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 32df38b2b9..38a70f0adf 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 6fb2d22be2..55c05b0363 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 f06c49ff08..22a2839869 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 e5754249e9..de2678adad 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 07c47144cb..50e33a5b04 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 4b8b849c79..59799158ad 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 898f571490..b68bd85968 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 a069a2b0ca..2cab481c3d 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 d4a745d985..2d13db2c9c 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 b35a9de1fc..9e867859da 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 8970e18a2c..fb5ab28bcf 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 549ab7ff21..1dff71a495 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 7faa6f411b..116f86feb8 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 7d08d0ecd5..e08b01b7ae 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 c872b2203f..659706a1d8 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 0d2ce83a8a..d21ce8d1a1 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) {