[LLVM/CPU] Add comments with origins of various runtime/backend types…

…, NFC (#9177)

LLVM codegen defines a bunch of LLVM IR types that correspond to various
runtime entities. Add comments showing what entity a given LLVM IR type
corresponds to wherever applicable. Since there is no direct connection
(e.g. using names, etc.) that could be extracted automatically (via ctags
or other mechanism), this can make the LLVM codegen a bit easier to read.

src/target/llvm/codegen_cpu.cc

@@ -44,16 +44,34 @@ void CodeGenCPU::Init(const std::string& module_name, llvm::TargetMachine* tm,
   static_assert(sizeof(TVMValue) == sizeof(double), "invariant");
   func_handle_map_.clear();
   export_system_symbols_.clear();
-  // TVM runtime types
+
+  // Runtime types.
+
   t_tvm_shape_index_ = llvm::Type::getIntNTy(*ctx, DataType::ShapeIndex().bits());
+  // Defined in 3rdparty/dlpack/include/dlpack/dlpack.h:
+  // typedef struct { DLDeviceType device_type; int device_id; } DLDevice;
   t_tvm_device_ = llvm::StructType::create({t_int_, t_int_});
+  // Defined in 3rdparty/dlpack/include/dlpack/dlpack.h:
+  // typedef struct { uint8_t code; uint8_t bits; uint16_t lanes; } DLDataType;
   t_tvm_type_ = llvm::StructType::create({t_int8_, t_int8_, t_int16_});
+  // Defined in include/tvm/runtime/c_runtime_api.h:
+  // typedef void* TVMFunctionHandle;
   t_tvm_func_handle_ = t_void_p_;
+  // Defined in 3rdparty/dlpack/include/dlpack/dlpack.h:
+  // typedef struct { ... } DLTensor;
   t_tvm_array_ = llvm::StructType::create({t_void_p_, t_tvm_device_, t_int_, t_tvm_type_,
                                            t_tvm_shape_index_->getPointerTo(),
                                            t_tvm_shape_index_->getPointerTo(), t_int64_});
+  // Defined in include/tvm/runtime/c_runtime_api.h:
+  // typedef union { ... } TVMValue;
   t_tvm_value_ = llvm::StructType::create({t_float64_});
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // typedef struct { void* sync_handle; int32_t num_task; } TVMParallelGroupEnv;
   t_tvm_parallel_group_env_ = llvm::StructType::create({t_int32_->getPointerTo(), t_int32_});
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // typedef int (*TVMBackendPackedCFunc)(TVMValue* args, int* type_codes, int num_args,
+  //                                      TVMValue* out_ret_value, int* out_ret_tcode,
+  //                                      void* resource_handle);
   ftype_tvm_backend_packed_c_func_ = llvm::FunctionType::get(
       t_int_,
       {t_tvm_func_handle_, t_tvm_value_->getPointerTo(), t_int_->getPointerTo(), t_int_,
@@ -62,21 +80,36 @@ void CodeGenCPU::Init(const std::string& module_name, llvm::TargetMachine* tm,
   t_tvm_crt_func_registry_ = llvm::StructType::create(
       {t_char_->getPointerTo(), ftype_tvm_backend_packed_c_func_->getPointerTo()});
   t_tvm_crt_module_ = llvm::StructType::create({t_tvm_crt_func_registry_->getPointerTo()});
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // typedef int (*FTVMParallelLambda)(int task_id, TVMParallelGroupEnv* penv, void* cdata);
   ftype_tvm_parallel_lambda_ = llvm::FunctionType::get(
       t_int_, {t_int_, t_tvm_parallel_group_env_->getPointerTo(), t_void_p_}, false);
   md_tbaa_ctx_ptr_ = md_builder_->createTBAAScalarTypeNode("ctx_ptr", md_tbaa_root_);
+
   // Runtime functions.
+
+  // Defined in include/tvm/runtime/c_runtime_api.h:
+  // int TVMFuncCall(TVMFunctionHandle func, TVMValue* arg_values, int* type_codes, int num_args,
+  //                 TVMValue* ret_val, int* ret_type_code);
   ftype_tvm_func_call_ = llvm::FunctionType::get(
       t_int_,
       {t_tvm_func_handle_, t_tvm_value_->getPointerTo(), t_int_->getPointerTo(), t_int_,
        t_tvm_value_->getPointerTo(), t_int_->getPointerTo()},
       false);
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // int TVMBackendGetFuncFromEnv(void* mod_node, const char* func_name, TVMFunctionHandle* out);
   ftype_tvm_get_func_from_env_ = llvm::FunctionType::get(
       t_int_, {t_void_p_, t_char_->getPointerTo(), t_tvm_func_handle_->getPointerTo()}, false);
+  // Defined in include/tvm/runtime/c_runtime_api.h:
+  // void TVMAPISetLastError(const char* msg);
   ftype_tvm_api_set_last_error_ =
       llvm::FunctionType::get(t_void_, {t_char_->getPointerTo()}, false);
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // int TVMBackendParallelLaunch(FTVMParallelLambda flambda, void* cdata, int num_task);
   ftype_tvm_parallel_launch_ = llvm::FunctionType::get(
       t_int_, {ftype_tvm_parallel_lambda_->getPointerTo(), t_void_p_, t_int_}, false);
+  // Defined in include/tvm/runtime/c_backend_api.h:
+  // int TVMBackendParallelBarrier(int task_id, TVMParallelGroupEnv* penv);
   ftype_tvm_parallel_barrier_ =
       llvm::FunctionType::get(t_int_, {t_int_, t_tvm_parallel_group_env_->getPointerTo()}, false);
   ftype_tvm_static_init_callback_ = llvm::FunctionType::get(t_int_, {t_void_p_}, false);
@@ -88,6 +121,8 @@ void CodeGenCPU::Init(const std::string& module_name, llvm::TargetMachine* tm,
   // initialize TVM runtime API
   if (system_lib && !target_c_runtime) {
     // We will need this in environment for backward registration.
+    // Defined in include/tvm/runtime/c_backend_api.h:
+    // int TVMBackendRegisterSystemLibSymbol(const char* name, void* ptr);
     f_tvm_register_system_symbol_ = llvm::Function::Create(
         llvm::FunctionType::get(t_int_, {t_char_->getPointerTo(), t_void_p_}, false),
         llvm::Function::ExternalLinkage, "TVMBackendRegisterSystemLibSymbol", module_.get());