[MC] Remove pending labels

This commit removes the complexity introduced by pending labels in
https://reviews.llvm.org/D5915 by using a simpler approach. D5915 aimed
to ensure padding placement before `.Ltmp0` for the following code, but
at the cost of expensive per-instruction `flushPendingLabels`.

```
// similar to llvm/test/MC/X86/AlignedBundling/labeloffset.s
.bundle_lock align_to_end
  calll   .L0$pb
.bundle_unlock
.L0$pb:
  popl    %eax
.Ltmp0:   //// padding should be inserted before this label instead of after
  addl    $_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %eax
```

(D5915 was adjusted by https://reviews.llvm.org/D8072 and
https://reviews.llvm.org/D71368)

This patch achieves the same goal by setting the offset of the empty
MCDataFragment (`Prev`) in `layoutBundle`. This eliminates the need for
pending labels and simplifies the code.

llvm/test/MC/MachO/pending-labels.s (D71368): relocation symbols are
changed, but the result is still supported by linkers.

Author: MaskRay

llvm/include/llvm/MC/MCAsmLayout.h

@@ -45,7 +45,7 @@ class MCAsmLayout {
   /// its bundle padding will be recomputed.
   void invalidateFragmentsFrom(MCFragment *F);
 
-  void layoutBundle(MCFragment *F);
+  void layoutBundle(MCFragment *Prev, MCFragment *F);
 
   /// \name Section Access (in layout order)
   /// @{

llvm/include/llvm/MC/MCObjectStreamer.h

@@ -91,7 +91,6 @@ class MCObjectStreamer : public MCStreamer {
   MCFragment *getCurrentFragment() const;
 
   void insert(MCFragment *F) {
-    flushPendingLabels(F);
     MCSection *CurSection = getCurrentSectionOnly();
     CurSection->addFragment(*F);
     F->setParent(CurSection);
@@ -106,24 +105,15 @@ class MCObjectStreamer : public MCStreamer {
 protected:
   bool changeSectionImpl(MCSection *Section, const MCExpr *Subsection);
 
-  /// Assign a label to the current Section and Subsection even though a
-  /// fragment is not yet present. Use flushPendingLabels(F) to associate
-  /// a fragment with this label.
-  void addPendingLabel(MCSymbol* label);
-
   /// If any labels have been emitted but not assigned fragments in the current
   /// Section and Subsection, ensure that they get assigned to fragment F.
   /// Optionally, one can provide an offset \p FOffset as a symbol offset within
   /// the fragment.
-  void flushPendingLabels(MCFragment *F, uint64_t FOffset = 0);
+  void flushPendingLabels(MCFragment *F, uint64_t FOffset = 0) {}
 
 public:
   void visitUsedSymbol(const MCSymbol &Sym) override;
 
-  /// Create a data fragment for any pending labels across all Sections
-  /// and Subsections.
-  void flushPendingLabels();
-
   MCAssembler &getAssembler() { return *Assembler; }
   MCAssembler *getAssemblerPtr() override;
   /// \name MCStreamer Interface

llvm/include/llvm/MC/MCSection.h

@@ -226,13 +226,6 @@ class MCSection {
   /// Add a pending label for the requested subsection. This label will be
   /// associated with a fragment in flushPendingLabels()
   void addPendingLabel(MCSymbol* label, unsigned Subsection = 0);
-
-  /// Associate all pending labels in a subsection with a fragment.
-  void flushPendingLabels(MCFragment *F, unsigned Subsection);
-
-  /// Associate all pending labels with empty data fragments. One fragment
-  /// will be created for each subsection as necessary.
-  void flushPendingLabels();
 };
 
 } // end namespace llvm

llvm/lib/MC/MCAssembler.cpp

@@ -403,7 +403,7 @@ uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
   llvm_unreachable("invalid fragment kind");
 }
 
