Revert "MCFragment: Use trailing data for fixed-size part" (#151383)

Reverts #150846 due to unsigned underflow identifier by
UBSan in several tests

Author: npanchen

llvm/include/llvm/MC/MCObjectStreamer.h

@@ -52,10 +52,6 @@ class MCObjectStreamer : public MCStreamer {
   DenseMap<const MCSymbol *, SmallVector<PendingAssignment, 1>>
       pendingAssignments;
 
-  SmallVector<std::unique_ptr<char[]>, 0> FragStorage;
-  // Available bytes in the current block for trailing data or new fragments.
-  size_t FragSpace = 0;
-
   void emitInstToData(const MCInst &Inst, const MCSubtargetInfo &);
   void emitCFIStartProcImpl(MCDwarfFrameInfo &Frame) override;
   void emitCFIEndProcImpl(MCDwarfFrameInfo &Frame) override;
@@ -88,18 +84,11 @@ class MCObjectStreamer : public MCStreamer {
   // Add a fragment with a variable-size tail and start a new empty fragment.
   void insert(MCFragment *F);
 
-  char *getCurFragEnd() const {
-    return reinterpret_cast<char *>(CurFrag + 1) + CurFrag->getFixedSize();
-  }
-  MCFragment *allocFragSpace(size_t Headroom);
   // Add a new fragment to the current section without a variable-size tail.
   void newFragment();
 
-  void ensureHeadroom(size_t Headroom);
   void appendContents(ArrayRef<char> Contents);
   void appendContents(size_t Num, char Elt);
-  // Add a fixup to the current fragment. Call ensureHeadroom beforehand to
-  // ensure the fixup and appended content apply to the same fragment.
   void addFixup(const MCExpr *Value, MCFixupKind Kind);
 
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;

llvm/include/llvm/MC/MCSection.h

@@ -93,7 +93,8 @@ class MCFragment {
   // Track content and fixups for the fixed-size part as fragments are
   // appended to the section. The content remains immutable, except when
   // modified by applyFixup.
-  uint32_t FixedSize = 0;
+  uint32_t ContentStart = 0;
+  uint32_t ContentEnd = 0;
   uint32_t FixupStart = 0;
   uint32_t FixupEnd = 0;
 
@@ -187,6 +188,18 @@ class MCFragment {
   //== Content-related functions manage parent's storage using ContentStart and
   // ContentSize.
 
+  // Get a SmallVector reference. The caller should call doneAppending to update
+  // `ContentEnd`.
+  SmallVectorImpl<char> &getContentsForAppending();
+  void doneAppending();
+  void appendContents(ArrayRef<char> Contents) {
+    getContentsForAppending().append(Contents.begin(), Contents.end());
+    doneAppending();
+  }
+  void appendContents(size_t Num, char Elt) {
+    getContentsForAppending().append(Num, Elt);
+    doneAppending();
+  }
   MutableArrayRef<char> getContents();
   ArrayRef<char> getContents() const;
 
@@ -195,10 +208,10 @@ class MCFragment {
   MutableArrayRef<char> getVarContents();
   ArrayRef<char> getVarContents() const;
 
-  size_t getFixedSize() const { return FixedSize; }
+  size_t getFixedSize() const { return ContentEnd - ContentStart; }
   size_t getVarSize() const { return VarContentEnd - VarContentStart; }
   size_t getSize() const {
-    return FixedSize + (VarContentEnd - VarContentStart);
+    return ContentEnd - ContentStart + (VarContentEnd - VarContentStart);
   }
 
   //== Fixup-related functions manage parent's storage using FixupStart and
@@ -621,11 +634,28 @@ class LLVM_ABI MCSection {
   bool isBssSection() const { return IsBss; }
 };
 
+inline SmallVectorImpl<char> &MCFragment::getContentsForAppending() {
+  SmallVectorImpl<char> &S = getParent()->ContentStorage;
+  if (LLVM_UNLIKELY(ContentEnd != S.size())) {
+    // Move the elements to the end. Reserve space to avoid invalidating
+    // S.begin()+I for `append`.
+    auto Size = ContentEnd - ContentStart;
+    auto I = std::exchange(ContentStart, S.size());
+    S.reserve(S.size() + Size);
+    S.append(S.begin() + I, S.begin() + I + Size);
+  }
+  return S;
+}
+inline void MCFragment::doneAppending() {
+  ContentEnd = getParent()->ContentStorage.size();
+}
 inline MutableArrayRef<char> MCFragment::getContents() {
-  return {reinterpret_cast<char *>(this + 1), FixedSize};
+  return MutableArrayRef(getParent()->ContentStorage)
+      .slice(ContentStart, ContentEnd - ContentStart);
 }
 inline ArrayRef<char> MCFragment::getContents() const {
-  return {reinterpret_cast<const char *>(this + 1), FixedSize};
+  return ArrayRef(getParent()->ContentStorage)
+      .slice(ContentStart, ContentEnd - ContentStart);
 }
 
