[MC] Cache current fragment in MCStreamer

This eliminates indirection through `getCurrentSectionOnly()->curFragList()->Tail`.

Author: MaskRay

llvm/include/llvm/MC/MCFragment.h

@@ -24,13 +24,15 @@
 namespace llvm {
 
 class MCAssembler;
+class MCObjectStreamer;
 class MCSection;
 class MCSubtargetInfo;
 class MCSymbol;
 
 class MCFragment {
   friend class MCAsmLayout;
   friend class MCAssembler;
+  friend class MCObjectStreamer;
   friend class MCSection;
 
 public:

llvm/include/llvm/MC/MCObjectStreamer.h

@@ -88,9 +88,12 @@ class MCObjectStreamer : public MCStreamer {
   MCFragment *getCurrentFragment() const;
 
   void insert(MCFragment *F) {
-    MCSection *CurSection = getCurrentSectionOnly();
-    CurSection->addFragment(*F);
-    F->setParent(CurSection);
+    auto *Sec = CurFrag->getParent();
+    F->setParent(Sec);
+    F->setLayoutOrder(CurFrag->getLayoutOrder() + 1);
+    CurFrag->Next = F;
+    CurFrag = F;
+    Sec->curFragList()->Tail = F;
   }
 
   /// Get a data fragment to write into, creating a new one if the current

llvm/include/llvm/MC/MCStreamer.h

@@ -19,6 +19,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCDwarf.h"
+#include "llvm/MC/MCFragment.h"
 #include "llvm/MC/MCLinkerOptimizationHint.h"
 #include "llvm/MC/MCPseudoProbe.h"
 #include "llvm/MC/MCWinEH.h"
@@ -255,6 +256,8 @@ class MCStreamer {
   bool AllowAutoPadding = false;
 
 protected:
+  MCFragment *CurFrag = nullptr;
+
   MCStreamer(MCContext &Ctx);
 
   virtual void emitCFIStartProcImpl(MCDwarfFrameInfo &Frame);

llvm/lib/MC/MCAsmStreamer.cpp

@@ -512,6 +512,7 @@ void MCAsmStreamer::emitExplicitComments() {
 
 void MCAsmStreamer::changeSection(MCSection *Section, uint32_t Subsection) {
   assert(Section && "Cannot switch to a null section!");
+  CurFrag = &Section->getDummyFragment();
   if (MCTargetStreamer *TS = getTargetStreamer()) {
     TS->changeSection(getCurrentSectionOnly(), Section, Subsection, OS);
   } else {

llvm/lib/MC/MCObjectStreamer.cpp

@@ -143,7 +143,8 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
 }
 
 MCFragment *MCObjectStreamer::getCurrentFragment() const {
-  return getCurrentSectionOnly()->curFragList()->Tail;
+  assert(CurFrag->getParent() == getCurrentSection().first);
+  return CurFrag;
 }
 
 static bool canReuseDataFragment(const MCDataFragment &F,
@@ -307,6 +308,7 @@ bool MCObjectStreamer::changeSectionImpl(MCSection *Section,
                        {Subsection, MCSection::FragList{F, F}});
   }
   Section->CurFragList = &Subsections[I].second;
+  CurFrag = Section->CurFragList->Tail;
 
   return getAssembler().registerSection(*Section);
 }

llvm/lib/MC/MCStreamer.cpp

@@ -105,6 +105,7 @@ void MCStreamer::reset() {
   SymbolOrdering.clear();
   SectionStack.clear();
   SectionStack.push_back(std::pair<MCSectionSubPair, MCSectionSubPair>());
+  CurFrag = nullptr;
 }
 
 raw_ostream &MCStreamer::getCommentOS() {