[lld] [lld-macho] Add support for category names in ConcatInputSection's (PR #90850)

[Vincent Lee via llvm-commits]

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@@ -186,13 +186,26 @@ ObjcCategoryChecker::ObjcCategoryChecker()
       roClassLayout(target->wordSize), listHeaderLayout(target->wordSize),
       methodLayout(target->wordSize) {}
 
-// \p r must point to an offset within a cstring section.
+// \p r must point to an offset within a CStringInputSection or a
+// ConcatInputSection
 static StringRef getReferentString(const Reloc &r) {
   if (auto *isec = r.referent.dyn_cast<InputSection *>())
     return cast<CStringInputSection>(isec)->getStringRefAtOffset(r.addend);
+
   auto *sym = cast<Defined>(r.referent.get<Symbol *>());
-  return cast<CStringInputSection>(sym->isec())
-      ->getStringRefAtOffset(sym->value + r.addend);
+  auto *symIsec = sym->isec();
+  auto symOffset = sym->value + r.addend;
+
+  if (auto *s = dyn_cast_or_null<CStringInputSection>(symIsec))
+    return s->getStringRefAtOffset(symOffset);
+
+  if (isa<ConcatInputSection>(symIsec)) {
+    auto strData = symIsec->data.slice(symOffset);
+    uint32_t len = strnlen((const char *)strData.data(), strData.size());
+    return StringRef((const char *)strData.data(), len);
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thevinster wrote:

nit: Please use `static_cast` instead of C-style casts. 

https://github.com/llvm/llvm-project/pull/90850