-void MCAsmLayout::layoutBundle(MCFragment *F) {
+void MCAsmLayout::layoutBundle(MCFragment *Prev, MCFragment *F) {
   // If bundling is enabled and this fragment has instructions in it, it has to
   // obey the bundling restrictions. With padding, we'll have:
   //
@@ -439,6 +439,9 @@ void MCAsmLayout::layoutBundle(MCFragment *F) {
     report_fatal_error("Padding cannot exceed 255 bytes");
   EF->setBundlePadding(static_cast<uint8_t>(RequiredBundlePadding));
   EF->Offset += RequiredBundlePadding;
+  if (auto *DF = dyn_cast_or_null<MCDataFragment>(Prev))
+    if (DF->getContents().empty())
+      DF->Offset = EF->Offset;
 }
 
 uint64_t MCAsmLayout::getFragmentOffset(const MCFragment *F) const {
@@ -451,14 +454,16 @@ void MCAsmLayout::ensureValid(const MCFragment *Frag) const {
   if (Sec.hasLayout())
     return;
   Sec.setHasLayout(true);
+  MCFragment *Prev = nullptr;
   uint64_t Offset = 0;
   for (MCFragment &F : Sec) {
     F.Offset = Offset;
     if (Assembler.isBundlingEnabled() && F.hasInstructions()) {
-      const_cast<MCAsmLayout *>(this)->layoutBundle(&F);
+      const_cast<MCAsmLayout *>(this)->layoutBundle(Prev, &F);
       Offset = F.Offset;
     }
     Offset += getAssembler().computeFragmentSize(*this, F);
+    Prev = &F;
   }
 }
 

llvm/lib/MC/MCObjectStreamer.cpp

@@ -46,59 +46,6 @@ MCAssembler *MCObjectStreamer::getAssemblerPtr() {
   return nullptr;
 }
 
-void MCObjectStreamer::addPendingLabel(MCSymbol* S) {
-  MCSection *CurSection = getCurrentSectionOnly();
-  if (CurSection) {
-    // Register labels that have not yet been assigned to a Section.
-    if (!PendingLabels.empty()) {
-      for (MCSymbol* Sym : PendingLabels)
-        CurSection->addPendingLabel(Sym);
-      PendingLabels.clear();
-    }
-
-    // Add this label to the current Section / Subsection.
-    CurSection->addPendingLabel(S, CurSubsectionIdx);
-
-    // Add this Section to the list of PendingLabelSections.
-    PendingLabelSections.insert(CurSection);
-  } else
-    // There is no Section / Subsection for this label yet.
-    PendingLabels.push_back(S);
-}
-
-void MCObjectStreamer::flushPendingLabels(MCFragment *F, uint64_t FOffset) {
-  assert(F);
-  MCSection *CurSection = getCurrentSectionOnly();
-  if (!CurSection) {
-    assert(PendingLabels.empty());
-    return;
-  }
-  // Register labels that have not yet been assigned to a Section.
-  if (!PendingLabels.empty()) {
-    for (MCSymbol* Sym : PendingLabels)
-      CurSection->addPendingLabel(Sym, CurSubsectionIdx);
-    PendingLabels.clear();
-  }
-
-  // Associate the labels with F.
-  CurSection->flushPendingLabels(F, CurSubsectionIdx);
-}
-
-void MCObjectStreamer::flushPendingLabels() {
-  // Register labels that have not yet been assigned to a Section.
-  if (!PendingLabels.empty()) {
-    MCSection *CurSection = getCurrentSectionOnly();
-    assert(CurSection);
-    for (MCSymbol* Sym : PendingLabels)
-      CurSection->addPendingLabel(Sym, CurSubsectionIdx);
-    PendingLabels.clear();
-  }
-
-  // Assign an empty data fragment to all remaining pending labels.
-  for (MCSection* Section : PendingLabelSections)
-    Section->flushPendingLabels();
-}
-
 // When fixup's offset is a forward declared label, e.g.:
 //
 //   .reloc 1f, R_MIPS_JALR, foo
@@ -113,7 +60,6 @@ void MCObjectStreamer::resolvePendingFixups() {
                                "unresolved relocation offset");
       continue;
     }
-    flushPendingLabels(PendingFixup.DF, PendingFixup.DF->getContents().size());
     PendingFixup.Fixup.setOffset(PendingFixup.Sym->getOffset() +
                                  PendingFixup.Fixup.getOffset());
 
