pytorch/torch/csrc/jit/interpreter.cpp

#include "interpreter.h"

#include "torch/csrc/autograd/edge.h"
#include "torch/csrc/autograd/function.h"
#include "torch/csrc/autograd/profiler.h"
#include "torch/csrc/autograd/variable.h"
#include "torch/csrc/jit/fusion_compiler.h"
#include "torch/csrc/jit/operator.h"
#include "torch/csrc/jit/graph_executor.h"
#include "torch/csrc/jit/ir.h"

#include "torch/csrc/jit/ivalue.h"
#include "torch/csrc/jit/constants.h"
#include "torch/csrc/variable_tensor_functions.h"
#include "torch/csrc/autograd/generated/variable_factories.h"

#include <exception>
#include <iostream>
#include <memory>
#include <mutex>
#include <ostream>
#include <stdexcept>
#include <typeinfo>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

namespace torch { namespace jit {

// Before we translate to intepreter instructions, we do
// some preprocessing of the graph to turn it into a form that is closer
// to what the instructions will look like.
// In particular we:
// * (TODO) desugar Loop trip counts into c = 0, c += 1 instructions in the loop
// * flatten stages so that each stage starts with a load from the stack
//   and ends with a store to the stack
// *. computes move_flags (see Outputs), and inserts
// *  Drop nodes are inserted for any node that is unused to create a dummy use
//    that will cause the interpreter to free the node.
//    A drop node is just a node with no outputs that just pops its inputs off the stack,
//    to ensure the interpreter release references to nodes that are never used.
//    Drop nodes are also inserted when the last use of a node is in some conditionally
//    run control flow (e.g. one side of an If) and the interpreter must free
//    the node only after the control flow has reconverged
// Outputs are:
// * graph - the post processed copy of g
// * move_flags[n] - a list of booleans, one for each input,
//   indicating whether this is the last use of the value. The interpreter
//   should generate a move rather than a copy in this case.
// * stage_input_types: the type annotations on the inputs to each stage
//   these can be removed once the the backward tracer is no longer used

namespace {

// new_cond = (i < max_trip_count) && cond
Value* createTripCountConjunctiveCondition(
    Graph* g,
    Value* cur_trip_count,
    Value* max_trip_count,
    Value* cond) {
  // Emit initial comparison -- initial_trip_count < max_trip_count
  Value* initial_comparison_value =
      g->insertNode(g->create(aten::lt, {cur_trip_count, max_trip_count}, 1))
          ->output()->setType(IntType::get());

  // Replace initial condition with logical `and` of trip count and
  // initial condition
  Value* new_cond =
      g->insertNode(
           g->create(aten::__and__, {initial_comparison_value, cond}, 1))
          ->output()->setType(IntType::get());
  return new_cond;
}

} // namespace

// this currently just _removes_ the trip count inputs and checks they are
// unused. In the future they will be desugared into normal arithmetic to
// provide a loop counter
void desugarTripCounts(Block * b) {
  for(auto n : b->nodes()) {

    if(n->kind() == prim::Loop) {
      auto g = n->owningGraph();
      auto body_block = n->blocks()[0];

      Value* block_trip_count_input = body_block->inputs()[0];
      // Treat loop iteration number as a loop-carried dependency. We emit an
      // increment at the end of the body block.
      n->insertOutput(0);

      Value* max_trip_count_value = n->input(0);
      {
        WithInsertPoint guard(n);
        // int i = 0
        Value* initial_trip_count = insertConstant(*g, 0);
        // Set up initial iteration number value for loop-carried dependency
        n->removeInput(0);
        // Input 0 is now initial termination condition, insert this after that.
        // LCD's start at index 1.
        n->insertInput(1, initial_trip_count);

        Value* new_cond = createTripCountConjunctiveCondition(
            g, initial_trip_count, max_trip_count_value, n->input(0));
        n->replaceInput(0, new_cond);
      }

      {
        WithInsertPoint guard(body_block);
        // Trip count is now a loop carried dependency. We emit an op to
        // increment the trip count at the end of the body. Then, emit the same
        // conjunctive stopping condition as above.

