[llvm] MC: Restructure MCFragment as a fixed part and a variable tail (PR #148544)

[Alexis Engelke via llvm-commits]

================
@@ -265,55 +263,55 @@ class MCFragment {
   /// MCRelaxableFragment: x86-specific
   bool AllowAutoPadding : 1;
 
-  LLVM_ABI MCFragment(FragmentType Kind, bool HasInstructions);
-
-public:
-  MCFragment() = delete;
-  MCFragment(const MCFragment &) = delete;
-  MCFragment &operator=(const MCFragment &) = delete;
-
-  MCFragment *getNext() const { return Next; }
-
-  FragmentType getKind() const { return Kind; }
-
-  MCSection *getParent() const { return Parent; }
-  void setParent(MCSection *Value) { Parent = Value; }
-
-  LLVM_ABI const MCSymbol *getAtom() const;
-
-  unsigned getLayoutOrder() const { return LayoutOrder; }
-  void setLayoutOrder(unsigned Value) { LayoutOrder = Value; }
-
-  /// Does this fragment have instructions emitted into it? By default
-  /// this is false, but specific fragment types may set it to true.
-  bool hasInstructions() const { return HasInstructions; }
-
-  bool isLinkerRelaxable() const { return LinkerRelaxable; }
-  void setLinkerRelaxable() { LinkerRelaxable = true; }
-
-  LLVM_ABI void dump() const;
-};
-
-/// Interface implemented by fragments that contain encoded instructions and/or
-/// data.
-class MCEncodedFragment : public MCFragment {
   uint8_t BundlePadding = 0;
+
   uint32_t ContentStart = 0;
   uint32_t ContentEnd = 0;
   uint32_t FixupStart = 0;
   uint32_t FixupEnd = 0;
 
-protected:
-  MCEncodedFragment(MCFragment::FragmentType FType, bool HasInstructions)
-      : MCFragment(FType, HasInstructions) {}
+  uint32_t VarContentStart = 0;
+  uint32_t VarContentEnd = 0;
+  uint32_t VarFixupStart = 0;
+  uint32_t VarFixupEnd = 0;
----------------
aengelke wrote:

Not sure I like this design, this seems to complicate things. Why not a single, contiguous region for all fixups and store just VarContentStart and VarFixupStart? This would avoid hacks like in MCAssembler::layout.

https://github.com/llvm/llvm-project/pull/148544