[clang] [CIR] Implement initial LoweringPrepare support for global ctors (PR #161452)

[Amr Hesham via cfe-commits]

================
@@ -589,6 +621,113 @@ void LoweringPreparePass::lowerUnaryOp(cir::UnaryOp op) {
   op.erase();
 }
 
+cir::FuncOp
+LoweringPreparePass::buildCXXGlobalVarDeclInitFunc(cir::GlobalOp op) {
+  // TODO(cir): Store this in the GlobalOp.
+  // This should come from the MangleContext, but for now I'm hardcoding it.
+  SmallString<256> fnName("__cxx_global_var_init");
+  // Get a unique name
+  uint32_t cnt = dynamicInitializerNames[fnName]++;
+  if (cnt)
+    fnName += "." + llvm::Twine(cnt).str();
+
+  // Create a variable initialization function.
+  CIRBaseBuilderTy builder(getContext());
+  builder.setInsertionPointAfter(op);
+  auto voidTy = cir::VoidType::get(builder.getContext());
+  auto fnType = cir::FuncType::get({}, voidTy);
+  FuncOp f = buildRuntimeFunction(builder, fnName, op.getLoc(), fnType,
+                                  cir::GlobalLinkageKind::InternalLinkage);
+
+  // Move over the initialzation code of the ctor region.
+  mlir::Block *entryBB = f.addEntryBlock();
+  if (!op.getCtorRegion().empty()) {
+    mlir::Block &block = op.getCtorRegion().front();
+    entryBB->getOperations().splice(entryBB->begin(), block.getOperations(),
+                                    block.begin(), std::prev(block.end()));
+  }
+
+  // Register the destructor call with __cxa_atexit
+  auto &dtorRegion = op.getDtorRegion();
+  if (!dtorRegion.empty()) {
+    assert(!cir::MissingFeatures::opGlobalDtorLowering());
+    llvm_unreachable("dtor region lowering is NYI");
+  }
+
+  // Replace cir.yield with cir.return
+  builder.setInsertionPointToEnd(entryBB);
+  mlir::Operation *yieldOp = nullptr;
+  if (!op.getCtorRegion().empty()) {
+    mlir::Block &block = op.getCtorRegion().front();
+    yieldOp = &block.getOperations().back();
+  } else {
+    assert(!cir::MissingFeatures::opGlobalDtorLowering());
+    llvm_unreachable("dtor region lowering is NYI");
+  }
+
+  assert(isa<YieldOp>(*yieldOp));
+  builder.create<ReturnOp>(yieldOp->getLoc());
+  return f;
+}
+
+void LoweringPreparePass::lowerGlobalOp(GlobalOp op) {
+  mlir::Region &ctorRegion = op.getCtorRegion();
+  mlir::Region &dtorRegion = op.getDtorRegion();
+
+  if (!ctorRegion.empty() || !dtorRegion.empty()) {
+    // Build a variable initialization function and move the initialzation code
+    // in the ctor region over.
+    cir::FuncOp f = buildCXXGlobalVarDeclInitFunc(op);
+
+    // Clear the ctor and dtor region
+    ctorRegion.getBlocks().clear();
+    dtorRegion.getBlocks().clear();
+
+    assert(!cir::MissingFeatures::astVarDeclInterface());
+    dynamicInitializers.push_back(f);
+  }
+
+  assert(!cir::MissingFeatures::opGlobalAnnotations());
+}
+
+void LoweringPreparePass::buildCXXGlobalInitFunc() {
+  if (dynamicInitializers.empty())
+    return;
+
+  assert(!cir::MissingFeatures::opGlobalCtorList());
+
+  SmallString<256> fnName;
+  // Include the filename in the symbol name. Including "sub_" matches gcc
+  // and makes sure these symbols appear lexicographically behind the symbols
+  // with priority (TBD).  Module implementation units behave the same
+  // way as a non-modular TU with imports.
+  // TODO: check CXX20ModuleInits
+  if (astCtx->getCurrentNamedModule() &&
+      !astCtx->getCurrentNamedModule()->isModuleImplementation()) {
+    llvm::raw_svector_ostream out(fnName);
+    std::unique_ptr<clang::MangleContext> mangleCtx(
+        astCtx->createMangleContext());
+    cast<clang::ItaniumMangleContext>(*mangleCtx)
+        .mangleModuleInitializer(astCtx->getCurrentNamedModule(), out);
+  } else {
+    fnName += "_GLOBAL__sub_I_";
+    fnName += getTransformedFileName(mlirModule);
+  }
+
+  CIRBaseBuilderTy builder(getContext());
+  builder.setInsertionPointToEnd(&mlirModule.getBodyRegion().back());
+  auto fnType =
+      cir::FuncType::get({}, cir::VoidType::get(builder.getContext()));
----------------
AmrDeveloper wrote:

```suggestion
      cir::FuncType::get({}, builder.getVoidTy());
```

https://github.com/llvm/llvm-project/pull/161452