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

[via llvm-commits]

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@@ -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 '" +
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alx32 wrote:

We could but I haven't encountered this scenario yet so we might not want to add the extra complexity without a way of testing it. 

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