@@ -245,7 +191,6 @@ void MCObjectStreamer::emitValueImpl(const MCExpr *Value, unsigned Size,
                                      SMLoc Loc) {
   MCStreamer::emitValueImpl(Value, Size, Loc);
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
 
   MCDwarfLineEntry::make(this, getCurrentSectionOnly());
 
@@ -291,17 +236,9 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   // If there is a current fragment, mark the symbol as pointing into it.
   // Otherwise queue the label and set its fragment pointer when we emit the
   // next fragment.
-  auto *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
-  if (F) {
-    Symbol->setFragment(F);
-    Symbol->setOffset(F->getContents().size());
-  } else {
-    // Assign all pending labels to offset 0 within the dummy "pending"
-    // fragment. (They will all be reassigned to a real fragment in
-    // flushPendingLabels())
-    Symbol->setOffset(0);
-    addPendingLabel(Symbol);
-  }
+  MCDataFragment *F = getOrCreateDataFragment();
+  Symbol->setFragment(F);
+  Symbol->setOffset(F->getContents().size());
 
   emitPendingAssignments(Symbol);
 }
@@ -598,11 +535,9 @@ void MCObjectStreamer::emitCVInlineLinetableDirective(
 void MCObjectStreamer::emitCVDefRangeDirective(
     ArrayRef<std::pair<const MCSymbol *, const MCSymbol *>> Ranges,
     StringRef FixedSizePortion) {
-  MCFragment *Frag =
-      getContext().getCVContext().emitDefRange(*this, Ranges, FixedSizePortion);
+  getContext().getCVContext().emitDefRange(*this, Ranges, FixedSizePortion);
   // Attach labels that were pending before we created the defrange fragment to
   // the beginning of the new fragment.
-  flushPendingLabels(Frag, 0);
   this->MCStreamer::emitCVDefRangeDirective(Ranges, FixedSizePortion);
 }
 
@@ -620,7 +555,6 @@ void MCObjectStreamer::emitCVFileChecksumOffsetDirective(unsigned FileNo) {
 void MCObjectStreamer::emitBytes(StringRef Data) {
   MCDwarfLineEntry::make(this, getCurrentSectionOnly());
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
   DF->getContents().append(Data.begin(), Data.end());
 }
 
