source/opt/value_number_table.cpp

// Copyright (c) 2017 Google Inc.
//
// 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 "source/opt/value_number_table.h"

#include <algorithm>

#include "source/opt/cfg.h"
#include "source/opt/ir_context.h"

namespace spvtools {
namespace opt {

uint32_t ValueNumberTable::GetValueNumber(Instruction* inst) const {
  assert(inst->result_id() != 0 &&
         "inst must have a result id to get a value number.");

  // Check if this instruction already has a value.
  auto result_id_to_val = id_to_value_.find(inst->result_id());
  if (result_id_to_val != id_to_value_.end()) {
    return result_id_to_val->second;
  }
  return 0;
}

uint32_t ValueNumberTable::GetValueNumber(uint32_t id) const {
  return GetValueNumber(context()->get_def_use_mgr()->GetDef(id));
}

uint32_t ValueNumberTable::AssignValueNumber(Instruction* inst) {
  // If it already has a value return that.
  uint32_t value = GetValueNumber(inst);
  if (value != 0) {
    return value;
  }

  // If the instruction has other side effects, then it must
  // have its own value number.
  // OpSampledImage and OpImage must remain in the same basic block in which
  // they are used, because of this we will assign each one it own value number.
  if (!context()->IsCombinatorInstruction(inst)) {
    value = TakeNextValueNumber();
    id_to_value_[inst->result_id()] = value;
    return value;
  }

  switch (inst->opcode()) {
    case SpvOpSampledImage:
    case SpvOpImage:
    case SpvOpVariable:
      value = TakeNextValueNumber();
      id_to_value_[inst->result_id()] = value;
      return value;
    default:
      break;
  }

  // If it is a load from memory that can be modified, we have to assume the
  // memory has been modified, so we give it a new value number.
  //
  // Note that this test will also handle volatile loads because they are not
  // read only.  However, if this is ever relaxed because we analyze stores, we
  // will have to add a new case for volatile loads.
  if (inst->IsLoad() && !inst->IsReadOnlyLoad()) {
    value = TakeNextValueNumber();
    id_to_value_[inst->result_id()] = value;
    return value;
  }

  // When we copy an object, the value numbers should be the same.
  if (inst->opcode() == SpvOpCopyObject) {
    value = GetValueNumber(inst->GetSingleWordInOperand(0));
    if (value != 0) {
      id_to_value_[inst->result_id()] = value;
      return value;
    }
  }

  // Phi nodes are a type of copy.  If all of the inputs have the same value
  // number, then we can assign the result of the phi the same value number.
  if (inst->opcode() == SpvOpPhi) {
    value = GetValueNumber(inst->GetSingleWordInOperand(0));
    if (value != 0) {
      for (uint32_t op = 2; op < inst->NumInOperands(); op += 2) {
        if (value != GetValueNumber(inst->GetSingleWordInOperand(op))) {
          value = 0;
          break;
        }
      }
      if (value != 0) {
        id_to_value_[inst->result_id()] = value;
        return value;
      }
    }
  }

  // Replace all of the operands by their value number.  The sign bit will be
  // set to distinguish between an id and a value number.
  Instruction value_ins(context(), inst->opcode(), inst->type_id(),
                        inst->result_id(), {});
  for (uint32_t o = 0; o < inst->NumInOperands(); ++o) {
    const Operand& op = inst->GetInOperand(o);
    if (spvIsIdType(op.type)) {
      uint32_t id_value = op.words[0];
      auto use_id_to_val = id_to_value_.find(id_value);
      if (use_id_to_val != id_to_value_.end()) {
        id_value = (1 << 31) | use_id_to_val->second;
      }
      value_ins.AddOperand(Operand(op.type, {id_value}));
    } else {
      value_ins.AddOperand(Operand(op.type, op.words));
    }
  }

  // TODO: Implement a normal form for opcodes that commute like integer
  // addition.  This will let us know that a+b is the same value as b+a.

  // Otherwise, we check if this value has been computed before.
  auto value_iterator = instruction_to_value_.find(value_ins);
  if (value_iterator != instruction_to_value_.end()) {
    value = id_to_value_[value_iterator->first.result_id()];
    id_to_value_[inst->result_id()] = value;
    return value;
  }

  // If not, assign it a new value number.
  value = TakeNextValueNumber();
  id_to_value_[inst->result_id()] = value;
  instruction_to_value_[value_ins] = value;
  return value;
}

void ValueNumberTable::BuildDominatorTreeValueNumberTable() {
  // First value number the headers.
  for (auto& inst : context()->annotations()) {
    if (inst.result_id() != 0) {
      AssignValueNumber(&inst);
    }
  }

  for (auto& inst : context()->capabilities()) {
    if (inst.result_id() != 0) {
      AssignValueNumber(&inst);
    }
  }

  for (auto& inst : context()->types_values()) {
    if (inst.result_id() != 0) {
      AssignValueNumber(&inst);
    }
  }

  for (auto& inst : context()->module()->ext_inst_imports()) {
    if (inst.result_id() != 0) {
      AssignValueNumber(&inst);
    }
  }

  for (Function& func : *context()->module()) {
    // For best results we want to traverse the code in reverse post order.
    // This happens naturally because of the forward referencing rules.
    for (BasicBlock& block : func) {
      for (Instruction& inst : block) {
        if (inst.result_id() != 0) {
          AssignValueNumber(&inst);
        }
      }
    }
  }
}

bool ComputeSameValue::operator()(const Instruction& lhs,
                                  const Instruction& rhs) const {
  if (lhs.result_id() == 0 || rhs.result_id() == 0) {
    return false;
  }

  if (lhs.opcode() != rhs.opcode()) {
    return false;
  }

  if (lhs.type_id() != rhs.type_id()) {
    return false;
  }

  if (lhs.NumInOperands() != rhs.NumInOperands()) {
    return false;
  }

  for (uint32_t i = 0; i < lhs.NumInOperands(); ++i) {
    if (lhs.GetInOperand(i) != rhs.GetInOperand(i)) {
      return false;
    }
  }

  return lhs.context()->get_decoration_mgr()->HaveTheSameDecorations(
      lhs.result_id(), rhs.result_id());
}

std::size_t ValueTableHash::operator()(const Instruction& inst) const {
  // We hash the opcode and in-operands, not the result, because we want
  // instructions that are the same except for the result to hash to the
  // same value.
  std::u32string h;
  h.push_back(inst.opcode());
  h.push_back(inst.type_id());
  for (uint32_t i = 0; i < inst.NumInOperands(); ++i) {
    const auto& opnd = inst.GetInOperand(i);
    for (uint32_t word : opnd.words) {
      h.push_back(word);
    }
  }
  return std::hash<std::u32string>()(h);
}
}  // namespace opt
}  // namespace spvtools