        Value* const_one = insertConstant(*g, 1);

        Value* inc_trip_count =
            g->insertNode(g->create(
                    aten::add, {block_trip_count_input, const_one}, 1))
             ->output()->setType(IntType::get());
        body_block->insertOutput(1, inc_trip_count);

        Value* body_cond = createTripCountConjunctiveCondition(
            g, inc_trip_count, max_trip_count_value, body_block->outputs()[0]);
        body_block->eraseOutput(0);
        body_block->insertOutput(0, body_cond);
      }
    }
    for(auto sb : n->blocks()) {
      desugarTripCounts(sb);
    }
  }
}

// removes all inputs and outputs to a graph, replacing them with nodes before of after each insertStage
static std::vector<std::vector<TypePtr>> flattenStages(Graph & graph) {
  // because JIT classic needs this to fix up gradients, remove when possible
  std::vector<std::vector<TypePtr>> stage_input_types;

  WithInsertPoint guard(*graph.nodes().begin());
  size_t input_pos = 0;
  size_t output_pos = 0;
  auto it = graph.nodes().begin();
  for(size_t i = 0; i <= graph.stage(); i++) {
    stage_input_types.emplace_back();
    auto store = graph.create(prim::Store, 0)->insertBefore(*it);
    while(input_pos < graph.inputs().size() && graph.inputs()[input_pos]->stage() == i) {
      auto nv = store->addOutput();
      auto old_node = graph.inputs()[input_pos];
      stage_input_types[i].push_back(old_node->type());
      old_node->replaceAllUsesWith(nv);
      input_pos++;
    }
    while(it != graph.nodes().end() && it->stage() == i)
      ++it;
    auto load = graph.create(prim::Load, 0)->insertBefore(*it);
    while(output_pos < graph.outputs().size() && graph.outputs()[output_pos]->stage() == i) {
      load->addInput(graph.outputs()[output_pos]);
      output_pos++;
    }
  }
  while (graph.inputs().size() > 0)
    graph.eraseInput(graph.inputs().size() - 1);
  while (graph.outputs().size() > 0)
    graph.eraseOutput(graph.outputs().size() - 1);

  return stage_input_types;
}


// insert Drop nodes to kill references for anything unused:
// this can happen in a few places, e.g. when a node returns
// many values but only one is used
// a, b = foo()
// return a
void dropUnused(Block *b) {
  auto createDropIfUnused = [&](ArrayRef<Value*> values) -> Node* {
    std::vector<Value*> to_drop;
    for(auto v : values) {
      if(v->uses().size() == 0)
        to_drop.push_back(v);
    }
    if(to_drop.size() == 0)
      return nullptr;
    return b->owningGraph()->create(prim::Drop, to_drop, 0);
  };

  if(auto d = createDropIfUnused(b->inputs())) {
    b->prependNode(d);
  }
  for(auto n : b->nodes()) {
    if(auto d = createDropIfUnused(n->outputs())) {
      d->insertAfter(n);
    }
    for(auto b : n->blocks())
      dropUnused(b);
  }
}


// for each input, should we move rather than copy the inputs
std::unordered_map<Node*, std::vector<uint8_t>> findLastUses(Graph & g) {
  // struct to share common data structures
  struct FindLastUses {
    Graph & graph;
    // have we seen this value, yet, if not, it is the last use of the value
    std::unordered_set<Value*> seen;

    std::unordered_map<Node*, std::vector<uint8_t>> move_flags;
    // A map from an If or Loop node to the optional Drop block that
    // occurs directly after it to release any tensors that go out of scope
    // when the If/Loop exits. These are created and inserted on demand.
    std::unordered_map<Node*, Node*> drop_for_node;

    FindLastUses(Graph & g)
    : graph(g) {
      scanBlock(graph.block());
    }
    void scanBlock(Block * b) {
      scanNode(b->return_node());
      for(auto n : b->nodes().reverse()) {
        scanNode(n);
      }
    }
    void scanNode(Node * n) {
      for(auto b : n->blocks()) {
        scanBlock(b);
      }
      move_flags[n].resize(n->inputs().size());
      // scan backwards so if a value is used twice in the list then it is a move
      for(size_t i = n->inputs().size(); i > 0; --i) {
        scanUse(n, i-1);
      }
    }
    void scanUse(Node * n, size_t i) {
      auto & move_flags_n = move_flags[n];
      auto v = n->inputs()[i];
      auto inserted = seen.insert(v).second;
      if(!inserted) {
        move_flags_n[i] = false;
        return;
      }

