[lld] [lld-macho] Fix compatibility between --icf=safe_thunks and --keep-icf-stabs (PR #116687)

[Ellis Hoag via llvm-commits]

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@@ -481,6 +481,34 @@ void macho::markAddrSigSymbols() {
   }
 }
 
+// Given a symbol that was folded into a thunk, return the symbol pointing to
+// the actual body of the function. We use this approach rather than storing the
+// needed info in the Defined itself in order to minimize memory usage.
+Defined *macho::getBodyForThunkFoldedSym(Defined *foldedSym) {
+  assert(isa<ConcatInputSection>(foldedSym->originalIsec) &&
+         "thunk-folded ICF symbol expected to be on a ConcatInputSection");
+  // foldedSec is the InputSection that was marked as deleted upon fold
+  ConcatInputSection *foldedSec =
+      cast<ConcatInputSection>(foldedSym->originalIsec);
+
+  // thunkBody is the actual live thunk, containing the code that branches to
+  // the actual body of the function.
+  InputSection *thunkBody = foldedSec->replacement;
+
+  // the actual (merged) body of the function that the thunk jumps to. This will
+  // end up in the final binary.
+  InputSection *functionBody = target->getThunkBranchTarget(thunkBody);
+
+  for (Symbol *sym : functionBody->symbols) {
+    Defined *d = dyn_cast<Defined>(sym);
+    // The symbol needs to be at the start of the InputSection
+    if (d && d->value == 0)
+      return d;
+  }
+
+  llvm_unreachable("could not find body symbol for ICF-generated thunk");
+  return nullptr;
----------------
ellishg wrote:

I don't see how `return nullptr` is any safer. Even on non-debug builds `llvm_unreachable()` will abort and I don't think there will be any compiler errors.

https://github.com/llvm/llvm-project/pull/116687