 inline MutableArrayRef<char> MCFragment::getVarContents() {

llvm/lib/MC/MCObjectStreamer.cpp

@@ -46,83 +46,27 @@ MCAssembler *MCObjectStreamer::getAssemblerPtr() {
   return nullptr;
 }
 
-constexpr size_t FragBlockSize = 16384;
-// Ensure the new fragment can at least store a few bytes.
-constexpr size_t NewFragHeadroom = 8;
-
-static_assert(NewFragHeadroom >= alignof(MCFragment));
-static_assert(FragBlockSize >= sizeof(MCFragment) + NewFragHeadroom);
-
-MCFragment *MCObjectStreamer::allocFragSpace(size_t Headroom) {
-  auto Size = std::max(FragBlockSize, sizeof(MCFragment) + Headroom);
-  FragSpace = Size - sizeof(MCFragment);
-  auto Chunk = std::unique_ptr<char[]>(new char[Size]);
-  auto *F = reinterpret_cast<MCFragment *>(Chunk.get());
-  FragStorage.push_back(std::move(Chunk));
-  return F;
-}
-
 void MCObjectStreamer::newFragment() {
-  MCFragment *F;
-  if (LLVM_LIKELY(sizeof(MCFragment) + NewFragHeadroom <= FragSpace)) {
-    auto End = reinterpret_cast<size_t>(getCurFragEnd());
-    F = reinterpret_cast<MCFragment *>(
-        alignToPowerOf2(End, alignof(MCFragment)));
-    FragSpace -= size_t(F) - End + sizeof(MCFragment);
-  } else {
-    F = allocFragSpace(0);
-  }
-  new (F) MCFragment();
-  addFragment(F);
-}
-
-void MCObjectStreamer::ensureHeadroom(size_t Headroom) {
-  if (Headroom <= FragSpace)
-    return;
-  auto *F = allocFragSpace(Headroom);
-  new (F) MCFragment();
-  addFragment(F);
+  addFragment(getContext().allocFragment<MCFragment>());
 }
 
-void MCObjectStreamer::insert(MCFragment *Frag) {
-  assert(Frag->getKind() != MCFragment::FT_Data &&
+void MCObjectStreamer::insert(MCFragment *F) {
+  assert(F->getKind() != MCFragment::FT_Data &&
          "F should have a variable-size tail");
-  // Frag is not connected to FragSpace. Before modifying CurFrag with
-  // addFragment(Frag), allocate an empty fragment to maintain FragSpace
-  // connectivity, potentially reusing CurFrag's associated space.
-  MCFragment *F;
-  if (LLVM_LIKELY(sizeof(MCFragment) + NewFragHeadroom <= FragSpace)) {
-    auto End = reinterpret_cast<size_t>(getCurFragEnd());
-    F = reinterpret_cast<MCFragment *>(
-        alignToPowerOf2(End, alignof(MCFragment)));
-    FragSpace -= size_t(F) - End + sizeof(MCFragment);
-  } else {
-    F = allocFragSpace(0);
-  }
-  new (F) MCFragment();
-
-  addFragment(Frag);
   addFragment(F);
+  newFragment();
 }
 
 void MCObjectStreamer::appendContents(ArrayRef<char> Contents) {
-  ensureHeadroom(Contents.size());
-  assert(FragSpace >= Contents.size());
-  llvm::copy(Contents, getCurFragEnd());
-  CurFrag->FixedSize += Contents.size();
-  FragSpace -= Contents.size();
+  CurFrag->appendContents(Contents);
 }
 
 void MCObjectStreamer::appendContents(size_t Num, char Elt) {
-  ensureHeadroom(Num);
-  MutableArrayRef<char> Data(getCurFragEnd(), Num);
-  llvm::fill(Data, Elt);
-  CurFrag->FixedSize += Num;
-  FragSpace -= Num;
+  CurFrag->appendContents(Num, Elt);
 }
 
 void MCObjectStreamer::addFixup(const MCExpr *Value, MCFixupKind Kind) {
-  CurFrag->addFixup(MCFixup::create(getCurFragSize(), Value, Kind));
+  CurFrag->addFixup(MCFixup::create(CurFrag->getFixedSize(), Value, Kind));
 }
 
