MC: Optimize getOrCreateDataFragment

... by eagerly allocating an empty fragment when adding a fragment
with a variable-size tail.

X86AsmBackend, The JCC erratum mitigation and x86-pad-for-align set a
flag for FT_Relaxable, which needs to be moved to emitInstructionBegin.
```
  if (CF->getKind() == MCFragment::FT_Relaxable)
    CF->setAllowAutoPadding(canPadInst(Inst, OS));
```

Follow-up to #148544

Author: MaskRay

llvm/include/llvm/MC/MCObjectStreamer.h

@@ -73,12 +73,14 @@ class MCObjectStreamer : public MCStreamer {
   MCSymbol *emitCFILabel() override;
   void emitCFISections(bool EH, bool Debug) override;
 
-  /// Get a data fragment to write into, creating a new one if the current
-  /// fragment is not FT_Data.
-  MCFragment *getOrCreateDataFragment();
+  // TODO: Change callers to use getCurrentFragment instead.
+  MCFragment *getOrCreateDataFragment() { return getCurrentFragment(); }
 
 protected:
   bool changeSectionImpl(MCSection *Section, uint32_t Subsection);
+  MCAlignFragment *createAlignFragment(Align Alignment, int64_t Fill,
+                                       uint8_t FillLen,
+                                       unsigned MaxBytesToEmit);
 
 public:
   void visitUsedSymbol(const MCSymbol &Sym) override;

llvm/include/llvm/MC/MCStreamer.h

@@ -270,6 +270,8 @@ class LLVM_ABI MCStreamer {
   /// section changes.
   virtual void changeSection(MCSection *, uint32_t);
 
+  void addFragment(MCFragment *F);
+
   virtual void emitCFIStartProcImpl(MCDwarfFrameInfo &Frame);
   virtual void emitCFIEndProcImpl(MCDwarfFrameInfo &CurFrame);
 
@@ -456,8 +458,10 @@ class LLVM_ABI MCStreamer {
 
   MCSymbol *endSection(MCSection *Section);
 
-  void insert(MCFragment *F);
+  /// Add a new fragment to the current section without a variable-size tail.
   void newFragment();
+  /// Add a fragment with a variable-size tail and start a new empty fragment.
+  void insert(MCFragment *F);
 
   /// Returns the mnemonic for \p MI, if the streamer has access to a
   /// instruction printer and returns an empty string otherwise.

llvm/lib/MC/MCCodeView.cpp

@@ -166,8 +166,8 @@ void CodeViewContext::emitStringTable(MCObjectStreamer &OS) {
   // somewhere else. If somebody wants two string tables in their .s file, one
   // will just be empty.
   if (!StrTabFragment) {
-    StrTabFragment = Ctx.allocFragment<MCFragment>();
-    OS.insert(StrTabFragment);
+    OS.newFragment();
+    StrTabFragment = OS.getCurrentFragment();
   }
 
   OS.emitValueToAlignment(Align(4), 0);

llvm/lib/MC/MCObjectStreamer.cpp

@@ -106,18 +106,6 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
     MCDwarfFrameEmitter::Emit(*this, MAB, false);
 }
 
-MCFragment *MCObjectStreamer::getOrCreateDataFragment() {
-  // TODO: Start a new fragment whenever finalizing the variable-size tail of a
-  // previous one, so that all getOrCreateDataFragment calls can be replaced
-  // with getCurrentFragment
-  auto *F = getCurrentFragment();
-  if (F->getKind() != MCFragment::FT_Data) {
-    F = getContext().allocFragment<MCFragment>();
-    insert(F);
-  }
-  return F;
-}
-
 void MCObjectStreamer::visitUsedSymbol(const MCSymbol &Sym) {
   Assembler->registerSymbol(Sym);
 }
@@ -379,6 +367,7 @@ void MCObjectStreamer::emitInstToFragment(const MCInst &Inst,
   F->setVarContents(Data);
   F->setVarFixups(Fixups);
   F->setInst(Inst);
+  newFragment();
 }
 
 void MCObjectStreamer::emitDwarfLocDirective(unsigned FileNo, unsigned Line,
@@ -444,6 +433,7 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
   F->Kind = MCFragment::FT_Dwarf;
   F->setDwarfAddrDelta(buildSymbolDiff(*this, Label, LastLabel, SMLoc()));
   F->setDwarfLineDelta(LineDelta);
+  newFragment();
 }
 
 void MCObjectStreamer::emitDwarfLineEndEntry(MCSection *Section,
@@ -474,6 +464,7 @@ void MCObjectStreamer::emitDwarfAdvanceFrameAddr(const MCSymbol *LastLabel,
   auto *F = getOrCreateDataFragment();
   F->Kind = MCFragment::FT_DwarfFrame;
   F->setDwarfAddrDelta(buildSymbolDiff(*this, Label, LastLabel, Loc));
+  newFragment();
 }
 
 void MCObjectStreamer::emitCVLocDirective(unsigned FunctionId, unsigned FileNo,
@@ -536,32 +527,38 @@ void MCObjectStreamer::emitBytes(StringRef Data) {
   DF->appendContents(ArrayRef(Data.data(), Data.size()));
 }
 
-void MCObjectStreamer::emitValueToAlignment(Align Alignment, int64_t Fill,
-                                            uint8_t FillLen,
-                                            unsigned MaxBytesToEmit) {
+MCAlignFragment *MCObjectStreamer::createAlignFragment(
+    Align Alignment, int64_t Fill, uint8_t FillLen, unsigned MaxBytesToEmit) {
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = Alignment.value();
-  insert(getContext().allocFragment<MCAlignFragment>(Alignment, Fill, FillLen,
-                                                     MaxBytesToEmit));
+  return getContext().allocFragment<MCAlignFragment>(Alignment, Fill, FillLen,
+                                                     MaxBytesToEmit);
+}
 
