variable.cpp

#include <torch/csrc/autograd/variable.h>

#include <torch/csrc/autograd/autograd.h>
#include <torch/csrc/autograd/edge.h>
#include <torch/csrc/autograd/engine.h>
#include <torch/csrc/autograd/function.h>
#include <torch/csrc/autograd/functions/accumulate_grad.h>
#include <torch/csrc/autograd/functions/tensor.h>
#include <torch/csrc/autograd/generated/Functions.h>

#include <ATen/core/VariableHooksInterface.h>

#include <ATen/ATen.h>
#include <c10/util/Exception.h>

#include <list>
#include <memory>
#include <mutex>
#include <stdexcept>
#include <string>
#include <vector>
#include <typeinfo>

namespace torch {
namespace autograd {


DifferentiableViewMeta::DifferentiableViewMeta(at::TensorImpl* self_impl, Variable base,
  c10::optional<std::function<at::Tensor(const at::Tensor&)>> view_fn,
  CreationMeta creation_meta)
    : AutogradMeta(self_impl), creation_meta(creation_meta) {
  base_ = std::move(base);
  view_fn_ = std::move(view_fn);
  TORCH_CHECK(base_.defined(), "base is undefined");
  if (base_.is_view()) {
    base_ = base_._base();
  }
  is_view_ = true;
  self_impl->set_version_counter(impl::version_counter(base_));
  attr_version = self_impl->version_counter().current_version();
}

DifferentiableViewMeta::~DifferentiableViewMeta() {
  base_.reset();
}

namespace {

at::Tensor singleton_undefined_tensor;

struct ConcreteAutogradMetaFactory : public c10::impl::AutogradMetaFactory {
  std::unique_ptr<c10::AutogradMetaInterface> make() const override {
    return std::make_unique<AutogradMeta>();
  }
  const at::Tensor& undefined_tensor() const override {
    return singleton_undefined_tensor;
  }
};

ConcreteAutogradMetaFactory meta_factory;

static c10::impl::AutogradMetaFactoryRegisterer meta_factory_registerer(&meta_factory);

}

namespace impl {

  AutogradMeta* materialize_autograd_meta(const Variable& self) {
    TORCH_CHECK(self.defined(), "cannot call materialize_autograd_meta() on undefined tensor");
    auto p = self.unsafeGetTensorImpl();
    if (!p->autograd_meta()) {
      p->set_autograd_meta(std::make_unique<AutogradMeta>());
    }
    return get_autograd_meta(self);
  }

  void rebase_history(const Variable& self, Edge gradient_edge) {
    TORCH_INTERNAL_ASSERT(gradient_edge.function != nullptr);
    if (self.is_view()) {
      // NB: is_view() ==> get_autograd_meta()
      auto diff_view_meta = static_cast<DifferentiableViewMeta*>(get_autograd_meta(self));

      // See NOTE [ View + Inplace detection ]
      if (diff_view_meta->creation_meta != CreationMeta::MULTI_OUTPUT_SAFE) {
        // Do not use handle_view_on_rebase here as check_inplace should have been called before this
        // and either throw an error or clear the warning
        TORCH_INTERNAL_ASSERT(diff_view_meta->creation_meta == CreationMeta::DEFAULT);
        TORCH_INTERNAL_ASSERT(gradient_edge.input_nr == 0);
        TORCH_INTERNAL_ASSERT(gradient_edge.function);
        TORCH_CHECK(
            gradient_edge.function->num_inputs() == 1,
            "Functions which modify views in-place must return a single Variable");
        diff_view_meta->output_nr_ = gradient_edge.input_nr;
        auto copy_slices = std::make_shared<CopySlices>(
            diff_view_meta->base_, at::TensorGeometry(self), diff_view_meta->view_fn_, std::move(gradient_edge.function));
        set_gradient_edge(diff_view_meta->base_, {std::move(copy_slices), 0});
        self.grad_fn(); // trigger an update to the view's grad_fn
        return;
      }
    }