 // As a compile-time optimization, avoid allocating and evaluating an MCExpr
@@ -171,8 +115,6 @@ void MCObjectStreamer::reset() {
   }
   EmitEHFrame = true;
   EmitDebugFrame = false;
-  FragStorage.clear();
-  FragSpace = 0;
   MCStreamer::reset();
 }
 
@@ -201,6 +143,7 @@ void MCObjectStreamer::emitCFISections(bool EH, bool Debug, bool SFrame) {
 void MCObjectStreamer::emitValueImpl(const MCExpr *Value, unsigned Size,
                                      SMLoc Loc) {
   MCStreamer::emitValueImpl(Value, Size, Loc);
+  MCFragment *DF = getCurrentFragment();
 
   MCDwarfLineEntry::make(this, getCurrentSectionOnly());
 
@@ -215,9 +158,9 @@ void MCObjectStreamer::emitValueImpl(const MCExpr *Value, unsigned Size,
     emitIntValue(AbsValue, Size);
     return;
   }
-  ensureHeadroom(Size);
-  addFixup(Value, MCFixup::getDataKindForSize(Size));
-  appendContents(Size, 0);
+  DF->addFixup(MCFixup::create(DF->getContents().size(), Value,
+                               MCFixup::getDataKindForSize(Size)));
+  DF->appendContents(Size, 0);
 }
 
 MCSymbol *MCObjectStreamer::emitCFILabel() {
@@ -251,7 +194,7 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   // section.
   MCFragment *F = CurFrag;
   Symbol->setFragment(F);
-  Symbol->setOffset(F->getFixedSize());
+  Symbol->setOffset(F->getContents().size());
 
   emitPendingAssignments(Symbol);
 }
@@ -317,38 +260,20 @@ void MCObjectStreamer::changeSection(MCSection *Section, uint32_t Subsection) {
     F0 = CurFrag;
   }
 
-  // To maintain connectivity between CurFrag and FragSpace when CurFrag is
-  // modified, allocate an empty fragment and append it to the fragment list.
-  // (Subsections[I].second.Tail is not connected to FragSpace.)
-  MCFragment *F;
-  if (LLVM_LIKELY(sizeof(MCFragment) + NewFragHeadroom <= FragSpace)) {
-    auto End = reinterpret_cast<size_t>(getCurFragEnd());
-    F = reinterpret_cast<MCFragment *>(
-        alignToPowerOf2(End, alignof(MCFragment)));
-    FragSpace -= size_t(F) - End + sizeof(MCFragment);
-  } else {
-    F = allocFragSpace(0);
-  }
-  new (F) MCFragment();
-  F->setParent(Section);
-
   auto &Subsections = Section->Subsections;
   size_t I = 0, E = Subsections.size();
   while (I != E && Subsections[I].first < Subsection)
     ++I;
   // If the subsection number is not in the sorted Subsections list, create a
   // new fragment list.
   if (I == E || Subsections[I].first != Subsection) {
+    auto *F = getContext().allocFragment<MCFragment>();
+    F->setParent(Section);
     Subsections.insert(Subsections.begin() + I,
                        {Subsection, MCSection::FragList{F, F}});
-    Section->CurFragList = &Subsections[I].second;
-    CurFrag = F;
-  } else {
-    Section->CurFragList = &Subsections[I].second;
-    CurFrag = Subsections[I].second.Tail;
-    // Ensure CurFrag is associated with FragSpace.
-    addFragment(F);
   }
+  Section->CurFragList = &Subsections[I].second;
+  CurFrag = Section->CurFragList->Tail;
 
   // Define the section symbol at subsection 0's initial fragment if required.
   if (!NewSec)
@@ -419,15 +344,11 @@ void MCObjectStreamer::emitInstToData(const MCInst &Inst,
   MCFragment *F = getCurrentFragment();
 
   // Append the instruction to the data fragment.
-  size_t CodeOffset = getCurFragSize();
-  SmallString<16> Content;
+  size_t CodeOffset = F->getContents().size();
   SmallVector<MCFixup, 1> Fixups;
-  getAssembler().getEmitter().encodeInstruction(Inst, Content, Fixups, STI);
-  appendContents(Content);
-  if (CurFrag != F) {
-    F = CurFrag;
-    CodeOffset = 0;
-  }
+  getAssembler().getEmitter().encodeInstruction(
+      Inst, F->getContentsForAppending(), Fixups, STI);
+  F->doneAppending();
   F->setHasInstructions(STI);
 
   if (Fixups.empty())

llvm/lib/MC/MCWin64EH.cpp

@@ -318,9 +318,6 @@ static void EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
 
