/* Copyright 2017 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/cpu/cpu_instruction_fusion.h"
#include "tensorflow/compiler/xla/service/hlo_opcode.h"

namespace xla {
namespace cpu {

namespace {

int64 BytesInDimension(const Shape& shape, int64 dimension) {
  return ShapeUtil::ByteSizeOfPrimitiveType(shape.element_type()) *
         shape.dimensions(dimension);
}

bool IsFusile(const HloInstruction& hlo) {
  // These are the only ones we fuse since we rely on effective elemental IR
  // generation.
  return hlo.IsElementwise() ||  //
         hlo.opcode() == HloOpcode::kBitcast ||
         hlo.opcode() == HloOpcode::kBroadcast ||
         hlo.opcode() == HloOpcode::kConcatenate ||
         hlo.opcode() == HloOpcode::kDynamicSlice ||
         hlo.opcode() == HloOpcode::kDynamicUpdateSlice ||
         hlo.opcode() == HloOpcode::kPad ||
         hlo.opcode() == HloOpcode::kReshape ||
         hlo.opcode() == HloOpcode::kReverse ||
         hlo.opcode() == HloOpcode::kSlice ||
         hlo.opcode() == HloOpcode::kTranspose;
}

}  // namespace

bool CpuInstructionFusion::ShouldFuse(HloInstruction* consumer,
                                      int64 operand_index) {
  HloInstruction* producer = consumer->mutable_operand(operand_index);
  VLOG(2) << "Considering for fusion: operand " << operand_index << " of "
          << consumer->ToString();

  constexpr int kFusionThresholdBytes = 16 * 1024;

  if (!IsFusile(*producer)) {
    VLOG(2) << "Producer is not fusile.";
    return false;
  }

  // Cost condition: not fuse (simple, expensive producers) and (consumers who
  // reuse operand elements).
  if (producer->opcode() != HloOpcode::kFusion &&
      consumer->ReusesOperandElements(operand_index) &&
      is_expensive(*producer)) {
    VLOG(2) << "Fusion is not profitable.";
    return false;
  }

  // TODO(b/28644064): see if the "producer->operand_count() == 0" check is
  // necessary.
  if (producer->operand_count() == 0 ||
      !InstructionFusion::ShouldFuse(consumer, operand_index)) {
    VLOG(2)
        << "Not fusing: producer has no operands, or !ShouldFuse(consumer).";
    return false;
  }

  // Output fusion is not currently supported on CPUs.
  if (producer->opcode() == HloOpcode::kFusion) {
    VLOG(2) << "Not fusing: producer is itself a fusion node.";
    return false;
  }

  if (consumer->opcode() == HloOpcode::kDot) {
    // In the general case we call out to optimized "black box" GEMM routines
    // for Dot, which precludes fusion.  However, in very specific cases, we try
    // to fuse Dot operations by generating an elemental dot implementation.
    //
    // We need to be careful and conservative here since any benefit we get from
    // fusion can easily be overshadowed by the overhead of a naive GEMM
    // algorithm in the IR.
    const Shape& output_shape = consumer->shape();
    if (output_shape.dimensions_size() == 2) {
      // We fuse in cases where we have dot([A,B],[B,1]) or dot([1,A],[A,B]) and
      // fusion can get rid of the larger tensor.  We assume that a naive
      // traversal of a small enough (to fit in L1) column or row tensor is
      // "good enough" from the perspective of cache management; and calling out
      // to an optimized GEMM kernel is not a huge win.
      if (output_shape.dimensions(0) == 1 && operand_index == 1 &&
          BytesInDimension(output_shape, 1) < kFusionThresholdBytes) {
        VLOG(2) << "Fusing small matrix-vector product.";
        return true;
      } else if (output_shape.dimensions(1) == 1 && operand_index == 0 &&
                 BytesInDimension(output_shape, 0) < kFusionThresholdBytes) {
        VLOG(2) << "Fusing small matrix-vector product.";
        return true;
      }
    }
  }

  if (consumer->opcode() == HloOpcode::kFusion) {
    // InstructionFusion::ShouldFuse above only allows kLoop and kInput fusions.
    // The CPU backend does not create kInput fusions, so we only expect to see
    // kLoop here.
    CHECK(consumer->fusion_kind() == HloInstruction::FusionKind::kLoop);
    VLOG(2) << "Fusing: consumer is a fusion node.";
    return true;
  }

  if (IsFusile(*consumer)) {
    VLOG(2) << "Fusing: consumer is elementwise or fusile.";
    return true;
  }

  VLOG(2) << "Not fusing.";
  return false;
}

}  // namespace cpu
}  // namespace xla
