Annotate entire body as target region if no subregions found

Otherwise, in cases of a custom codegen, the device specification may
be dropped entirely.

src/tir/transforms/annotate_device_regions.cc

@@ -34,16 +34,37 @@ namespace tir {
 
 class DeviceRegionAnnotater : public StmtMutator {
  public:
+  static Stmt Apply(Target host_target, Target device_target, Stmt body) {
+    DeviceRegionAnnotater mutator(device_target);
+    body = mutator(body);
+
+    bool same_host_and_device = host_target->str() == device_target->str();
+
+    // If no region was found that must be on the device, but the
+    // device and host differ (e.g. `T.target('c', host='llvm')`),
+    // then the entire region should be annotated.  This preserves the
+    // host-side handling of DLTensor arguments, while ensuring that
+    // any device targets are used for the codegen.
+    if (!mutator.found_target_region_ && !same_host_and_device) {
+      body = AttrStmt(device_target, tvm::attr::kTarget, 0, body);
+    }
+
+    return body;
+  }
+
+ private:
   explicit DeviceRegionAnnotater(Target device_target) : device_target_(device_target) {}
 
   Stmt VisitStmt_(const AttrStmtNode* op) final {
     if (op->attr_key == tvm::attr::kTarget) {
       // If a target attribute already exists, use it as-is.
+      found_target_region_ = true;
       return GetRef<Stmt>(op);
     } else if (op->attr_key == attr::thread_extent || op->attr_key == attr::pipeline_exec_scope ||
                op->attr_key == attr::device_scope) {
       // These attributes are only allowed in device-side code, so
       // they should be annotated with the function's default target.
+      found_target_region_ = true;
       Stmt body = GetRef<Stmt>(op);
       return AttrStmt(device_target_, tvm::attr::kTarget, 0, body);
     } else {
@@ -52,8 +73,8 @@ class DeviceRegionAnnotater : public StmtMutator {
     }
   }
 
- private:
   Target device_target_;
+  bool found_target_region_{false};
 };
 
 namespace transform {
@@ -64,9 +85,12 @@ Pass AnnotateDeviceRegions() {
     ICHECK(opt_target) << "AnnotateDeviceRegions: Require the target attribute";
     Target target = opt_target.value();
 
-    if (target->GetHost()) {
-      DeviceRegionAnnotater mutator(target.WithoutHost());
-      func.CopyOnWrite()->body = mutator(func->body);
+    if (auto opt_host = target->GetHost()) {
+      auto new_body =
+          DeviceRegionAnnotater::Apply(opt_host.value(), target.WithoutHost(), func->body);
+      if (!new_body.same_as(func->body)) {
+        func.CopyOnWrite()->body = new_body;
+      }
     }
     return func;
   };