[lld-macho] Implement ObjC category merging (-objc_category_merging)

lld/MachO/Driver.cpp

@@ -1437,6 +1437,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     resetOutputSegments();
     resetWriter();
     InputFile::resetIdCount();
+
+    objc::doCleanup();
   };
 
   ctx->e.logName = args::getFilenameWithoutExe(argsArr[0]);
@@ -1979,9 +1981,16 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     if (config->deadStrip)
       markLive();
 
+    // Categories are not subject to dead-strip. The __objc_catlist section is
+    // marked as NO_DEAD_STRIP and that propagates into all category data.
     if (args.hasArg(OPT_check_category_conflicts))
       objc::checkCategories();
 
+    // Category merging uses "->live = false" to erase old category data, so
+    // it has to run after dead-stripping (markLive).
+    if (args.hasArg(OPT_objc_category_merging, OPT_no_objc_category_merging))
+      objc::mergeCategories();
+
     // ICF assumes that all literals have been folded already, so we must run
     // foldIdenticalLiterals before foldIdenticalSections.
     foldIdenticalLiterals();

lld/MachO/InputSection.h

@@ -93,9 +93,9 @@ class InputSection {
   // .subsections_via_symbols, there is typically only one element here.
   llvm::TinyPtrVector<Defined *> symbols;
 
-protected:
   const Section &section;
 
+protected:
   const Defined *getContainingSymbol(uint64_t off) const;
 };