[lld] [lld-macho][ObjC] Implement category merging into base class (PR #92448)

[Kyungwoo Lee via llvm-commits]

================
@@ -1302,3 +1357,101 @@ void objc::mergeCategories() {
 }
 
 void objc::doCleanup() { ObjcCategoryMerger::doCleanup(); }
+
+void ObjcCategoryMerger::mergeCategoriesIntoBaseClass(
+    const Defined *baseClass, std::vector<InfoInputCategory> &categories) {
+  assert(categories.size() >= 1 && "Expected at least one category to merge");
+
+  // Collect all the info from the categories
+  ClassExtensionInfo extInfo(catLayout);
+  for (auto &catInfo : categories) {
+    parseCatInfoToExtInfo(catInfo, extInfo);
+  }
+
+  // Get metadata for the base class
+  Defined *metaRo = getClassRo(baseClass, /*getMetaRo=*/true);
+  ConcatInputSection *metaIsec = dyn_cast<ConcatInputSection>(metaRo->isec());
+  Defined *classRo = getClassRo(baseClass, /*getMetaRo=*/false);
+  ConcatInputSection *classIsec = dyn_cast<ConcatInputSection>(classRo->isec());
+
+  // Now collect the info from the base class from the various lists in the
+  // class metadata
+
+  // Protocol lists are a special case - the same protocol list is in classRo
+  // and metaRo, so we only need to parse it once
+  parseProtocolListInfo(classIsec, roClassLayout.baseProtocolsOffset,
+                        extInfo.protocols);
+
+  // Check that the classRo and metaRo protocol lists are identical
+  assert(
+      parseProtocolListInfo(classIsec, roClassLayout.baseProtocolsOffset) ==
+          parseProtocolListInfo(metaIsec, roClassLayout.baseProtocolsOffset) &&
+      "Category merger expects classRo and metaRo to have the same protocol "
+      "list");
+
+  parsePointerListInfo(metaIsec, roClassLayout.baseMethodsOffset,
+                       extInfo.classMethods);
+  parsePointerListInfo(classIsec, roClassLayout.baseMethodsOffset,
+                       extInfo.instanceMethods);
+
+  parsePointerListInfo(metaIsec, roClassLayout.basePropertiesOffset,
+                       extInfo.classProps);
+  parsePointerListInfo(classIsec, roClassLayout.basePropertiesOffset,
+                       extInfo.instanceProps);
+
+  // Erase the old lists - these will be generated and replaced
+  eraseSymbolAtIsecOffset(metaIsec, roClassLayout.baseMethodsOffset);
+  eraseSymbolAtIsecOffset(metaIsec, roClassLayout.baseProtocolsOffset);
+  eraseSymbolAtIsecOffset(metaIsec, roClassLayout.basePropertiesOffset);
+  eraseSymbolAtIsecOffset(classIsec, roClassLayout.baseMethodsOffset);
+  eraseSymbolAtIsecOffset(classIsec, roClassLayout.baseProtocolsOffset);
+  eraseSymbolAtIsecOffset(classIsec, roClassLayout.basePropertiesOffset);
+
+  // Emit the newly merged lists - first into the meta RO then into the class RO
+  // First we emit and link the protocol list into the meta RO. Then we link it
+  // in the classRo as well (they're supposed to be identical)
+  if (Defined *protoListSym =
+          emitAndLinkProtocolList(metaRo, roClassLayout.baseProtocolsOffset,
+                                  extInfo, extInfo.protocols)) {
+    createSymbolReference(classRo, protoListSym,
+                          roClassLayout.baseProtocolsOffset,
+                          infoCategoryWriter.catBodyInfo.relocTemplate);
+  }
+
+  emitAndLinkPointerList(metaRo, roClassLayout.baseMethodsOffset, extInfo,
+                         extInfo.classMethods);
----------------
kyulee-com wrote:

This code pattern seems unfortunate from the previous/existing work.
All these repetitive operations are related to `extInfo` and its fields `classMethods`, ...
I think the right abstraction is to do these operations in `ClassExtensionInfo` so that we can call `extInfo.emitAndLinkPointerList(metaRO, roClassLayout)` here. 
I guess we might do the similar thing for `parsePointerListInfo` as it's about collecting info into `ClassExtensInfo`.  Basically, it'd be much cleaner by implementing reader/writer parts within it and enumerate each fields.

https://github.com/llvm/llvm-project/pull/92448