MCFragment: Remove setContents/setFixups

Make the fixed-size part of MCFragment append-only to support allocating
content as trailing data. Update CodeView callers to use setVarContents
instead of setContents. Remove unused setFixups.

Author: MaskRay

llvm/include/llvm/MC/MCSection.h

@@ -231,11 +231,16 @@ class MCFragment {
   /// FT_Relaxable, x86-specific
   bool AllowAutoPadding : 1;
 
+  // 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 ContentStart = 0;
   uint32_t ContentEnd = 0;
   uint32_t FixupStart = 0;
   uint32_t FixupEnd = 0;
 
+  // Track content and fixups for the optional variable-size tail part,
+  // typically modified during relaxation.
   uint32_t VarContentStart = 0;
   uint32_t VarContentEnd = 0;
   uint32_t VarFixupStart = 0;
@@ -364,7 +369,6 @@ class MCFragment {
     getContentsForAppending().append(Num, Elt);
     doneAppending();
   }
-  LLVM_ABI void setContents(ArrayRef<char> Contents);
   MutableArrayRef<char> getContents() {
     return MutableArrayRef(getParent()->ContentStorage)
         .slice(ContentStart, ContentEnd - ContentStart);
@@ -396,7 +400,6 @@ class MCFragment {
   void clearFixups() { FixupEnd = FixupStart; }
   LLVM_ABI void addFixup(MCFixup Fixup);
   LLVM_ABI void appendFixups(ArrayRef<MCFixup> Fixups);
-  LLVM_ABI void setFixups(ArrayRef<MCFixup> Fixups);
   MutableArrayRef<MCFixup> getFixups() {
     return MutableArrayRef(getParent()->FixupStorage)
         .slice(FixupStart, FixupEnd - FixupStart);

llvm/lib/MC/MCAssembler.cpp

@@ -923,15 +923,15 @@ bool MCAssembler::relaxDwarfCallFrameFragment(MCFragment &F) {
 }
 
 bool MCAssembler::relaxCVInlineLineTable(MCCVInlineLineTableFragment &F) {
-  unsigned OldSize = F.getContents().size();
+  unsigned OldSize = F.getVarContents().size();
   getContext().getCVContext().encodeInlineLineTable(*this, F);
-  return OldSize != F.getContents().size();
+  return OldSize != F.getVarContents().size();
 }
 
 bool MCAssembler::relaxCVDefRange(MCCVDefRangeFragment &F) {
-  unsigned OldSize = F.getContents().size();
+  unsigned OldSize = F.getVarContents().size();
   getContext().getCVContext().encodeDefRange(*this, F);
-  return OldSize != F.getContents().size();
+  return OldSize != F.getVarContents().size();
 }
 
 bool MCAssembler::relaxFill(MCFillFragment &F) {

llvm/lib/MC/MCCodeView.cpp

@@ -26,8 +26,10 @@ using namespace llvm;
 using namespace llvm::codeview;
 
 void CodeViewContext::finish() {
-  if (StrTabFragment)
-    StrTabFragment->setContents(StrTab);
+  if (!StrTabFragment)
+    return;
+  assert(StrTabFragment->getKind() == MCFragment::FT_Data);
+  StrTabFragment->setVarContents(StrTab);
 }
 
 /// This is a valid number for use with .cv_loc if we've already seen a .cv_file
@@ -168,6 +170,7 @@ void CodeViewContext::emitStringTable(MCObjectStreamer &OS) {
   if (!StrTabFragment) {
     OS.newFragment();
     StrTabFragment = OS.getCurrentFragment();
+    OS.newFragment();
   }
 
   OS.emitValueToAlignment(Align(4), 0);
@@ -603,7 +606,7 @@ void CodeViewContext::encodeInlineLineTable(const MCAssembler &Asm,
 
   compressAnnotation(BinaryAnnotationsOpCode::ChangeCodeLength, Buffer);
   compressAnnotation(std::min(EndSymLength, LocAfterLength), Buffer);
-  Frag.setContents(Buffer);
+  Frag.setVarContents(Buffer);
 }
 
 void CodeViewContext::encodeDefRange(const MCAssembler &Asm,
@@ -691,6 +694,6 @@ void CodeViewContext::encodeDefRange(const MCAssembler &Asm,
     }
   }
 
-  Frag.setContents(Contents);
-  Frag.setFixups(Fixups);
+  Frag.setVarContents(Contents);
+  Frag.setVarFixups(Fixups);
 }

llvm/lib/MC/MCSection.cpp

@@ -58,16 +58,6 @@ LLVM_DUMP_METHOD void MCSection::dump(
 }
 #endif
 
-void MCFragment::setContents(ArrayRef<char> Contents) {
-  auto &S = getParent()->ContentStorage;
-  if (ContentStart + Contents.size() > ContentEnd) {
-    ContentStart = S.size();
-    S.resize_for_overwrite(S.size() + Contents.size());
-  }
-  ContentEnd = ContentStart + Contents.size();
-  llvm::copy(Contents, S.begin() + ContentStart);
-}
-
 void MCFragment::setVarContents(ArrayRef<char> Contents) {
   auto &S = getParent()->ContentStorage;
   if (VarContentStart + Contents.size() > VarContentEnd) {
@@ -94,16 +84,6 @@ void MCFragment::appendFixups(ArrayRef<MCFixup> Fixups) {
   FixupEnd = S.size();
 }
 
-void MCFragment::setFixups(ArrayRef<MCFixup> Fixups) {
-  auto &S = getParent()->FixupStorage;
-  if (FixupStart + Fixups.size() > FixupEnd) {
-    FixupStart = S.size();
-    S.resize_for_overwrite(S.size() + Fixups.size());
-  }
-  FixupEnd = FixupStart + Fixups.size();
-  llvm::copy(Fixups, S.begin() + FixupStart);
-}
-
 void MCFragment::setVarFixups(ArrayRef<MCFixup> Fixups) {
   auto &S = getParent()->FixupStorage;
   if (VarFixupStart + Fixups.size() > VarFixupEnd) {