      // the last use of v may be in a nested block of an If or Loop statement
      // find the node 'same_depth_node' at the same depth as the definition of v,
      // and consider that node to be the last use of v.
      // This ensures we do not delete nodes in nested scopes
      // that may be executed multiple times
      // and that nodes used on one side of an if
      // but not the other get deleted regardless of the branch
      // e.g.
      // a = 4
      // while <...>:
      //   y = a + a
      // drop(a)
      // In other words, we find the first program point for v that
      // _reverse_ dominates the definition of v, and add a drop point there.
      Node * same_depth_node = findOwnerInBlock(n, v->node()->owningBlock());
      JIT_ASSERT(same_depth_node); // failure means v is not in scope for n, use lint!

      // In the case where v and n are in the same block, just mark
      // its move_flags to be true
      if(same_depth_node == n) {
        move_flags_n[i] = true;
        return;
      }

      // in the case where the use is nested in a block
      // add a Drop node after that block which will drop 'v'.
      move_flags_n[i] = false;
      addToDropIfNotExists(findOrCreateDropInstructionForNode(same_depth_node), v);
    }

    // finds the node in block 'block' that contains in 'n'
    // or nullptr if no such node exists, e.g.:
    // n0: a = 4
    // n1: if <cond>:
    // n2:    b = a + a
    // findOwnerInBlock(n2, n0.block()) == n1
    Node * findOwnerInBlock(Node * n, Block * block) {
      while(n != nullptr && block != n->owningBlock()) {
        n = n->owningBlock()->owningNode();
      }
      return n;
    }

    Node * findOrCreateDropInstructionForNode(Node * n) {
      auto it = drop_for_node.find(n);
      if(it == drop_for_node.end()) {
        auto drop_node = graph.create(prim::Drop, 0);
        drop_node->insertAfter(n);
        it = drop_for_node.emplace(n, drop_node).first;
      }
      return it->second;
    }

    void addToDropIfNotExists(Node * drop, Value * v) {
      for(auto i : drop->inputs()) {
        // we already accounted for this use
        if(i == v)
          return;
      }
      drop->addInput(v);
      move_flags[drop].push_back(true);
    }
  };

  return FindLastUses(g).move_flags;
}

// pre-processing that happens once per graph
struct PreprocessGraph {
  PreprocessGraph(Graph & g)
  : graph(g.copy()) {
    desugarTripCounts(graph->block());
    stage_input_types = flattenStages(*graph);
    dropUnused(graph->block());
    // fill in move_flags by scanning blocks;
    move_flags = findLastUses(*graph);
    //TODO: desugar Loop trip counts, for now we drop trip counts
  }
  // Outputs of the preprocessing:
  std::shared_ptr<Graph> graph;
  // for each input, should we move rather than copy the inputs
  std::unordered_map<Node*, std::vector<uint8_t>> move_flags;
  std::vector<std::vector<TypePtr>> stage_input_types;

};

// previously the interpreter worked with at::Retainable values,
// which are annoying to handle since 99% of values are at::Tensor anyway
// instead we create a fake subclass of TensorImpl that can be subclassed
// to hold arbitrary things
// Note: this is currently unused but will probably be useful in the future,
// so we keep it around
struct ContainerTensor : public at::TensorImpl {
public:
  ContainerTensor()
  : TensorImpl(&(at::globalContext().getType(at::Backend::Undefined,at::ScalarType::Undefined))) {}

  virtual ~ContainerTensor() {}
  virtual const char * toString() const override {
    throw std::runtime_error("toString() on ContainerTensor");
  }
  virtual at::IntList sizes() const override {
    throw std::runtime_error("sizes() on ContainerTensor");
  }
  virtual at::IntList strides() const override {
    throw std::runtime_error("strides() on ContainerTensor");
  }
  virtual int64_t dim() const override {
    throw std::runtime_error("dim() on ContainerTensor");
  }
  virtual at::Scalar localScalar() override {
    throw std::runtime_error("localScalar() on ContainerTensor");
  }
  virtual void * unsafeGetTH(bool retain) override {
    throw std::runtime_error("unsafeGetTH() on ContainerTensor");
  }
  virtual std::unique_ptr<at::Storage> storage() override {
    throw std::runtime_error("storage() on ContainerTensor");
  }
};