    set_gradient_edge(self, std::move(gradient_edge));
  }

  void create_cpp_hook(const Variable& self) {
    auto &list = materialize_autograd_meta(self)->cpp_hooks_list;
    list.reset(new hooks_list());
    std::unique_ptr<FunctionPreHook> hook_ptr(new CppFunctionPreHook(list, self.output_nr()));
    clear_hooks(self);
    add_hook(self, std::make_shared<CppFunctionPreHook>(list, 0));
    auto fn = self.grad_fn();
    if (fn) {
      fn->add_pre_hook(std::move(hook_ptr));
    }
  }

  void set_grad_accumulator(const Variable& self,
      std::weak_ptr<Node> grad_accumulator) {
    materialize_autograd_meta(self)->grad_accumulator_ = std::move(grad_accumulator);
  }

  std::shared_ptr<Node> try_get_grad_accumulator(const Variable& self) {
    if (get_autograd_meta(self)) {
      return get_autograd_meta(self)->grad_accumulator_.lock();
    } else {
      return nullptr;
    }
  }

  std::shared_ptr<Node> grad_accumulator(const Variable& self) {
    auto autograd_meta = get_autograd_meta(self);
    if (!autograd_meta) {
      return nullptr;
    }
    if (autograd_meta->grad_fn_) {
      throw std::logic_error(
          "grad_accumulator() should be only called on leaf Variables");
    }
    if (!autograd_meta->requires_grad_) {
      return nullptr;
    }

    std::lock_guard<std::mutex> lock(autograd_meta->mutex_);

    auto result = autograd_meta->grad_accumulator_.lock();
    if (result)
      return result;

    c10::raw::intrusive_ptr::incref(self.unsafeGetTensorImpl());
    auto intrusive_from_this = c10::intrusive_ptr<at::TensorImpl>::reclaim(self.unsafeGetTensorImpl());
    result = std::make_shared<AccumulateGrad>(Variable(std::move(intrusive_from_this)));
    autograd_meta->grad_accumulator_ = result;
    return result;
  }

  Edge gradient_edge(const Variable& self) {
    // If grad_fn is null (as is the case for a leaf node), we instead
    // interpret the gradient function to be a gradient accumulator, which will
    // accumulate its inputs into the grad property of the variable. These
    // nodes get suppressed in some situations, see "suppress gradient
    // accumulation" below. Note that only variables which have `requires_grad =
    // True` can have gradient accumulators.
    if (const auto& gradient = self.grad_fn()) {
      return Edge(gradient, self.output_nr());
    } else {
      return Edge(grad_accumulator(self), 0);
    }
  }

  void set_gradient_edge(const Variable& self, Edge edge) {
    auto* meta = materialize_autograd_meta(self);
    meta->grad_fn_ = std::move(edge.function);
    meta->output_nr_ = edge.input_nr;
    // For views, make sure this new grad_fn_ is not overwritten unless it is necessary
    // in the VariableHooks::grad_fn below.
    // This logic is only relevant for custom autograd Functions for which multiple
    // operations can happen on a given Tensor before its gradient edge is set when
    // exiting the custom Function.
    if (self.is_view()) {
      // NB: is_view() ==> get_autograd_meta()
      auto diff_view_meta = static_cast<torch::autograd::DifferentiableViewMeta*>(meta);
      diff_view_meta->attr_version = self._version();
    }
  }

  Node* grad_fn_unsafe(const Variable& self) {
    if (get_autograd_meta(self)) {
      return get_autograd_meta(self)->grad_fn_.get();
    } else {
      return nullptr;
    }
  }

  // Versions
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  void set_version_counter(
      const Variable& self,
      const c10::VariableVersion& version_counter) {
    TORCH_CHECK(self.defined(), "cannot call set_version_counter() on undefined tensor");
    self.unsafeGetTensorImpl()->set_version_counter(version_counter);
  }

  void bump_version(const Variable& self) {
    TORCH_CHECK(self.defined(), "cannot call bump_version() on undefined tensor");
    self.unsafeGetTensorImpl()->bump_version();
  }

  const c10::VariableVersion& version_counter(const Variable& self) {
    TORCH_CHECK(self.defined(), "cannot call version_counter() on undefined tensor");
    return self.unsafeGetTensorImpl()->version_counter();
  }

