[lld] [LLD][COFF] Add support for ARM64X same-address thunks (PR #151255)

[Martin Storsjö via llvm-commits]

================
@@ -2668,12 +2677,46 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
     createECExportThunks();
 
   // Resolve remaining undefined symbols and warn about imported locals.
+  std::vector<Undefined *> aliases;
   ctx.forEachSymtab(
-      [&](SymbolTable &symtab) { symtab.resolveRemainingUndefines(); });
+      [&](SymbolTable &symtab) { symtab.resolveRemainingUndefines(aliases); });
 
   if (errorCount())
     return;
 
+  ctx.forEachActiveSymtab([](SymbolTable &symtab) {
+    symtab.initializeECThunks();
+    symtab.initializeLoadConfig();
+  });
+
+  // Identify unreferenced COMDAT sections.
+  if (config->doGC) {
+    if (config->mingw) {
+      // markLive doesn't traverse .eh_frame, but the personality function is
+      // only reached that way. The proper solution would be to parse and
+      // traverse the .eh_frame section, like the ELF linker does.
+      // For now, just manually try to retain the known possible personality
+      // functions. This doesn't bring in more object files, but only marks
+      // functions that already have been included to be retained.
+      ctx.forEachSymtab([&](SymbolTable &symtab) {
+        for (const char *n : {"__gxx_personality_v0", "__gcc_personality_v0",
+                              "rust_eh_personality"}) {
+          Defined *d = dyn_cast_or_null<Defined>(symtab.findUnderscore(n));
+          if (d && !d->isGCRoot) {
+            d->isGCRoot = true;
+            config->gcroot.push_back(d);
+          }
+        }
+      });
+    }
+
+    markLive(ctx);
+  }
+
+  ctx.symtab.initializeSameAddressThunks();
+  for (auto alias : aliases)
+    alias->resolveWeakAlias();
----------------
mstorsjo wrote:

What if `resolveWeakAlias()` fails? Previously that would cause `resolveRemainingUndefines` to go on to do other things, but those are entirely skipped, if the symbo had a weak alias now?

https://github.com/llvm/llvm-project/pull/151255