path: root/tensorflow/compiler/xla/service/while_util.cc

/* Copyright 2018 The TensorFlow Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#include "tensorflow/compiler/xla/service/while_util.h"
#include "absl/algorithm/container.h"
#include "absl/strings/str_cat.h"
#include "tensorflow/compiler/xla/literal_util.h"
#include "tensorflow/compiler/xla/service/hlo_computation.h"
#include "tensorflow/compiler/xla/service/hlo_creation_utils.h"
#include "tensorflow/compiler/xla/service/tuple_util.h"

namespace xla {

using absl::StrCat;

static StatusOr<HloComputation*> WidenWhileCondition(
    HloComputation* narrow_condition, const Shape& wide_shape) {
  const Shape& narrow_shape =
      narrow_condition->parameter_instruction(0)->shape();

  HloComputation* wide_while_cond = [&]() {
    HloComputation::Builder builder(StrCat("wide.", narrow_condition->name()));
    builder.AddInstruction(
        HloInstruction::CreateParameter(0, wide_shape, "wide_param"));

    // This is needed so that the root instruction is shaped as a PRED[] -- we
    // need to get this right to begin with since we can't mutate the type of
    // the root instruction later.  We later change the root instruction to
    // something more appropriate.
    builder.AddInstruction(
        HloInstruction::CreateConstant(LiteralUtil::CreateR0<bool>(false)));
    return narrow_condition->parent()->AddEmbeddedComputation(builder.Build());
  }();

  HloInstruction* truncated_parameter =
      TupleUtil::ExtractPrefix(wide_while_cond->parameter_instruction(0),
                               narrow_shape.tuple_shapes_size());
  HloInstruction* call_narrow_cond = wide_while_cond->AddInstruction(
      HloInstruction::CreateCall(ShapeUtil::MakeShape(PRED, {}),
                                 {truncated_parameter}, narrow_condition));

  wide_while_cond->set_root_instruction(call_narrow_cond);

  TF_RETURN_IF_ERROR(CallInliner::Inline(call_narrow_cond).status());
  return wide_while_cond;
}

static StatusOr<std::pair<HloComputation*, CallInliner::InlinedInstructionMap>>
WidenWhileBody(HloComputation* narrow_body, const Shape& wide_shape) {
  const Shape& narrow_shape = narrow_body->parameter_instruction(0)->shape();

  HloComputation* wide_while_body = [&]() {
    HloComputation::Builder builder(StrCat("wide.", narrow_body->name()));
    builder.AddInstruction(
        HloInstruction::CreateParameter(0, wide_shape, "wide_param"));
    return narrow_body->parent()->AddEmbeddedComputation(builder.Build());
  }();

  HloInstruction* wide_parameter = wide_while_body->parameter_instruction(0);
  HloInstruction* truncated_parameter = TupleUtil::ExtractPrefix(
      wide_parameter, narrow_shape.tuple_shapes_size());
  HloInstruction* call_narrow_body =
      wide_while_body->AddInstruction(HloInstruction::CreateCall(
          narrow_shape, {truncated_parameter}, narrow_body));

  std::vector<HloInstruction*> live_through_values;
  for (int i = narrow_shape.tuple_shapes_size();
       i < wide_shape.tuple_shapes_size(); i++) {
    live_through_values.push_back(
        wide_while_body->AddInstruction(HloInstruction::CreateGetTupleElement(
            wide_shape.tuple_shapes(i), wide_parameter, i)));
  }

  wide_while_body->set_root_instruction(
      TupleUtil::AppendSuffix(call_narrow_body, live_through_values));

  TF_ASSIGN_OR_RETURN(auto inlined_instructions_map,
                      CallInliner::Inline(call_narrow_body));
  return {{wide_while_body, std::move(inlined_instructions_map)}};
}

/*static*/ StatusOr<WhileUtil::MakeInstructionsLiveInResult>
WhileUtil::MakeInstructionsLiveIn(
    HloInstruction* while_instr,
    absl::Span<HloInstruction* const> instructions) {
  CHECK(ShapeUtil::IsTuple(while_instr->shape()));

  int64 elements_in_old_while_shape = while_instr->shape().tuple_shapes_size();
  Shape new_while_shape = while_instr->shape();
  for (auto* instruction : instructions) {
    *new_while_shape.add_tuple_shapes() = instruction->shape();
  }

