[Relax] Support callback as argument

Prior to this commit, calls from Relax to external PackedFuncs could
only be done through the TVM global registry.  While Relax functions
accepting a callback could be written as `callback_arg:
R.Callable(arg_struct_info, ret_struct_info)`, attempting to compile
these functions would raise an error during the `CodeGenVM` step of
`relax.build`.  In addition, the global registry is only queried when
initializing the `relax.VirtualMachine`, and so later changes requires
restarting the VM.

This commit updates both the `CodeGenVM` lowering pass and the relax
VM to support callbacks.  The is primarily intended for use with the
`LazyTransformParams` pass, to improve flexibility by avoiding use of
the global registry.

Author: Lunderberg

include/tvm/runtime/relax_vm/bytecode.h

@@ -58,6 +58,7 @@ enum class Opcode {
   Ret = 2U,
   Goto = 3U,
   If = 4U,
+  CallFromRegister = 5U,
 };
 
 /*! \brief A single virtual machine instruction.
@@ -183,10 +184,15 @@ struct Instruction {
   /*! \brief The instruction opcode. */
   Opcode op;
   union {
-    struct /* Call */ {
+    struct /* Call, CallFromRegister */ {
       /*! \brief The destination register. */
       RegName dst;
-      /*! \brief The index into the packed function table. */
+      /*! \brief The index of the function.
+       *
+       * For `OpCode::Call`, this is an index into the table of static
+       * functions.  For `OpCode::CallFromRegister`, this is an index
+       * of a register.
+       */
       Index func_idx;
       /*! \brief The number of arguments to the packed function. */
       Index num_args;
@@ -208,27 +214,43 @@ struct Instruction {
       Index false_offset;
     };
   };
+
   /*!
    * \brief Construct a Call instruction.
-   * \param func_idx The index of the function to call.
+   * \param func_idx The index of the function to call within the
+   *                 static function table
    * \param num_args The number of arguments.
    * \param args The input arguments.
    * \param dst The destination register.
    * \return The call instruction.
    */
   static Instruction Call(Index func_idx, Index num_args, Arg* args, RegName dst);
+
+  /*!
+   * \brief Construct a Call instruction.
+   * \param func_idx The index of the function to call within the
+   *                 current stack frame's registers.
+   * \param num_args The number of arguments.
+   * \param args The input arguments.
+   * \param dst The destination register.
+   * \return The call instruction.
+   */
+  static Instruction CallFromRegister(Index func_idx, Index num_args, Arg* args, RegName dst);
+
   /*!
    * \brief Construct a return instruction.
    * \param result The register containing the return value.
    * \return The return instruction.
    */
   static Instruction Ret(RegName result);
+
   /*!
    * \brief Construct a goto instruction.
    * \param pc_offset The register containing the jump offset.
    * \return The goto instruction.
    */
   static Instruction Goto(RegName pc_offset);
+
   /*!
    * \brief Construct an If instruction.
    * \param cond The register containing the cond value.

src/relax/backend/vm/exec_builder.cc

@@ -138,10 +138,20 @@ void ExecBuilderNode::EndFunction(const std::string& func_name) {
 
 void ExecBuilderNode::EmitCall(vm::Instruction::Arg func, std::vector<vm::Instruction::Arg> args,
                                vm::RegName dst) {
-  ICHECK(func.kind() == vm::Instruction::ArgKind::kFuncIdx);
+  Opcode op_code;
+  if (func.kind() == vm::Instruction::ArgKind::kFuncIdx) {
+    op_code = Opcode::Call;
+  } else if (func.kind() == vm::Instruction::ArgKind::kRegister) {
+    op_code = Opcode::CallFromRegister;
+  } else {
+    LOG(FATAL) << "VM instruction for a function must be either "
+               << "kFuncIdx (static function ) "
+               << "or kRegister (function passed as parameter), "
+               << "but instead found " << func.kind();
+  }
   // store instruction
   exec_->instr_offset.push_back(exec_->instr_data.size());
-  exec_->instr_data.push_back(static_cast<ExecWord>(Opcode::Call));
+  exec_->instr_data.push_back(static_cast<ExecWord>(op_code));
   exec_->instr_data.push_back(dst);
   exec_->instr_data.push_back(func.value());
   exec_->instr_data.push_back(args.size());
@@ -228,7 +238,8 @@ void ExecBuilderNode::CheckExecutable() {
     for (size_t idx = start_instr; idx < end_instr; ++idx) {
       Instruction instr = exec_->GetInstruction(idx);
       switch (instr.op) {
-        case Opcode::Call: {
+        case Opcode::Call:
+        case Opcode::CallFromRegister: {
           check_func_defined(Instruction::Arg::FuncIdx(instr.func_idx));
           for (int i = 0; i < instr.num_args; ++i) {
             check_reg_defined(instr.args[i]);
@@ -280,7 +291,8 @@ void ExecBuilderNode::Formalize() {
     for (size_t idx = start_instr; idx < end_instr; ++idx) {
       Instruction instr = this->exec_->GetInstruction(idx);
       switch (instr.op) {
-        case Opcode::Call: {
+        case Opcode::Call:
+        case Opcode::CallFromRegister: {
           // rewrite args
           for (int i = 0; i < instr.num_args; ++i) {
             if (instr.args[i].kind() == Instruction::ArgKind::kRegister &&