  // Hooks
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  void add_hook(const Variable& self, std::shared_ptr<FunctionPreHook> hook) {
    materialize_autograd_meta(self)->hooks_.push_back(std::move(hook));
  }

  namespace {
    std::vector<std::shared_ptr<FunctionPreHook>> empty_singleton;
  }

  // TODO: Return an ArrayRef instead (and delete the singleton while you're at
  // it
  const std::vector<std::shared_ptr<FunctionPreHook>>& hooks(const Variable& self)
      {
    if (get_autograd_meta(self)) {
      return get_autograd_meta(self)->hooks_;
    } else {
      return empty_singleton;
    }
  }

  void clear_hooks(const Variable& self) {
    // This is a little goofy, but usually this should be a no oop
    materialize_autograd_meta(self)->hooks_.clear();
  }

  void set_name(const Variable& self, const std::string& name) {
    materialize_autograd_meta(self)->name_ = name;
  }

  // Miscellaneous
  //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  void set_pyobj(const Variable& self, PyObject* pyobj) {
    TORCH_CHECK(self.defined(), "cannot call set_pyobj() on undefined tensor");
    self.unsafeGetTensorImpl()->set_pyobj(pyobj);
  }

  PyObject* pyobj(const Variable& self) {
    TORCH_CHECK(self.defined(), "cannot call pyobj() on undefined tensor");
    return self.unsafeGetTensorImpl()->pyobj();
  }

  AutogradMeta* get_autograd_meta(const Variable& self) {
    // NB: could return null
    TORCH_CHECK(self.defined(), "cannot call get_autograd_meta() on undefined tensor");
    return static_cast<AutogradMeta*>(self.unsafeGetTensorImpl()->autograd_meta());
  }

} // namespace impl

using at::Tensor;

struct VariableHooks final : at::impl::VariableHooksInterface {
  Tensor tensor_data(const Tensor&) const override;
  Tensor variable_data(const Tensor&) const override;
  const std::shared_ptr<torch::autograd::Node>& grad_fn(const Tensor&) const override;
  unsigned _register_hook(const Tensor&, std::function<Tensor(const Tensor&)> hook) const override;
  void remove_hook(const Tensor&, unsigned pos) const override;
  bool is_view(const Tensor&) const override;
  const Tensor& base(const Tensor&) const override;
  const std::string& name(const Tensor&) const override;
};

VariableHooks variableHooks;
at::impl::VariableHooksRegisterer registerVariableHooks(&variableHooks);

Tensor VariableHooks::variable_data(const Tensor& self) const {
  TORCH_CHECK(self.defined(), "cannot call variable_data() on undefined tensor");
  auto self_impl_copy = self.unsafeGetTensorImpl()->shallow_copy_and_detach(
    /*version_counter=*/0,
    /*allow_tensor_metadata_change=*/false);
  self_impl_copy->set_autograd_meta(nullptr);
  return at::Tensor(self_impl_copy);
}

Tensor VariableHooks::tensor_data(const Tensor& self) const {
  TORCH_CHECK(self.defined(), "cannot call tensor_data() on undefined tensor");
  auto self_impl_copy = self.unsafeGetTensorImpl()->shallow_copy_and_detach(
    /*version_counter=*/self.unsafeGetTensorImpl()->version_counter(),
    /*allow_tensor_metadata_change=*/self.unsafeGetTensorImpl()->allow_tensor_metadata_change());
  return at::Tensor(self_impl_copy);
}

// View Variables
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

bool VariableHooks::is_view(const Tensor& self) const {
  if (torch::autograd::impl::get_autograd_meta(self)) {
    return torch::autograd::impl::get_autograd_meta(self)->is_view_;
  } else {
    return false;
  }
}

const Tensor& VariableHooks::base(const Tensor& self) const {
  if (self.is_view()) {
    // is_view() implies get_autograd_meta()
    auto diff_view_meta = static_cast<torch::autograd::DifferentiableViewMeta*>(torch::autograd::impl::get_autograd_meta(self));
    return diff_view_meta->base_;
  } else {
    throw std::runtime_error("Can't get base of non-view Variable");
  }
}