  TF_ASSIGN_OR_RETURN(
      HloComputation * new_while_condition,
      WidenWhileCondition(while_instr->while_condition(), new_while_shape));

  HloComputation* new_while_body;
  CallInliner::InlinedInstructionMap inlined_instructions_map;
  TF_ASSIGN_OR_RETURN(
      std::tie(new_while_body, inlined_instructions_map),
      WidenWhileBody(while_instr->while_body(), new_while_shape));

  HloInstruction* new_while_init =
      TupleUtil::AppendSuffix(while_instr->mutable_operand(0), instructions);
  HloComputation* containing_computation = while_instr->parent();
  HloInstruction* new_while = containing_computation->AddInstruction(
      HloInstruction::CreateWhile(new_while_shape, new_while_condition,
                                  new_while_body, new_while_init));

  // We want to get rid of the old while instruction even if it has side
  // effecting operations so we do a manual HloComputation::RemoveInstruction
  // instead of relying on HloComputation::ReplaceInstruction.
  TF_RETURN_IF_ERROR(while_instr->ReplaceAllUsesWith(TupleUtil::ExtractPrefix(
      new_while, while_instr->shape().tuple_shapes_size())));
  TF_RETURN_IF_ERROR(containing_computation->RemoveInstruction(while_instr));

  HloInstruction* while_body_param = new_while_body->parameter_instruction(0);
  std::vector<HloInstruction*> live_in_instructions;
  for (int64 i = elements_in_old_while_shape;
       i < new_while_shape.tuple_shapes_size(); i++) {
    live_in_instructions.push_back(
        new_while_body->AddInstruction(HloInstruction::CreateGetTupleElement(
            instructions[i - elements_in_old_while_shape]->shape(),
            while_body_param, i)));
  }

  WhileUtil::MakeInstructionsLiveInResult result;

  result.new_while_instr = new_while;
  result.while_body_live_in_values = std::move(live_in_instructions);
  result.while_body_instruction_map = std::move(inlined_instructions_map);

  return std::move(result);
}

static StatusOr<std::unique_ptr<HloComputation>>
MakeCountedLoopConditionComputation(const Shape& loop_state_shape,
                                    int32 trip_count) {
  Shape scalar_pred = ShapeUtil::MakeShape(PRED, {});

  TF_ASSIGN_OR_RETURN(std::unique_ptr<HloComputation> cond_computation,
                      CreateComputationWithSignature(
                          {&loop_state_shape}, scalar_pred, "while_cond"));

  HloInstruction* trip_count_constant = cond_computation->AddInstruction(
      HloInstruction::CreateConstant(LiteralUtil::CreateR0<int32>(trip_count)));

  HloInstruction* param = cond_computation->parameter_instruction(0);
  TF_ASSIGN_OR_RETURN(HloInstruction * indvar,
                      MakeGetTupleElementHlo(param, 0));

  TF_ASSIGN_OR_RETURN(
      HloInstruction * compare,
      MakeBinaryHlo(HloOpcode::kLt, indvar, trip_count_constant));
  cond_computation->set_root_instruction(compare);
  return std::move(cond_computation);
}

static StatusOr<std::unique_ptr<HloComputation>> MakeCountedLoopBodyComputation(
    const Shape& loop_state_shape,
    const std::function<StatusOr<WhileUtil::LoopStateTy>(
        HloInstruction*, const WhileUtil::LoopStateTy&)>& loop_body_generator) {
  TF_ASSIGN_OR_RETURN(std::unique_ptr<HloComputation> body_computation,
                      CreateComputationWithSignature(
                          {&loop_state_shape}, loop_state_shape, "while_body"));
  HloInstruction* one = body_computation->AddInstruction(
      HloInstruction::CreateConstant(LiteralUtil::CreateR0<int32>(1)));
  HloInstruction* param = body_computation->parameter_instruction(0);
  TF_ASSIGN_OR_RETURN(HloInstruction * indvar,
                      MakeGetTupleElementHlo(param, 0));
  TF_ASSIGN_OR_RETURN(HloInstruction * next_indvar,
                      MakeBinaryHlo(HloOpcode::kAdd, indvar, one));