// We need some lists for inputs and outputs. To keep all the memory
// contiguous we allocate a single vector and use offsets into the vector
// which are stored in the ListHandle struct
// start is an offset into int_data of Code for ListHandle<int>
// and bool_data of Code for ListHandle<bool>
template<typename T>
struct ListHandle {
  int start;
  int size;
};

struct UseList {
  // values to be used
  ListHandle<int> values;
  // boolean flags indicating whether to free the Tensor after this use
  ListHandle<bool> free_flags;
};

// one instruction plus meta-data
struct Instruction {
  Operation callback;
  UseList inputs;
  ListHandle<int> outputs;
  Symbol debug_name; // used in dump to understand the generated code
  std::shared_ptr<SourceLocation> debug_location; // for error reporting
};


int relativeJump(int from_inst, int to_inst) {
  return to_inst - (from_inst + 1);
}

struct CodeImpl {
  CodeImpl(std::shared_ptr<Graph>& graph_)
      : preprocess(*graph_) {
    graph = preprocess.graph;
    // std::cout << "into code graph:\n" << *graph << "\n";
    insertNodesFromBlock(graph->block());
  }

  // jump when input is 0
  void createJumpZ(int from_inst, int to_inst) {
    auto & inst = instructions[from_inst];
    JIT_ASSERT(inst.debug_name == prim::Placeholder);
    auto offset = relativeJump(from_inst, to_inst);
    inst.callback = [offset](Stack & stack) {
      auto t = pop(stack).toInt();
      return (t == 0) ? offset : 0;
    };
    inst.debug_name = prim::JumpZ;
  }

  // jump when input is not 0
  void createJumpNZ(int from_inst, int to_inst) {
    auto & inst = instructions[from_inst];
    JIT_ASSERT(inst.debug_name == prim::Placeholder);
    auto offset = relativeJump(from_inst, to_inst);
    inst.callback = [offset](Stack & stack) {
      auto t = pop(stack).toInt();
      return (t != 0) ? offset : 0;
    };
    inst.debug_name = prim::JumpNZ;
  }

  void createJump(int from_inst, int to_inst) {
    auto & inst = instructions[from_inst];
    JIT_ASSERT(inst.debug_name == prim::Placeholder);
    auto offset = relativeJump(from_inst, to_inst);
    inst.callback = [=](Stack & stack) {
      return offset;
    };
    inst.debug_name = prim::Jump;
  }

  void insertNodesFromBlock(Block* block) {
    for(auto node : block->nodes()) {
      const auto & source_location = node->getSourceLocation();
      switch(node->kind()) {
        case prim::If: {
          // x = if c:
          //   <then_block>
          //   -> (vt)
          // else:
          //    <else_block>
          //   -> (vf)

          // turns into:
          //   JumpNZ c, then
          //   <else_block>
          //   x = vf
          //   Jump end
          // then:
          //   <then_block>
          //   x = vt
          // end:

          // prim::Placeholder instructions are replaced with branch instructions
          // when the branch target locations are known
          auto cond_branch = insertInstruction(prim::Placeholder, source_location, node->inputs(), moveFlags(node), {});
          auto then_block = node->blocks()[0];
          auto else_block = node->blocks()[1];
          insertNodesFromBlock(else_block);
          insertAssign(source_location,else_block->outputs(), moveFlags(else_block), node->outputs());
          auto jump = insertInstruction(prim::Placeholder, source_location, {}, {}, {});
          auto then_block_start = instructions.size();
          insertNodesFromBlock(then_block);
          insertAssign(source_location, then_block->outputs(), moveFlags(then_block), node->outputs());
          createJump(jump, instructions.size());
          createJumpNZ(cond_branch, then_block_start);
        } break;
        case prim::Loop: {
          // o0 = while c i0
          //        block 0: l0
          //          <body>
          //          -> (v0, v1)