@@ -653,8 +587,6 @@ void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset,
 // Associate DTPRel32 fixup with data and resize data area
 void MCObjectStreamer::emitDTPRel32Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(MCFixup::create(DF->getContents().size(),
                                             Value, FK_DTPRel_4));
   DF->getContents().resize(DF->getContents().size() + 4, 0);
@@ -663,8 +595,6 @@ void MCObjectStreamer::emitDTPRel32Value(const MCExpr *Value) {
 // Associate DTPRel64 fixup with data and resize data area
 void MCObjectStreamer::emitDTPRel64Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(MCFixup::create(DF->getContents().size(),
                                             Value, FK_DTPRel_8));
   DF->getContents().resize(DF->getContents().size() + 8, 0);
@@ -673,8 +603,6 @@ void MCObjectStreamer::emitDTPRel64Value(const MCExpr *Value) {
 // Associate TPRel32 fixup with data and resize data area
 void MCObjectStreamer::emitTPRel32Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(MCFixup::create(DF->getContents().size(),
                                             Value, FK_TPRel_4));
   DF->getContents().resize(DF->getContents().size() + 4, 0);
@@ -683,8 +611,6 @@ void MCObjectStreamer::emitTPRel32Value(const MCExpr *Value) {
 // Associate TPRel64 fixup with data and resize data area
 void MCObjectStreamer::emitTPRel64Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(MCFixup::create(DF->getContents().size(),
                                             Value, FK_TPRel_8));
   DF->getContents().resize(DF->getContents().size() + 8, 0);
@@ -693,8 +619,6 @@ void MCObjectStreamer::emitTPRel64Value(const MCExpr *Value) {
 // Associate GPRel32 fixup with data and resize data area
 void MCObjectStreamer::emitGPRel32Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(
       MCFixup::create(DF->getContents().size(), Value, FK_GPRel_4));
   DF->getContents().resize(DF->getContents().size() + 4, 0);
@@ -703,8 +627,6 @@ void MCObjectStreamer::emitGPRel32Value(const MCExpr *Value) {
 // Associate GPRel64 fixup with data and resize data area
 void MCObjectStreamer::emitGPRel64Value(const MCExpr *Value) {
   MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   DF->getFixups().push_back(
       MCFixup::create(DF->getContents().size(), Value, FK_GPRel_4));
   DF->getContents().resize(DF->getContents().size() + 8, 0);
@@ -789,8 +711,6 @@ MCObjectStreamer::emitRelocDirective(const MCExpr &Offset, StringRef Name,
         MCSymbolRefExpr::create(getContext().createTempSymbol(), getContext());
 
   MCDataFragment *DF = getOrCreateDataFragment(&STI);
-  flushPendingLabels(DF, DF->getContents().size());
-
   MCValue OffsetVal;
   if (!Offset.evaluateAsRelocatable(OffsetVal, nullptr, nullptr))
     return std::make_pair(false,
@@ -830,9 +750,6 @@ MCObjectStreamer::emitRelocDirective(const MCExpr &Offset, StringRef Name,
 
 void MCObjectStreamer::emitFill(const MCExpr &NumBytes, uint64_t FillValue,
                                 SMLoc Loc) {
-  MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   assert(getCurrentSectionOnly() && "need a section");
   insert(
       getContext().allocFragment<MCFillFragment>(FillValue, 1, NumBytes, Loc));
@@ -861,22 +778,14 @@ void MCObjectStreamer::emitFill(const MCExpr &NumValues, int64_t Size,
   }
 
   // Otherwise emit as fragment.
-  MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   assert(getCurrentSectionOnly() && "need a section");
   insert(
       getContext().allocFragment<MCFillFragment>(Expr, Size, NumValues, Loc));
 }
 
 void MCObjectStreamer::emitNops(int64_t NumBytes, int64_t ControlledNopLength,
                                 SMLoc Loc, const MCSubtargetInfo &STI) {
-  // Emit an NOP fragment.
-  MCDataFragment *DF = getOrCreateDataFragment();
-  flushPendingLabels(DF, DF->getContents().size());
-
   assert(getCurrentSectionOnly() && "need a section");
-
   insert(getContext().allocFragment<MCNopsFragment>(
       NumBytes, ControlledNopLength, Loc, STI));
 }
@@ -916,9 +825,6 @@ void MCObjectStreamer::finishImpl() {
   // Emit pseudo probes for the current module.
   MCPseudoProbeTable::emit(this);
 
-  // Update any remaining pending labels with empty data fragments.
-  flushPendingLabels();
-
   resolvePendingFixups();
   getAssembler().Finish();
 }

llvm/lib/MC/MCSection.cpp

@@ -80,33 +80,6 @@ void MCSection::switchSubsection(unsigned Subsection) {
 StringRef MCSection::getVirtualSectionKind() const { return "virtual"; }
 
