Return empty CSourceModule when no lowered_funcs exists in Relay mod (#4847)

* Use dummy func when no lowered_funcs exists in Relay mod

* Dummy func -> CSourceModule with empty code str

* Added comments describing the empty CSouceModule

* Always import external modules w/o assertions

* Use CSourceModule as a fallback for LLVMModule

* Changed cond for target == llvm

* Create an empty LLVM module w/o using dummy func

* Avoid using IR str concat to create LLVM module

* Improved comments for codegen.LLVMModuleCreate

* Satisfy the linter for LLVMModuleCreate

src/relay/backend/build_module.cc

@@ -28,8 +28,10 @@
 #include <tvm/relay/expr.h>
 #include <tvm/relay/transform.h>
 #include <tvm/relay/qnn/transform.h>
+#include <tvm/tir/ir_pass.h>
 #include <memory>
 
+#include "../../target/source/codegen_source_base.h"
 #include "utils.h"
 
 namespace tvm {
@@ -451,28 +453,51 @@ class RelayBuildModule : public runtime::ModuleNode {
     ret_.params = graph_codegen_->GetParams();
 
     auto lowered_funcs = graph_codegen_->GetLoweredFunc();
+
+    // When there is no lowered_funcs due to reasons such as optimization.
     if (lowered_funcs.size() == 0) {
-      LOG(WARNING) << "no lowered funcs exist in the compiled module";
+      Target target_host = GetTargetHost();
+
+      // If no target_host has been set, we choose a default one, which is
+      // llvm if "codegen.LLVMModuleCreate" is accessible.
+      const runtime::PackedFunc* pf = runtime::Registry::Get("codegen.LLVMModuleCreate");
+      if (!target_host.defined())
+        target_host = (pf != nullptr) ? target::llvm() : target::stackvm();
+
+      if (target_host.defined() && target_host->target_name == "llvm") {
+        // If we can decide the target is LLVM, we then create an empty LLVM module.
+        ret_.mod = (*pf)(target_host->str(), "empty_module");
+      } else {
+        // If we cannot decide the target is LLVM, we create an empty CSourceModule.
+        // The code content is initialized with ";" to prevent complaining
+        // from CSourceModuleNode::SaveToFile.
+        ret_.mod = tvm::codegen::CSourceModuleCreate(";", "");
+      }
     } else {
       ret_.mod = tvm::build(
         lowered_funcs,
         target_host_,
         BuildConfig::Current());
     }
+
     Array<tvm::runtime::Module> ext_mods = graph_codegen_->GetExternalModules();
-    if (!ext_mods.empty()) {
-      CHECK(lowered_funcs.size() > 0 || ext_mods.size() == 1)
-          << "Expect to have a TVM DSOModule when multiple external runtime modules exist";
-      if (lowered_funcs.size() == 0) {
-        // Execute the whole module using external runtime.
-        ret_.mod = ext_mods[0];
-      } else {
     // Import all external runtime modules.
-        for (const auto& it : ext_mods) {
+    for (const auto& it : ext_mods)
       ret_.mod.Import(it);
   }
+
+ private:
+  Target GetTargetHost() {
+    Target target_host = target_host_;
+    if (!target_host_.defined()) {
+      for (const auto &it : targets_) {
+        if (it.second->device_type == kDLCPU) {
+          target_host = it.second;
+          break;
+        }
       }
     }
+    return target_host;
   }
 
  protected:

src/target/llvm/llvm_module.cc

@@ -356,6 +356,28 @@ TVM_REGISTER_GLOBAL("codegen.build_llvm")
     *rv = runtime::Module(n);
   });
 
+TVM_REGISTER_GLOBAL("codegen.LLVMModuleCreate")
+.set_body([](TVMArgs args, TVMRetValue *rv) {
+  auto n = make_object<LLVMModuleNode>();
+  auto target = args[0].operator std::string();
+  auto module_name = args[1].operator std::string();
+
+  // Generate a LLVM module from an input target string
+  InitializeLLVM();
+  auto tm = GetLLVMTargetMachine(target);
+  auto ctx = std::make_shared<llvm::LLVMContext>();
+  std::unique_ptr<llvm::Module> module(new llvm::Module(module_name, *ctx));
+
+  // Use a default data layout and target triple
+  auto triple = tm->getTargetTriple();
+  module->setTargetTriple(triple.str());
+  module->setDataLayout(tm->createDataLayout());
+
+  n->Init(std::move(module), ctx);
+
+  *rv = runtime::Module(n);
+});
+
 TVM_REGISTER_GLOBAL("target.llvm_lookup_intrinsic_id")
 .set_body([](TVMArgs args, TVMRetValue* rv) {
     *rv = static_cast<int64_t>(LookupLLVMIntrinsic(args[0]));