src/runtime/library_module.cc

@@ -71,11 +71,15 @@ PackedFunc WrapPackedFunc(TVMBackendPackedCFunc faddr, const ObjectPtr<Object>&
   return PackedFunc([faddr, sptr_to_self](TVMArgs args, TVMRetValue* rv) {
     TVMValue ret_value;
     int ret_type_code = kTVMNullptr;
-    int ret = (*faddr)(const_cast<TVMValue*>(args.values), const_cast<int*>(args.type_codes),
-                       args.num_args, &ret_value, &ret_type_code, nullptr);
-    // NOTE: important to keep the original error message.
+    auto arg_values = const_cast<TVMValue*>(args.values);
+    auto arg_type_codes = const_cast<int*>(args.type_codes);
+    int ret =
+        (*faddr)(arg_values, arg_type_codes, args.num_args, &ret_value, &ret_type_code, nullptr);
+    // NOTE: It is important to keep the original error message.
+    // Using the `TVMThrowLastError()` function will also preserve the
+    // full stack trace for debugging in pdb.
     if (ret != 0) {
-      LOG(FATAL) << TVMGetLastError();
+      TVMThrowLastError();
     }
     if (ret_type_code != kTVMNullptr) {
       *rv = TVMRetValue::MoveFromCHost(ret_value, ret_type_code);

src/runtime/relax_vm/bytecode.cc

@@ -42,6 +42,17 @@ Instruction Instruction::Call(Index func_idx, Index num_args, Instruction::Arg*
   return instr;
 }
 
+Instruction Instruction::CallFromRegister(Index func_idx, Index num_args, Instruction::Arg* args,
+                                          RegName dst) {
+  Instruction instr;
+  instr.op = Opcode::CallFromRegister;
+  instr.dst = dst;
+  instr.func_idx = func_idx;
+  instr.num_args = num_args;
+  instr.args = args;
+  return instr;
+}
+
 Instruction Instruction::Ret(RegName result) {
   Instruction instr;
   instr.op = Opcode::Ret;

src/runtime/relax_vm/executable.cc

@@ -134,6 +134,14 @@ Instruction Executable::GetInstruction(Index i) const {
       ExecWord* args = const_cast<ExecWord*>(&instr_data[offset + 4]);
       return Instruction::Call(func_idx, num_args, reinterpret_cast<Instruction::Arg*>(args), dst);
     }
+    case Opcode::CallFromRegister: {
+      RegName dst = instr_data[offset + 1];
+      Index func_idx = instr_data[offset + 2];
+      Index num_args = instr_data[offset + 3];
+      ExecWord* args = const_cast<ExecWord*>(&instr_data[offset + 4]);
+      return Instruction::CallFromRegister(func_idx, num_args,
+                                           reinterpret_cast<Instruction::Arg*>(args), dst);
+    }
     case Opcode::Ret: {
       RegName result = instr_data[offset + 1];
       return Instruction::Ret(result);

src/runtime/relax_vm/vm.cc

@@ -372,9 +372,10 @@ class VirtualMachineImpl : public VirtualMachine {
   /*!
    * \brief Run call instruction.
    * \param curr_frame The current frame.
+   * \param callable The callable object, either PackedFunc or closure
    * \param inst The call instruction.
    */
-  virtual void RunInstrCall(VMFrame* curr_frame, Instruction inst);
+  virtual void RunInstrCall(VMFrame* curr_frame, const ObjectRef& callable, Instruction inst);
 
   /*! \brief Run VM dispatch loop. */
   void RunLoop();
@@ -506,14 +507,18 @@ void VirtualMachineImpl::SetInput(std::string func_name, bool with_param_module,
 //------------------------------------------
 void VirtualMachineImpl::InvokeClosurePacked(const ObjectRef& closure_or_packedfunc, TVMArgs args,
                                              TVMRetValue* rv) {
+  ICHECK(closure_or_packedfunc.defined())
+      << "InvokeClosurePacked requires the callable object to be defined";
+
   // run packed call if it is a packed func.
   if (auto* packed = closure_or_packedfunc.as<PackedFunc::ContainerType>()) {
     packed->CallPacked(args, rv);
     return;
   }
   // run closure call.
   auto* clo = closure_or_packedfunc.as<VMClosureObj>();
-  ICHECK(clo != nullptr) << "Function expects a closure or PackedFunc ";
+  ICHECK(clo != nullptr) << "Function expects a closure or PackedFunc, "
+                         << "but received " << closure_or_packedfunc->GetTypeKey();
 
