[lld] [lld-macho] Implement ObjC category merging (-objc_category_merging) (PR #82928)

[Kyungwoo Lee via llvm-commits]

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@@ -320,3 +342,1007 @@ void objc::checkCategories() {
       }
   }
 }
+
+namespace {
+
+class ObjcCategoryMerger {
+  // Information about an input category
+  struct InfoInputCategory {
+    ConcatInputSection *catBodyIsec;
+    ConcatInputSection *catListIsec;
+    uint32_t offCatListIsec = 0;
+
+    bool wasMerged = false;
+  };
+
+  // To write new (merged) categories or classes, we will try make limited
+  // assumptions about the alignment and the sections the various class/category
+  // info are stored in and . So we'll just reuse the same sections and
+  // alignment as already used in existing (input) categories. To do this we
+  // have InfoCategoryWriter which contains the various sections that the
+  // generated categories will be written to.
+  template <typename T> struct InfroWriteSection {
+    bool valid = false; // Data has been successfully collected from input
+    uint32_t align = 0;
+    const Section *inputSection;
+    Reloc relocTemplate;
+    T *outputSection;
+  };
+
+  struct InfoCategoryWriter {
+    InfroWriteSection<ConcatOutputSection> catListInfo;
+    InfroWriteSection<CStringSection> catNameInfo;
+    InfroWriteSection<ConcatOutputSection> catBodyInfo;
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kyulee-com wrote:

Can we switch `catNameInfo` and `catBodyInfo`, based on the code flow?

https://github.com/llvm/llvm-project/pull/82928