          // turns into:
          // l0 = i0
          // JumpZ c, end
          // begin:
          //   <body>
          //   c, l0 = v0, v1
          //   JumpNZ c, begin
          // end:

          auto body_block = node->blocks()[0];

          // before assign op: stack: ... <cond> <loop-carried-depdencies>
          insertAssign(source_location, node->inputs(), moveFlags(node), body_block->inputs());
          // after assign op: stack: ... <cond>
          // cond_branch consumes <cond> from top of the stack
          auto cond_branch = insertInstruction(prim::Placeholder, source_location,{}, {}, {});
          // after branch: stack: ...

          auto entry = instructions.size();
          insertNodesFromBlock(body_block);
          // before assign op: stack: ... <cond> <loop-carried-depdencies>
          insertAssign(source_location, body_block->outputs(), moveFlags(body_block), body_block->inputs());
          // after assign op: stack: ... <cond>
          auto cond_branch_end = insertInstruction(prim::Placeholder, source_location, {}, {}, {});
          // after branch: stack: ...

          aliasRegistersTo(node->outputs(), body_block->inputs());
          createJumpZ(cond_branch, instructions.size());
          createJumpNZ(cond_branch_end, entry);
        } break;
        default: {
          insertInstruction(node);
        } break;
      }
      // each stage ends with a load instruction
      // we record where these instructions occur, and use them to
      // exit the interpreter
      if(node->kind() == prim::Load) {
        stage_end.push_back(instructions.size());
      }
    }
  }

  size_t insertInstruction(Node * n) {
    auto inst = insertInstruction(n->kind(), n->getSourceLocation(), n->inputs(), moveFlags(n) , n->outputs());
    instructions[inst].callback = getInterpreterOperation(n);
    return inst;
  }
  size_t insertInstruction(Symbol sym,
                           std::shared_ptr<SourceLocation> debug_location,
                                 ArrayRef<Value*> inputs,
                                 ArrayRef<uint8_t> move_flags,
                                 ArrayRef<Value*> outputs) {
    instructions.emplace_back();
    auto & inst = instructions.back();
    inst.debug_name = sym;
    inst.debug_location = std::move(debug_location);
    listBegin(inst.inputs.values);
    for(auto input : inputs) {
      listInsert(inst.inputs.values, getOrAllocateRegister(input, true));
    }
    listBegin(inst.inputs.free_flags);
    for(auto flag : move_flags) {
      listInsert(inst.inputs.free_flags, flag);
    }
    listBegin(inst.outputs);
    for(auto output : outputs) {
      listInsert(inst.outputs, getOrAllocateRegister(output));
    }
    return instructions.size() - 1;
  }
  ArrayRef<uint8_t> moveFlags(Node * n) {
    return preprocess.move_flags.at(n);
  }
  ArrayRef<uint8_t> moveFlags(Block *b) {
    return moveFlags(b->return_node());
  }

  size_t insertAssign(std::shared_ptr<SourceLocation> debug_location, ArrayRef<Value*> inputs, ArrayRef<uint8_t> move_flags, ArrayRef<Value*> outputs) {
    auto inst = insertInstruction(prim::Assign, std::move(debug_location),inputs, move_flags, outputs);
    // This node effectively forwards its inputs into different places in a register list.
    // We don't need to manipulate the stack in any way, because all inputs are also outputs,
    // and the interpreter will take care of putting them in correct places.
    instructions[inst].callback = [](Stack& stack) { return 0; };
    return inst;
  }