+void MCObjectStreamer::emitValueToAlignment(Align Alignment, int64_t Fill,
+                                            uint8_t FillLen,
+                                            unsigned MaxBytesToEmit) {
+  auto *F = createAlignFragment(Alignment, Fill, FillLen, MaxBytesToEmit);
+  insert(F);
   // Update the maximum alignment on the current section if necessary.
-  MCSection *CurSec = getCurrentSectionOnly();
-  CurSec->ensureMinAlignment(Alignment);
+  F->getParent()->ensureMinAlignment(Alignment);
 }
 
 void MCObjectStreamer::emitCodeAlignment(Align Alignment,
                                          const MCSubtargetInfo *STI,
                                          unsigned MaxBytesToEmit) {
-  emitValueToAlignment(Alignment, 0, 1, MaxBytesToEmit);
-  auto *F = cast<MCAlignFragment>(getCurrentFragment());
+  auto *F = createAlignFragment(Alignment, 0, 1, MaxBytesToEmit);
   F->setEmitNops(true, STI);
+  insert(F);
+  // Update the maximum alignment on the current section if necessary.
+  F->getParent()->ensureMinAlignment(Alignment);
+
   // With RISC-V style linker relaxation, mark the section as linker-relaxable
   // if the alignment is larger than the minimum NOP size.
   unsigned Size;
   if (getAssembler().getBackend().shouldInsertExtraNopBytesForCodeAlign(*F,
                                                                         Size)) {
-    getCurrentSectionOnly()->setLinkerRelaxable();
-    newFragment();
+    F->getParent()->setLinkerRelaxable();
   }
 }
 

llvm/lib/MC/MCStreamer.cpp

@@ -1404,7 +1404,7 @@ MCSymbol *MCStreamer::endSection(MCSection *Section) {
   return Sym;
 }
 
-void MCStreamer::insert(MCFragment *F) {
+void MCStreamer::addFragment(MCFragment *F) {
   auto *Sec = CurFrag->getParent();
   F->setParent(Sec);
   F->setLayoutOrder(CurFrag->getLayoutOrder() + 1);
@@ -1414,7 +1414,14 @@ void MCStreamer::insert(MCFragment *F) {
 }
 
 void MCStreamer::newFragment() {
-  insert(getContext().allocFragment<MCFragment>());
+  addFragment(getContext().allocFragment<MCFragment>());
+}
+
+void MCStreamer::insert(MCFragment *F) {
+  assert(F->getKind() != MCFragment::FT_Data &&
+         "F should have a variable-size tail");
+  addFragment(F);
+  newFragment();
 }
 
 static VersionTuple

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -456,8 +456,6 @@ bool X86AsmBackend::canPadInst(const MCInst &Inst, MCObjectStreamer &OS) const {
 }
 
 bool X86AsmBackend::canPadBranches(MCObjectStreamer &OS) const {
-  if (!OS.getAllowAutoPadding())
-    return false;
   assert(allowAutoPadding() && "incorrect initialization!");
 
   // We only pad in text section.
@@ -491,8 +489,15 @@ void X86AsmBackend::emitInstructionBegin(MCObjectStreamer &OS,
   // fragment will have changed.
   IsRightAfterData =
       isRightAfterData(OS.getCurrentFragment(), PrevInstPosition);
+  bool CanPadInst = false;
+  bool AutoPadding = OS.getAllowAutoPadding();
+  if (LLVM_UNLIKELY(AutoPadding || X86PadForAlign)) {
+    CanPadInst = canPadInst(Inst, OS);
+    if (CanPadInst)
+      OS.getCurrentFragment()->setAllowAutoPadding(true);
+  }
 
-  if (!canPadBranches(OS))
+  if (!AutoPadding || !canPadBranches(OS))
     return;
 
   // NB: PrevInst only valid if canPadBranches is true.
@@ -504,7 +509,7 @@ void X86AsmBackend::emitInstructionBegin(MCObjectStreamer &OS,
   // we call canPadInst (not cheap) twice. However, in the common case, we can
   // avoid unnecessary calls to that, as this is otherwise only used for
   // relaxable fragments.
-  if (!canPadInst(Inst, OS))
+  if (!CanPadInst)
     return;
 
   if (PendingBA && PendingBA->getNext() == OS.getCurrentFragment()) {
@@ -542,15 +547,12 @@ void X86AsmBackend::emitInstructionBegin(MCObjectStreamer &OS,
 /// Set the last fragment to be aligned for the BoundaryAlignFragment.
 void X86AsmBackend::emitInstructionEnd(MCObjectStreamer &OS,
                                        const MCInst &Inst) {
-  MCFragment *CF = OS.getCurrentFragment();
-  if (CF->getKind() == MCFragment::FT_Relaxable)
-    CF->setAllowAutoPadding(canPadInst(Inst, OS));
-
   // Update PrevInstOpcode here, canPadInst() reads that.
+  MCFragment *CF = OS.getCurrentFragment();
   PrevInstOpcode = Inst.getOpcode();
   PrevInstPosition = std::make_pair(CF, getSizeForInstFragment(CF));
 
-  if (!canPadBranches(OS))
+  if (!OS.getAllowAutoPadding() || !canPadBranches(OS))
     return;
 
   // PrevInst is only needed if canPadBranches. Copying an MCInst isn't cheap.