namespace {
  std::string singleton_string;
}

const std::string& VariableHooks::name(const Tensor& self) const {
  TORCH_CHECK(self.defined(), "cannot call variable_data() on undefined tensor");
  if (torch::autograd::impl::get_autograd_meta(self)) {
    return torch::autograd::impl::get_autograd_meta(self)->name_;
  } else {
    return singleton_string;
  }
}

namespace {
  std::shared_ptr<torch::autograd::Node> singleton_shared_ptr;
}

const std::shared_ptr<torch::autograd::Node>& VariableHooks::grad_fn(const Tensor& self) const {
  if (self.is_view()) {
    // NB: is_view() ==> get_autograd_meta()
    auto diff_view_meta = static_cast<torch::autograd::DifferentiableViewMeta*>(torch::autograd::impl::get_autograd_meta(self));

    // See NOTE [ View + Inplace detection ]
    if (diff_view_meta->creation_meta != CreationMeta::MULTI_OUTPUT_SAFE) {
      std::lock_guard<std::mutex> lock(diff_view_meta->mutex_);
      if (!diff_view_meta->grad_fn_ && !diff_view_meta->base_.requires_grad()) {
        return diff_view_meta->grad_fn_;
      }
      auto current_version = self._version();
      if (diff_view_meta->attr_version != current_version) {
        // This is an indirect rebase_history due to another view or the base being modified inplace
        handle_view_on_rebase(diff_view_meta, /* indirect */ true);
        TORCH_INTERNAL_ASSERT(diff_view_meta->output_nr_ == 0);
        // Note [View + Inplace update for view tensor]
        // An inplace update happened on Tensor `self` (which is a view).
        // For example:
        //   view_1 = view_op_1(diff_view_meta->base_)
        //   view_2 = view_op_2(view_1)
        //   ...
        //   self = view_op_n(view_n-1)
        //   self = inplace_op(self)
        //
        // For CPU/CUDA backends, we employ one AsStridedBackward Node to represent the chain of
        // view backward ops for effienciency.
        //
        // However in XLA backend we don't have full support of AsStridedBackward, we instead run a full
        // forward pass with a tensor that requires gradient to get proper grad_fn setup,
        // then save it to DifferentiableViewMeta for future use.
        // This is fairly cheap for XLA lazy tensor approach (but would be really expensive for CPU/CUDA).
        // XLA Tensor only run thorugh VariableType dispatch and lower the forward pass to a XLA HLO graph,
        // then we take grad_fn and never materialize the tensor content.
        // So we only construct the graph but not execute it, which is a fairly cheap operation to do.
        //
        // See Note [View + Inplace update for base tensor] for what we do to base tensor when
        // an in-place operation happens.
        //
        // TODO: Potentially the following logic can be replaced by special logic in VariableType_x.cpp
        //       that would provide a way to recreate the grad_fn chain.
        if (diff_view_meta->has_view_fn()) {
          auto view_fn = diff_view_meta->view_fn();
          auto diff_view = view_fn(diff_view_meta->base_);
          diff_view_meta->grad_fn_ = diff_view.grad_fn();
        } else {
          auto fn = std::make_shared<torch::autograd::generated::AsStridedBackward>();
          fn->self_geometry = at::TensorGeometry(diff_view_meta->base_);
          fn->size = self.sizes().vec();
          fn->stride = self.strides().vec();
          fn->storage_offset = self.storage_offset();
          fn->set_next_edges(torch::autograd::collect_next_edges(diff_view_meta->base_));
          fn->add_input_metadata(
            diff_view_meta->base_.options(),
            self.sizes(), // Note: sizes(), not base_.sizes(), is intentional
            diff_view_meta->base_.device());
          diff_view_meta->grad_fn_ = std::move(fn);
        }
        diff_view_meta->attr_version = current_version;
      }
      return diff_view_meta->grad_fn_;
    }
  }
  
  if (torch::autograd::impl::get_autograd_meta(self)) {
    return torch::autograd::impl::get_autograd_meta(self)->grad_fn_;
  } else {
    return singleton_shared_ptr;
  }
}