 void MCSection::addPendingLabel(MCSymbol *label, unsigned Subsection) {
-  PendingLabels.push_back(PendingLabel(label, Subsection));
-}
-
-void MCSection::flushPendingLabels(MCFragment *F, unsigned Subsection) {
-  // Set the fragment and fragment offset for all pending symbols in the
-  // specified Subsection, and remove those symbols from the pending list.
-  for (auto It = PendingLabels.begin(); It != PendingLabels.end(); ++It) {
-    PendingLabel& Label = *It;
-    if (Label.Subsection == Subsection) {
-      Label.Sym->setFragment(F);
-      assert(Label.Sym->getOffset() == 0);
-      PendingLabels.erase(It--);
-    }
-  }
-}
-
-void MCSection::flushPendingLabels() {
-  // Make sure all remaining pending labels point to data fragments, by
-  // creating new empty data fragments for each Subsection with labels pending.
-  while (!PendingLabels.empty()) {
-    PendingLabel& Label = PendingLabels[0];
-    switchSubsection(Label.Subsection);
-    MCFragment *F = new MCDataFragment();
-    addFragment(*F);
-    F->setParent(this);
-    flushPendingLabels(F, Label.Subsection);
-  }
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

llvm/test/MC/MachO/pending-labels.s

@@ -32,17 +32,17 @@ __data:
 	.quad L8-.
 // CHECK: 0000000000000038 X86_64_RELOC_SUBTRACTOR _function1-__data
 // CHECK: 0000000000000038 X86_64_RELOC_UNSIGNED _function1
-// CHECK: 0000000000000030 X86_64_RELOC_SUBTRACTOR _function1-__data
-// CHECK: 0000000000000030 X86_64_RELOC_UNSIGNED _function1
+// CHECK: 0000000000000030 X86_64_RELOC_SUBTRACTOR __text-__data
+// CHECK: 0000000000000030 X86_64_RELOC_UNSIGNED __text
 // CHECK: 0000000000000028 X86_64_RELOC_SUBTRACTOR _function2-__data
 // CHECK: 0000000000000028 X86_64_RELOC_UNSIGNED _function2
 // CHECK: 0000000000000020 X86_64_RELOC_SUBTRACTOR _function2-__data
 // CHECK: 0000000000000020 X86_64_RELOC_UNSIGNED _function2
-// CHECK: 0000000000000018 X86_64_RELOC_SUBTRACTOR _function2-__data
-// CHECK: 0000000000000018 X86_64_RELOC_UNSIGNED _function2
-// CHECK: 0000000000000010 X86_64_RELOC_SUBTRACTOR _function1-__data
-// CHECK: 0000000000000010 X86_64_RELOC_UNSIGNED _function1
-// CHECK: 0000000000000008 X86_64_RELOC_SUBTRACTOR _function2-__data
-// CHECK: 0000000000000008 X86_64_RELOC_UNSIGNED _function2
-// CHECK: 0000000000000000 X86_64_RELOC_SUBTRACTOR _function1-__data
-// CHECK: 0000000000000000 X86_64_RELOC_UNSIGNED _function1
+// CHECK: 0000000000000018 X86_64_RELOC_SUBTRACTOR __text_cold-__data
+// CHECK: 0000000000000018 X86_64_RELOC_UNSIGNED __text_cold
+// CHECK: 0000000000000010 X86_64_RELOC_SUBTRACTOR __text-__data
+// CHECK: 0000000000000010 X86_64_RELOC_UNSIGNED __text
+// CHECK: 0000000000000008 X86_64_RELOC_SUBTRACTOR __text_cold-__data
+// CHECK: 0000000000000008 X86_64_RELOC_UNSIGNED __text_cold
+// CHECK: 0000000000000000 X86_64_RELOC_SUBTRACTOR __text-__data
+// CHECK: 0000000000000000 X86_64_RELOC_UNSIGNED __text

llvm/tools/dsymutil/MachOUtils.cpp

@@ -381,7 +381,6 @@ bool generateDsymCompanion(
   auto &Writer = static_cast<MachObjectWriter &>(MCAsm.getWriter());
 
   // Layout but don't emit.
-  ObjectStreamer.flushPendingLabels();
   MCAsmLayout Layout(MCAsm);
   MCAsm.layout(Layout);