forward_grad.cpp

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

namespace torch::autograd {

namespace {
// See discussion in forward_grad.h for why these are global variables and not
// thread local

std::mutex all_forward_levels_mutex_;
std::vector<std::shared_ptr<ForwardADLevel>> all_forward_levels_;

const static at::Tensor singleton_undefined_tensor;
} // namespace

uint64_t ForwardADLevel::get_next_idx() {
  std::lock_guard<std::mutex> lock(all_forward_levels_mutex_);
  auto next_idx = all_forward_levels_.size();
  TORCH_CHECK(
      next_idx == 0, "Nested forward mode AD is not supported at the moment");
  all_forward_levels_.push_back(std::make_shared<ForwardADLevel>(next_idx));
  return next_idx;
}

void ForwardADLevel::release_idx(uint64_t idx) {
  std::unique_lock<std::mutex> lock(all_forward_levels_mutex_);
  TORCH_CHECK(
      idx + 1 == all_forward_levels_.size(),
      "Exiting a forward AD level that is not the "
      "last that was created is not support. Ensure they are released in the reverse "
      "order they were created.");
  TORCH_INTERNAL_ASSERT(!all_forward_levels_.empty());
  // Keep the level alive until we have released the lock
  auto lvl = std::move(all_forward_levels_.back());
  all_forward_levels_.pop_back();
  lock.unlock();
}

std::shared_ptr<ForwardADLevel> ForwardADLevel::get_by_idx(uint64_t idx) {
  std::lock_guard<std::mutex> lock(all_forward_levels_mutex_);
  TORCH_CHECK(
      idx < all_forward_levels_.size(),
      "Trying to access a forward AD level with an invalid index. "
      "This index was either not created or is already deleted.");
  return all_forward_levels_[idx];
}

std::shared_ptr<ForwardADLevel> ForwardADLevel::try_get_by_idx(uint64_t idx) {
  std::lock_guard<std::mutex> lock(all_forward_levels_mutex_);
  if (idx < all_forward_levels_.size()) {
    return all_forward_levels_[idx];
  } else {
    return nullptr;
  }
}

ForwardADLevel::~ForwardADLevel() {
  std::lock_guard<std::mutex> lock(mutex_);
  auto it = grads_.begin();
  while (it != grads_.end()) {
    // Warning this will lock *it mutex
    // This is ok as this function is the *only* one to call back into another
    // class's method.
    (*it)->reset(idx_, /* update_level */ false);
    it = grads_.erase(it);
  }
}

const at::Tensor& ForwardGrad::value(uint64_t level) const {
  std::lock_guard<std::mutex> lock(mutex_);
  const auto& it = content_.find(level);
  return it == content_.end() ? singleton_undefined_tensor : (*it).second;
}

const at::Tensor& ForwardGrad::undef_grad() {
  return singleton_undefined_tensor;
}

} // namespace torch::autograd