[lld] [LLD][COFF] Generate X64 thunks for ARM64EC entry points and patchable functions. (PR #105499)

[Martin Storsjö via llvm-commits]

================
@@ -1317,6 +1317,72 @@ void LinkerDriver::convertResources() {
   f->includeResourceChunks();
 }
 
+void LinkerDriver::maybeCreateECExportThunk(StringRef name, Symbol *&sym) {
+  Defined *def;
+  if (!sym)
+    return;
+  if (auto undef = dyn_cast<Undefined>(sym))
+    def = undef->getWeakAlias();
+  else
+    def = dyn_cast<Defined>(sym);
+  if (!def)
+    return;
+
+  if (def->getChunk()->getArm64ECRangeType() != chpe_range_type::Arm64EC)
+    return;
+  StringRef expName;
+  if (auto mangledName = getArm64ECMangledFunctionName(name))
+    expName = saver().save("EXP+" + *mangledName);
+  else
+    expName = saver().save("EXP+" + name);
+  sym = addUndefined(expName);
+  if (auto undef = dyn_cast<Undefined>(sym)) {
+    if (!undef->getWeakAlias()) {
+      auto thunk = make<ECExportThunkChunk>(def);
+      replaceSymbol<DefinedSynthetic>(undef, undef->getName(), thunk);
+    }
+  }
+}
+
+void LinkerDriver::createECExportThunks() {
+  // Check if EXP+ symbols have corresponding $hp_target symbols and use them
+  // to create export thunks when available.
+  for (Symbol *s : ctx.symtab.expSymbols) {
+    if (!s->isUsedInRegularObj)
+      continue;
+    assert(s->getName().starts_with "EXP+");
+    std::string targetName =
+        (s->getName().substr(strlen("EXP+")) + "$hp_target").str();
+    Symbol *sym = ctx.symtab.find(targetName);
+    if (!sym)
+      continue;
+    Defined *targetSym;
+    if (auto undef = dyn_cast<Undefined>(sym))
+      targetSym = undef->getWeakAlias();
+    else
+      targetSym = dyn_cast<Defined>(sym);
+    if (!targetSym)
+      continue;
+
+    auto *undef = dyn_cast<Undefined>(s);
+    if (undef && !undef->getWeakAlias()) {
+      auto thunk = make<ECExportThunkChunk>(targetSym);
+      replaceSymbol<DefinedSynthetic>(undef, undef->getName(), thunk);
+    }
+    if (!targetSym->isGCRoot) {
+      targetSym->isGCRoot = true;
+      ctx.config.gcroot.push_back(targetSym);
+    }
+  }
+
+  if (ctx.config.entry)
+    maybeCreateECExportThunk(ctx.config.entry->getName(), ctx.config.entry);
+  for (Export &e : ctx.config.exports) {
+    if (!e.data)
----------------
mstorsjo wrote:

For the case of mingw and exporting all (or practically, most) symbols, I guess `ctx.config.exports` already has been populated here, and `e.data` is set according to the heuristic there?

https://github.com/llvm/llvm-project/pull/105499