Value = MCSymbolRefExpr::Create(Start, getContext());
     Symbol->setVariableValue(Value);
diff --git a/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp b/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
index 52d9237..6c3d247 100644
--- a/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
+++ b/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
@@ -83,14 +83,15 @@ public:
   virtual void EmitRegSave(const SmallVectorImpl<unsigned> &RegList,
                            bool isVector);
 
-  virtual void ChangeSection(const MCSection *Section) {
+  virtual void ChangeSection(const MCSection *Section,
+                             const MCExpr *Subsection) {
     // We have to keep track of the mapping symbol state of any sections we
     // use. Each one should start off as EMS_None, which is provided as the
     // default constructor by DenseMap::lookup.
-    LastMappingSymbols[getPreviousSection()] = LastEMS;
+    LastMappingSymbols[getPreviousSection().first] = LastEMS;
     LastEMS = LastMappingSymbols.lookup(Section);
 
-    MCELFStreamer::ChangeSection(Section);
+    MCELFStreamer::ChangeSection(Section, Subsection);
   }
 
   /// This function is the one used to emit instruction data into the ELF
@@ -183,7 +184,7 @@ private:
     MCELF::SetType(SD, ELF::STT_NOTYPE);
     MCELF::SetBinding(SD, ELF::STB_LOCAL);
     SD.setExternal(false);
-    Symbol->setSection(*getCurrentSection());
+    Symbol->setSection(*getCurrentSection().first);
 
     const MCExpr *Value = MCSymbolRefExpr::Create(Start, getContext());
     Symbol->setVariableValue(Value);
diff --git a/lib/Target/NVPTX/NVPTXSection.h b/lib/Target/NVPTX/NVPTXSection.h
index e166be5..e57ace9 100644
--- a/lib/Target/NVPTX/NVPTXSection.h
+++ b/lib/Target/NVPTX/NVPTXSection.h
@@ -32,7 +32,8 @@ public:
   /// Override this as NVPTX has its own way of printing switching
   /// to a section.
   virtual void PrintSwitchToSection(const MCAsmInfo &MAI,
-                                    raw_ostream &OS) const {}
+                                    raw_ostream &OS,
+                                    const MCExpr *Subsection) const {}
 
   /// Base address of PTX sections is zero.
   virtual bool isBaseAddressKnownZero() const { return true; }
diff --git a/lib/Target/PowerPC/PPCAsmPrinter.cpp b/lib/Target/PowerPC/PPCAsmPrinter.cpp
index 96a9f0a..3c7cc4e 100644
--- a/lib/Target/PowerPC/PPCAsmPrinter.cpp
+++ b/lib/Target/PowerPC/PPCAsmPrinter.cpp
@@ -721,7 +721,7 @@ void PPCLinuxAsmPrinter::EmitFunctionEntryLabel() {
     return AsmPrinter::EmitFunctionEntryLabel();
     
   // Emit an official procedure descriptor.
-  const MCSection *Current = OutStreamer.getCurrentSection();
+  MCSectionSubPair Current = OutStreamer.getCurrentSection();
   const MCSectionELF *Section = OutStreamer.getContext().getELFSection(".opd",
       ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC,
       SectionKind::getReadOnly());
@@ -741,7 +741,7 @@ void PPCLinuxAsmPrinter::EmitFunctionEntryLabel() {
                         8/*size*/);
   // Emit a null environment pointer.
   OutStreamer.EmitIntValue(0, 8 /* size */);
-  OutStreamer.SwitchSection(Current);
+  OutStreamer.SwitchSection(Current.first, Current.second);
 
   MCSymbol *RealFnSym = OutContext.GetOrCreateSymbol(
                           ".L." + Twine(CurrentFnSym->getName()));
diff --git a/test/MC/ELF/subsection.s b/test/MC/ELF/subsection.s
new file mode 100644
index 0000000..d437cac
--- /dev/null
+++ b/test/MC/ELF/subsection.s
@@ -0,0 +1,37 @@
+// RUN: llvm-mc -filetype=obj %s -o - -triple x86_64-pc-linux | llvm-objdump -s - | FileCheck %s
+
+// CHECK: Contents of section .text:
+// CHECK-NEXT: 0000 03042502 00000003 04250100 0000ebf7
+.text 1
+add 1, %eax
+jmp label
+.subsection
+add 2, %eax
+label:
+
+// CHECK-NOT: Contents of section .rela.text:
+
+// CHECK: Contents of section .data:
+// CHECK-NEXT: 0000 01030402 74657374
+.data
+l0:
+.byte 1
+.subsection 1+1
+l1:
+.byte 2
+l2:
+.subsection l2-l1
+.byte l1-l0
+.subsection 3
+.ascii "test"
+.previous
+.byte 4
+
+// CHECK: Contents of section test:
+// CHECK-NEXT: 0000 010302
+.section test
+.byte 1
+.pushsection test, 1
+.byte 2
+.popsection
+.byte 3
diff --git a/tools/lto/LTOModule.cpp b/tools/lto/LTOModule.cpp
index ff67769..d805f49 100644
--- a/tools/lto/LTOModule.cpp
+++ b/tools/lto/LTOModule.cpp
@@ -743,7 +743,7 @@ namespace {
           AddValueSymbols(Inst.getOperand(i).getExpr());
     }
     virtual void EmitLabel(MCSymbol *Symbol) {
-      Symbol->setSection(*getCurrentSection());
+      Symbol->setSection(*getCurrentSection().first);
       markDefined(*Symbol);
     }
     virtual void EmitDebugLabel(MCSymbol *Symbol) {
@@ -771,7 +771,8 @@ namespace {
     virtual void EmitBundleUnlock() {}
 
     // Noop calls.
-    virtual void ChangeSection(const MCSection *Section) {}
+    virtual void ChangeSection(const MCSection *Section,
+                               const MCExpr *Subsection) {}
     virtual void InitToTextSection() {}
     virtual void InitSections() {}
     virtual void EmitAssemblerFlag(MCAssemblerFlag Flag) {}