  std::vector<HloInstruction*> loop_body_generator_args;
  for (int64 i = 1, e = loop_state_shape.tuple_shapes_size(); i < e; i++) {
    TF_ASSIGN_OR_RETURN(HloInstruction * tuple_element,
                        MakeGetTupleElementHlo(param, i));
    loop_body_generator_args.push_back(tuple_element);
  }
  TF_ASSIGN_OR_RETURN(std::vector<HloInstruction*> next_state,
                      loop_body_generator(indvar, loop_body_generator_args));
  next_state.insert(next_state.begin(), next_indvar);
  HloInstruction* next_state_tuple =
      body_computation->AddInstruction(HloInstruction::CreateTuple(next_state));
  body_computation->set_root_instruction(next_state_tuple);

  return std::move(body_computation);
}

static StatusOr<HloInstruction*> MakeInitTupleFromInitValues(
    HloComputation* computation, const WhileUtil::LoopStateTy& init_values) {
  std::vector<HloInstruction*> init_values_with_indvar;
  init_values_with_indvar.reserve(init_values.size() + 1);
  HloInstruction* zero = computation->AddInstruction(
      HloInstruction::CreateConstant(LiteralUtil::CreateR0<int32>(0)));
  init_values_with_indvar.push_back(zero);
  absl::c_copy(init_values, std::back_inserter(init_values_with_indvar));
  return computation->AddInstruction(
      HloInstruction::CreateTuple(init_values_with_indvar));
}

static Shape MakeLoopStateShape(const WhileUtil::LoopStateTy& init_values) {
  std::vector<Shape> loop_state_shape_components;
  loop_state_shape_components.reserve(init_values.size() + 1);
  loop_state_shape_components.push_back(ShapeUtil::MakeShape(S32, {}));
  absl::c_transform(init_values,
                    std::back_inserter(loop_state_shape_components),
                    [](HloInstruction* instr) { return instr->shape(); });
  return ShapeUtil::MakeTupleShape(loop_state_shape_components);
}

/*static*/ StatusOr<WhileUtil::LoopStateTy> WhileUtil::MakeCountedLoop(
    HloComputation* computation, int32 trip_count,
    const WhileUtil::LoopStateTy& init_values,
    const WhileUtil::LoopBodyGeneratorTy& loop_body_generator) {
  CHECK_GE(trip_count, 0);

  Shape loop_state_shape = MakeLoopStateShape(init_values);
  TF_ASSIGN_OR_RETURN(
      std::unique_ptr<HloComputation> cond,
      MakeCountedLoopConditionComputation(loop_state_shape, trip_count));
  TF_ASSIGN_OR_RETURN(
      std::unique_ptr<HloComputation> body,
      MakeCountedLoopBodyComputation(loop_state_shape, loop_body_generator));
  TF_ASSIGN_OR_RETURN(HloInstruction * init_tuple,
                      MakeInitTupleFromInitValues(computation, init_values));
  HloModule* module = computation->parent();
  HloInstruction* while_instr =
      computation->AddInstruction(HloInstruction::CreateWhile(
          loop_state_shape, module->AddEmbeddedComputation(std::move(cond)),
          module->AddEmbeddedComputation(std::move(body)), init_tuple));

  std::vector<HloInstruction*> result;
  for (int64 i = 0, e = init_values.size(); i < e; i++) {
    TF_ASSIGN_OR_RETURN(HloInstruction * user_state,
                        MakeGetTupleElementHlo(while_instr, i + 1));
    result.push_back(user_state);
  }
  return result;
}

/*static*/ std::vector<HloInstruction*> WhileUtil::GetInvariantGTEsForWhileBody(
    const HloComputation& while_body) {
  std::vector<HloInstruction*> result;
  const HloInstruction::InstructionVector root_operands =
      while_body.root_instruction()->operands();
  for (int i = 0; i < root_operands.size(); i++) {
    HloInstruction* instr = root_operands[i];
    if (instr->opcode() == HloOpcode::kGetTupleElement &&
        instr->tuple_index() == i &&
        instr->operand(0) == while_body.parameter_instruction(0)) {
      result.push_back(instr);
    }
  }
  return result;
}

}  // namespace xla