   // Emit the epilog instructions.
   if (EnableUnwindV2) {
-    // Ensure the fixups and appended content apply to the same fragment.
-    OS->ensureHeadroom(info->EpilogMap.size() * 2);
-
     bool IsLast = true;
     for (const auto &Epilog : llvm::reverse(info->EpilogMap)) {
       if (IsLast) {

llvm/lib/MC/MCWinCOFFStreamer.cpp

@@ -280,7 +280,6 @@ void MCWinCOFFStreamer::emitCOFFSymbolIndex(MCSymbol const *Symbol) {
 void MCWinCOFFStreamer::emitCOFFSectionIndex(const MCSymbol *Symbol) {
   visitUsedSymbol(*Symbol);
   const MCSymbolRefExpr *SRE = MCSymbolRefExpr::create(Symbol, getContext());
-  ensureHeadroom(2);
   addFixup(SRE, FK_SecRel_2);
   appendContents(2, 0);
 }
@@ -294,7 +293,6 @@ void MCWinCOFFStreamer::emitCOFFSecRel32(const MCSymbol *Symbol,
   if (Offset)
     MCE = MCBinaryExpr::createAdd(
         MCE, MCConstantExpr::create(Offset, getContext()), getContext());
-  ensureHeadroom(4);
   addFixup(MCE, FK_SecRel_4);
   // Emit 4 bytes (zeros) to the object file.
   appendContents(4, 0);
@@ -310,7 +308,6 @@ void MCWinCOFFStreamer::emitCOFFImgRel32(const MCSymbol *Symbol,
   if (Offset)
     MCE = MCBinaryExpr::createAdd(
         MCE, MCConstantExpr::create(Offset, getContext()), getContext());
-  ensureHeadroom(4);
   addFixup(MCE, FK_Data_4);
   // Emit 4 bytes (zeros) to the object file.
   appendContents(4, 0);
@@ -321,7 +318,6 @@ void MCWinCOFFStreamer::emitCOFFSecNumber(MCSymbol const *Symbol) {
   // Create Symbol for section number.
   const MCExpr *MCE = MCCOFFSectionNumberTargetExpr::create(
       *Symbol, this->getWriter(), getContext());
-  ensureHeadroom(4);
   addFixup(MCE, FK_Data_4);
   // Emit 4 bytes (zeros) to the object file.
   appendContents(4, 0);
@@ -332,7 +328,6 @@ void MCWinCOFFStreamer::emitCOFFSecOffset(MCSymbol const *Symbol) {
   // Create Symbol for section offset.
   const MCExpr *MCE =
       MCCOFFSectionOffsetTargetExpr::create(*Symbol, getContext());
-  ensureHeadroom(4);
   addFixup(MCE, FK_Data_4);
   // Emit 4 bytes (zeros) to the object file.
   appendContents(4, 0);

llvm/lib/Target/Mips/MCTargetDesc/MipsTargetStreamer.cpp

@@ -1034,43 +1034,37 @@ MCELFStreamer &MipsTargetELFStreamer::getStreamer() {
 
 void MipsTargetELFStreamer::emitGPRel32Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(4);
   S.addFixup(Value, Mips::fixup_Mips_GPREL32);
   S.appendContents(4, 0);
 }
 
 void MipsTargetELFStreamer::emitGPRel64Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(8);
   // fixup_Mips_GPREL32 desginates R_MIPS_GPREL32+R_MIPS_64 on MIPS64.
   S.addFixup(Value, Mips::fixup_Mips_GPREL32);
   S.appendContents(8, 0);
 }
 
 void MipsTargetELFStreamer::emitDTPRel32Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(4);
   S.addFixup(Value, Mips::fixup_Mips_DTPREL32);
   S.appendContents(4, 0);
 }
 
 void MipsTargetELFStreamer::emitDTPRel64Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(8);
   S.addFixup(Value, Mips::fixup_Mips_DTPREL64);
   S.appendContents(8, 0);
 }
 
 void MipsTargetELFStreamer::emitTPRel32Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(4);
   S.addFixup(Value, Mips::fixup_Mips_TPREL32);
   S.appendContents(4, 0);
 }
 
 void MipsTargetELFStreamer::emitTPRel64Value(const MCExpr *Value) {
   auto &S = getStreamer();
-  S.ensureHeadroom(8);
   S.addFixup(Value, Mips::fixup_Mips_TPREL64);
   S.appendContents(8, 0);
 }