[llvm] MC: Store fragment content and fixups out-of-line (PR #146307)

[Fangrui Song via llvm-commits]

================
@@ -20,65 +20,102 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
+#include <type_traits>
 #include <utility>
 
 using namespace llvm;
 
+static_assert(std::is_trivially_destructible_v<MCDataFragment>,
+              "fragment classes must be trivially destructible");
+
 MCFragment::MCFragment(FragmentType Kind, bool HasInstructions)
     : Kind(Kind), HasInstructions(HasInstructions), AlignToBundleEnd(false),
       LinkerRelaxable(false), AllowAutoPadding(false) {}
 
-void MCFragment::destroy() {
-  switch (Kind) {
-    case FT_Align:
-      cast<MCAlignFragment>(this)->~MCAlignFragment();
-      return;
-    case FT_Data:
-      cast<MCDataFragment>(this)->~MCDataFragment();
-      return;
-    case FT_Fill:
-      cast<MCFillFragment>(this)->~MCFillFragment();
-      return;
-    case FT_Nops:
-      cast<MCNopsFragment>(this)->~MCNopsFragment();
-      return;
-    case FT_Relaxable:
-      cast<MCRelaxableFragment>(this)->~MCRelaxableFragment();
-      return;
-    case FT_Org:
-      cast<MCOrgFragment>(this)->~MCOrgFragment();
-      return;
-    case FT_Dwarf:
-      cast<MCDwarfLineAddrFragment>(this)->~MCDwarfLineAddrFragment();
-      return;
-    case FT_DwarfFrame:
-      cast<MCDwarfCallFrameFragment>(this)->~MCDwarfCallFrameFragment();
-      return;
-    case FT_LEB:
-      cast<MCLEBFragment>(this)->~MCLEBFragment();
-      return;
-    case FT_BoundaryAlign:
-      cast<MCBoundaryAlignFragment>(this)->~MCBoundaryAlignFragment();
-      return;
-    case FT_SymbolId:
-      cast<MCSymbolIdFragment>(this)->~MCSymbolIdFragment();
-      return;
-    case FT_CVInlineLines:
-      cast<MCCVInlineLineTableFragment>(this)->~MCCVInlineLineTableFragment();
-      return;
-    case FT_CVDefRange:
-      cast<MCCVDefRangeFragment>(this)->~MCCVDefRangeFragment();
-      return;
-    case FT_PseudoProbe:
-      cast<MCPseudoProbeAddrFragment>(this)->~MCPseudoProbeAddrFragment();
-      return;
+const MCSymbol *MCFragment::getAtom() const {
+  return cast<MCSectionMachO>(Parent)->getAtom(LayoutOrder);
+}
+
+SmallVectorImpl<char> &MCEncodedFragment::getContentsForAppending() {
+  SmallVectorImpl<char> &S = getParent()->ContentStorage;
+  if (ContentSize == 0) {
+    ContentStart = S.size();
+  } else if (ContentStart + ContentSize != S.size()) {
+    // If not empty and not at the storage end, move to the storage end.
+    auto I = std::exchange(ContentStart, S.size());
+    S.reserve(S.size() + ContentSize);
----------------
MaskRay wrote:

https://reviews.llvm.org/D91744
`S.begin() + I` used by append is invalidated upon reallocation. So we have to call `reserve` first. I'll improve the comment.

https://github.com/llvm/llvm-project/pull/146307