  // helpers to build/access RegList objects
  int get(const ListHandle<int> & list, int i)  const {
    return int_data[list.start + i];
  }
  bool get(const ListHandle<bool> & list, int i) const {
    return bool_data[list.start + i];
  }
  void listBegin(ListHandle<int> & list) {
    list.start = int_data.size();
    list.size = 0;
  }
  void listInsert(ListHandle<int> & list, int value) {
    JIT_ASSERTM(list.start + list.size == (int)int_data.size(), "another list already started");
    int_data.push_back(value);
    list.size++;
  }
  void listBegin(ListHandle<bool> & list) {
    list.start = bool_data.size();
    list.size = 0;
  }
  void listInsert(ListHandle<bool> & list, int value) {
    JIT_ASSERTM(list.start + list.size == (int)bool_data.size(), "another list already started");
    bool_data.push_back(value);
    list.size++;
  }
  // must be called before any new_allocations are used, otherwise they will
  // already have registers assigned
  void aliasRegistersTo(ArrayRef<Value*> new_allocations, ArrayRef<Value*> existing_allocations) {
    JIT_ASSERT(new_allocations.size() == existing_allocations.size());
    for(size_t i = 0; i < new_allocations.size(); ++i) {
      auto n = new_allocations[i]->unique();
      auto e = existing_allocations[i]->unique();
      JIT_ASSERT(unique_to_reg.count(e) > 0 && unique_to_reg.count(n) == 0);
      unique_to_reg[n] = unique_to_reg[e];
    }
  }
  int getOrAllocateRegister(Value * n, bool required = false) {
    size_t u = n->unique();
    if(unique_to_reg.count(u) > 0)
      return unique_to_reg[u];
    JIT_ASSERT(!required);
    int r = register_size++;
    unique_to_reg[u] = r;
    return r;
  }

  // Returns a function implementing functionality of a given node,
  // or nullptr if it's a no-op for autograd.
  Operation getInterpreterOperation(jit::Node* node) {
    if(node->kind() != prim::GraphExecutor) {
      return getOperation(node);
    }
    // recursive graph executors cannot be Operators because they
    // have to register themselves with the interpreter so that
    // we can provide useful debugging information

    auto executor = std::make_shared<GraphExecutor>(node->g(attr::Subgraph));
    graph_executors.emplace_back(executor.get());
    auto num_inputs = node->inputs().size();
    return [=](Stack& stack) mutable {
      autograd::profiler::RecordFunction record("GraphExecutor");
      auto inputs = last(stack, num_inputs);
      variable_tensor_list tinputs(
          fmap(inputs, [](const IValue& v) { return v.toTensor(); }));
      drop(stack, num_inputs);
      //TODO: has graph executor work from a stack as well
      variable_tensor_list toutputs = executor->run(variable_tensor_list(std::move(tinputs)));
      stack.insert(stack.end(), toutputs.begin(), toutputs.end());
      return 0;
    };
  }

  const std::vector<GraphExecutor*>& executors() {
    return graph_executors;
  }

  void dumpInstruction(std::ostream & out, size_t pc) const {
    auto writeList = [&](const ListHandle<int> & list) {
      for(int i = 0; i < list.size; i++) {
        if(i > 0)
          out << ", ";
        out << get(list, i);
      }
    };
    auto writeUseList = [&](const UseList & list) {
      for(int i = 0; i < list.values.size; i++) {
        if(i > 0)
          out << ", ";
        if(get(list.free_flags, i))
          out << "move(" << get(list.values, i) << ")";
        else
          out << get(list.values, i);
      }
    };
    auto & inst = instructions.at(pc);
    writeList(inst.outputs);
    // NB: debug names are the kind of operator used to select
    // dispatch
    out << " = " << inst.debug_name.toUnqualString() << " ";
    writeUseList(inst.inputs);
  }
  void dump(std::ostream & out) const {
    for(size_t i = 0; i < instructions.size(); ++i) {
      dumpInstruction(out, i);
      out << "\n";
    }
  }

  // We MUST hold onto graph here because some Operators stored in the
  // instruction lists have dependencies on meta-data stored in the graph
  // that would be dead otherwise.
  // It is also very useful for debugging interpreter problems to
  // keep this around.
  std::shared_ptr<Graph> graph;
  std::vector<GraphExecutor*> graph_executors; // for debugging
  PreprocessGraph preprocess;

  std::unordered_map<size_t, int> unique_to_reg; // map from unique of nodes to register in register table

  friend struct InterpreterState;
  std::vector<Instruction> instructions;
  std::vector<size_t> stage_end; // each stage runs while(pc < stage_end[stage])
  int register_size = 0;

  // all memory ArrayRef<int> are slices of this, to make sure
  // the interpreter is mostly linearly scanning through memory
  std::vector<int> int_data;
  std::vector<bool> bool_data;
};