void VariableHooks::remove_hook(const Tensor& self, unsigned pos) const {
  auto &list = torch::autograd::impl::materialize_autograd_meta(self)->cpp_hooks_list;
  TORCH_CHECK(list && pos < list->size() , "Invalid index, no hook at position ", pos);
  // Hook will be ignored
  (*list)[pos] = nullptr;
}

unsigned VariableHooks::_register_hook(const Tensor& self, std::function<Tensor(const Tensor&)> hook) const {
  TORCH_CHECK(self.requires_grad(), "cannot register a hook on a variable that "
                           "doesn't require gradient");
  // NB: materialize_autograd_meta unnecessary due to requires grad check
  auto &list = torch::autograd::impl::get_autograd_meta(self)->cpp_hooks_list;
  if(!list) {
    torch::autograd::impl::create_cpp_hook(self);
  }
  unsigned idx = list->size();
  list->push_back(hook);
  return idx;
}

void handle_view_on_rebase(DifferentiableViewMeta* diff_view_meta, bool indirect) {
  /// See NOTE [ View + Inplace detection ] for justification of the logic below
  if (diff_view_meta->creation_meta != CreationMeta::DEFAULT) {
    auto grad_fn = diff_view_meta->grad_fn_.get();
    std::string msg;
    std::string modified_obj;
    // Create the header for the error message.
    if (indirect) {
      modified_obj = "its base or another view of its base has been";
    } else {
      modified_obj = "is being";
    }
    if (grad_fn) {
      msg = c10::str("Output ", diff_view_meta->output_nr_, " of ", grad_fn->name(), " is a view and ",
                     modified_obj, " modified inplace.");
    } else {
      msg = c10::str("A view was created in no_grad mode and ", modified_obj, " modified inplace with grad mode enabled.");
    }

    if (diff_view_meta->creation_meta == CreationMeta::MULTI_OUTPUT_NODE) {
      TORCH_CHECK(false, msg, " This view is the output of a function that returns multiple views. Such functions do not"
                         " allow the output views to be modified inplace. You should replace the inplace operation by an"
                         " out-of-place one.");
    } else {
      if (diff_view_meta->creation_meta == CreationMeta::NO_GRAD_MODE) {
        TORCH_INTERNAL_ASSERT(!grad_fn);
        msg = c10::str(msg, " Given that this use case is ambiguous and error-prone, it is deprecated and will be forbidden"
                       "  starting 1.6 (see https://github.com/pytorch/pytorch/pull/32839 for more details about this). You"
                       " can clarify your code and remove this warning by moving both the view and the inplace either both"
                       " inside the no_grad block (if you don't want the inplace to be tracked) or both outside (if you want"
                       " the inplace to be tracked).");
      } else if (diff_view_meta->creation_meta == CreationMeta::IN_CUSTOM_FUNCTION) {
        msg = c10::str(msg, " This view was created inside a custom Function (or because an input was returned as-is) and the"
                       " autograd logic to handle view+inplace would override the custom backward associated with the custom"
                       " Function, leading to incorrect gradients. This behavior is deprecated and will be forbidden starting"
                       " version 1.6. You can remove this warning by cloning the output of the custom Function.");
      } else if (diff_view_meta->creation_meta == CreationMeta::MULTI_OUTPUT_SAFE) {
        msg = c10::str(msg, " This view is an output of a function that "
                       "returns multiple views. Inplace operators on such "
                       "views are being deprecated and will be forbidden "
                       "starting from version 1.8. Consider using `unsafe_` "
                       "version of the function that produced this view or "
                       "don't modify this view inplace.");
      } else {
        TORCH_INTERNAL_ASSERT(false, "Invalid CreationMeta state");
      }

      if (!indirect && !grad_fn) {
        // This view is (wrongly) detected as a leaf that requires grad and would raise the surprising: "a leaf Variable that
        // requires grad is being used in an in-place operation." after the warning. So we make the warning an error directly.
        TORCH_CHECK(false, msg);
      } else {
        TORCH_WARN(msg);
      }
    }

    // We warn only once per view
    // Note that if a Tensor is modified inplace from two threads at the same time, this is not thread safe and can warn
    // multiple time. This is ok as it should be a rare event.
    diff_view_meta->creation_meta = CreationMeta::DEFAULT;
  }
}

}} // namespace torch::autograd