[lld] [lld-macho] Handle InputSection targets in branch range extension logic (PR #126347)

[Ellis Hoag via llvm-commits]

================
@@ -116,6 +116,30 @@ void ConcatOutputSection::addInput(ConcatInputSection *input) {
 
 DenseMap<Symbol *, ThunkInfo> lld::macho::thunkMap;
 
+// Returns the target Symbol that a relocation refers to.
+// A Reloc can refer to either a Symbol directly, or to an InputSection.
+// For InputSection referents, we return the first Symbol at offset 0.
+// This conversion is necessary because the thunk generation algorithm
+// can only handle Symbols as branch targets, not InputSections.
+static Symbol *getReferentSymbol(const Reloc &r) {
+  if (auto *sym = r.referent.dyn_cast<Symbol *>()) {
+    return sym;
+  } else if (auto *isec = r.referent.dyn_cast<InputSection *>()) {
+    // Use the first symbol at offset 0 in the InputSection
+    for (Defined *sym : isec->symbols) {
+      if (sym->value == 0) {
+        return sym;
+      }
+    }
+    // Handle absence of suitable symbol
+    warn("Branch-range extension: No symbol at offset 0 in InputSection '" +
+         toString(isec) + "', possible branch out of range errors may occur.");
+    return nullptr;
+  } else {
+    llvm_unreachable("Unexpected referent type");
+  }
----------------
ellishg wrote:

```suggestion
  }
  llvm_unreachable("Unexpected referent type");
```

https://github.com/llvm/llvm-project/pull/126347