// InterpreterState state that is held across stages and used to compute a Code
struct InterpreterStateImpl {
  InterpreterStateImpl(const Code & function_)
  : function(function_.pImpl),
    int_data(function->int_data.data()),
    bool_data(function->bool_data),
    registers(function->register_size) {
  }
  void runOneStage(Stack & stack) {
    // std::cout << "running stage: " << current_stage << " of " << function->stage_end.size() << "\n";
    // std::cout << *function->graph << "\n";
    // function->dump(std::cout);
    size_t pc = current_pc;
    size_t last = function->stage_end[current_stage];
    auto & instructions = function->instructions;
    while(pc < last) {
        // std::cout << "executing " << pc << ": ";
        // function->dumpInstruction(std::cout, pc);
        // std::cout << "\n";
        try {
          auto & inst = instructions[pc];
          loadTensorsFromRegisters(inst.inputs, stack);
          size_t new_pc = pc + 1 + inst.callback(stack);
          for(int i = inst.outputs.size - 1; i >= 0; i--) {
            int reg = get(inst.outputs,i);
            registers[reg] = pop(stack);
            // std::cout << "pop reg[" << reg << "];\n" << registers[reg].pImpl << "\n";
          }
          pc = new_pc;
        } catch(std::exception & e) {
          if(!instructions[pc].debug_location)
            throw; // rethrow original exception
          // throw a new exception with enhanced debugging information
          instructions[pc].debug_location->wrapAndRethrowException(e, "operation failed in interpreter");
        }
    }
    current_pc = pc;
    current_stage++;
  }
  const TensorType & tensorTypeForInput(size_t i) const {
    return *function->preprocess.stage_input_types.at(current_stage).at(i)->expect<TensorType>();
  }
  int get(const ListHandle<int> & list, int i) {
    return int_data[list.start + i];
  };
  bool get(const ListHandle<bool> & list, int i) {
    return bool_data[list.start + i];
  }
  void loadTensorsFromRegisters(const UseList & uses, Stack & stack) {
    for(int i = 0; i < uses.values.size; i++) {
      int reg = get(uses.values,i);
      // std::cout << "push reg[" << reg << "];\n" << registers[reg] << "\n\n";
      if(get(uses.free_flags,i)) {
        stack.push_back(std::move(registers[reg]));
      } else {
        stack.push_back(registers[reg]);
      }

    }
  }
  size_t current_stage = 0;
  size_t current_pc = 0;
  std::shared_ptr<CodeImpl> function; // keep function alive
  // these are just copies of function to prevent indirections in interpreter
  int * int_data;
  const std::vector<bool> & bool_data;


  // this holds all the tensors for this interpreter run
  // we don't bother minimizing the size of this vector, since the extra
  // memory used by the pointers in this will be small
  // instead we are very aggresive about releasing tensors when they become dead
  // to make sure memory management happens efficiently.

  // We optimize for the case where derivatives are run with retain_graph=False
  // in the case where it is true, then the interpreter and this array get copied
  // if this every becomes a bottleneck then we _should_ consider minimizing the
  // total number or register
  std::vector<IValue> registers;

  // single buffer for input/output calls to ATen functions, so that we do not reallocate
  Stack stack;
};

std::ostream & operator<<(std::ostream & out, const Code & code) {
  out << *code.pImpl->graph << "\n";
  code.pImpl->dump(out);
  return out;
}

Code::Code(std::shared_ptr<Graph>& graph)
    : pImpl(new CodeImpl(graph)) {}
Code::~Code() {}

const std::vector<GraphExecutor*>& Code::executors() {
  return pImpl->executors();
}

InterpreterState::InterpreterState(const Code & function)
  : pImpl(new InterpreterStateImpl(function)) {}
InterpreterState::~InterpreterState() {}

void InterpreterState::runOneStage(Stack & stack) {
  return pImpl->runOneStage(stack);
}

const TensorType & InterpreterState::tensorTypeForInput(size_t i) const {
  return pImpl->tensorTypeForInput(i);
}

InterpreterState InterpreterState::clone() const {
  return InterpreterState(new InterpreterStateImpl(*pImpl));
}

InterpreterState::InterpreterState(InterpreterStateImpl * pImpl) : pImpl(pImpl) {}

}}