   std::vector<TVMValue> values(args.size() + 1);
   std::vector<int> tcodes(args.size() + 1);
@@ -595,6 +600,8 @@ Optional<VMClosure> VirtualMachineImpl::GetClosureInternal(const String& func_na
     auto impl = PackedFunc([gf_idx](TVMArgs args, TVMRetValue* rv) {
       // Per convention, ctx ptr is a VirtualMachine*
       VirtualMachine* ctx_ptr = static_cast<VirtualMachine*>(args[0].operator void*());
+      ICHECK(ctx_ptr) << "Context pointer for relax VM closure should be a VirtualMachine*, "
+                      << "but was NULL";
 
       std::vector<RegType> inputs(args.size() - 1);
       for (size_t i = 0; i < inputs.size(); ++i) {
@@ -644,7 +651,7 @@ RegType VirtualMachineImpl::InvokeBytecode(Index gf_idx, const std::vector<RegTy
   auto guard = PushFrame(this->pc_, gfunc);
   // Get new frame and set the caller info.
   VMFrame* curr_frame = frames_.back().get();
-  if (curr_instr.op == Opcode::Call) {
+  if (curr_instr.op == Opcode::Call || curr_instr.op == Opcode::CallFromRegister) {
     curr_frame->caller_return_register = curr_instr.dst;
   }
 
@@ -688,8 +695,12 @@ void VirtualMachineImpl::InitFuncPool() {
   }
 }
 
-void VirtualMachineImpl::RunInstrCall(VMFrame* curr_frame, Instruction instr) {
-  DLOG(INFO) << "\n  pc = " << pc_ << ", execute: " << GetFuncName(instr.func_idx);
+void VirtualMachineImpl::RunInstrCall(VMFrame* curr_frame, const ObjectRef& callable,
+                                      Instruction instr) {
+  ICHECK(callable.defined()) << "RunInstrCall requires the callable object to be defined";
+  auto func_name = instr.op == Opcode::Call ? GetFuncName(instr.func_idx) : "<dynamic>";
+
+  DLOG(INFO) << "\n  pc = " << pc_ << ", execute: " << func_name;
   int args_begin_offset = instrument_ != nullptr ? 4 : 0;
   // Use the call arg stack from the current frame to increase reuse
   // and avoid re-allocation
@@ -735,11 +746,11 @@ void VirtualMachineImpl::RunInstrCall(VMFrame* curr_frame, Instruction instr) {
   ICHECK_LT(static_cast<size_t>(instr.func_idx), this->func_pool_.size());
 
   if (instrument_ == nullptr) {
-    this->InvokeClosurePacked(func_pool_[instr.func_idx], args, &ret);
+    this->InvokeClosurePacked(callable, args, &ret);
   } else {
     // insert light-weight instrument callback
-    setter(0, func_pool_[instr.func_idx]);
-    setter(1, GetFuncName(instr.func_idx));
+    setter(0, callable);
+    setter(1, func_name);
     setter(2, true);
     setter(3, nullptr);
     TVMRetValue rv;
@@ -758,7 +769,7 @@ void VirtualMachineImpl::RunInstrCall(VMFrame* curr_frame, Instruction instr) {
       ret_kind = rv;
     }
     if (ret_kind != static_cast<int>(VMInstrumentReturnKind::kSkipRun)) {
-      this->InvokeClosurePacked(func_pool_[instr.func_idx], args, &ret);
+      this->InvokeClosurePacked(callable, args, &ret);
       setter(2, false);
       setter(3, ret);
       instrument_.CallPacked(TVMArgs(values.data(), tcodes.data(), values.size()), &rv);
@@ -782,7 +793,11 @@ void VirtualMachineImpl::RunLoop() {
     Instruction instr = exec_->GetInstruction(pc_);
     switch (instr.op) {
       case Opcode::Call: {
-        this->RunInstrCall(curr_frame, instr);
+        this->RunInstrCall(curr_frame, func_pool_[instr.func_idx], instr);
+        break;
+      }
+      case Opcode::CallFromRegister: {
+        this->RunInstrCall(curr_frame, ReadRegister(curr_frame, instr.func_idx), instr);
         break;
       }
       case Opcode::Ret: {
@@ -1000,7 +1015,7 @@ class VirtualMachineProfiler : public VirtualMachineImpl {
   }
 
  protected:
-  void RunInstrCall(VMFrame* curr_frame, Instruction inst) override {
+  void RunInstrCall(VMFrame* curr_frame, const ObjectRef& callable, Instruction inst) override {
     bool profiling = false;
     if (prof_ && prof_->IsRunning()) {
       auto f_name = GetFuncName(inst.func_idx);
@@ -1036,7 +1051,7 @@ class VirtualMachineProfiler : public VirtualMachineImpl {
       }
     }
 
-    VirtualMachineImpl::RunInstrCall(curr_frame, inst);
+    VirtualMachineImpl::RunInstrCall(curr_frame, callable, inst);
 
     if (profiling) {
       prof_->StopCall();