compiler/optimizing/code_generator_utils.cc

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * 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 "code_generator_utils.h"

#include <android-base/logging.h>

#include "nodes.h"

namespace art HIDDEN {

void CalculateMagicAndShiftForDivRem(int64_t divisor, bool is_long,
                                     int64_t* magic, int* shift) {
  // It does not make sense to calculate magic and shift for zero divisor.
  DCHECK_NE(divisor, 0);

  /* Implementation according to H.S.Warren's "Hacker's Delight" (Addison Wesley, 2002)
   * Chapter 10 and T.Grablund, P.L.Montogomery's "Division by Invariant Integers Using
   * Multiplication" (PLDI 1994).
   * The magic number M and shift S can be calculated in the following way:
   * Let nc be the most positive value of numerator(n) such that nc = kd - 1,
   * where divisor(d) >= 2.
   * Let nc be the most negative value of numerator(n) such that nc = kd + 1,
   * where divisor(d) <= -2.
   * Thus nc can be calculated like:
   * nc = exp + exp % d - 1, where d >= 2 and exp = 2^31 for int or 2^63 for long
   * nc = -exp + (exp + 1) % d, where d >= 2 and exp = 2^31 for int or 2^63 for long
   *
   * So the shift p is the smallest p satisfying
   * 2^p > nc * (d - 2^p % d), where d >= 2
   * 2^p > nc * (d + 2^p % d), where d <= -2.
   *
   * The magic number M is calculated by
   * M = (2^p + d - 2^p % d) / d, where d >= 2
   * M = (2^p - d - 2^p % d) / d, where d <= -2.
   *
   * Notice that p is always bigger than or equal to 32 (resp. 64), so we just return 32 - p
   * (resp. 64 - p) as the shift number S.
   */

  int64_t p = is_long ? 63 : 31;
  const uint64_t exp = is_long ? (UINT64_C(1) << 63) : (UINT32_C(1) << 31);

  // Initialize the computations.
  uint64_t abs_d = (divisor >= 0) ? divisor : -divisor;
  uint64_t sign_bit = is_long ? static_cast<uint64_t>(divisor) >> 63 :
                                static_cast<uint32_t>(divisor) >> 31;
  uint64_t tmp = exp + sign_bit;
  uint64_t abs_nc = tmp - 1 - (tmp % abs_d);
  uint64_t quotient1 = exp / abs_nc;
  uint64_t remainder1 = exp % abs_nc;
  uint64_t quotient2 = exp / abs_d;
  uint64_t remainder2 = exp % abs_d;

  /*
   * To avoid handling both positive and negative divisor, "Hacker's Delight"
   * introduces a method to handle these 2 cases together to avoid duplication.
   */
  uint64_t delta;
  do {
    p++;
    quotient1 = 2 * quotient1;
    remainder1 = 2 * remainder1;
    if (remainder1 >= abs_nc) {
      quotient1++;
      remainder1 = remainder1 - abs_nc;
    }
    quotient2 = 2 * quotient2;
    remainder2 = 2 * remainder2;
    if (remainder2 >= abs_d) {
      quotient2++;
      remainder2 = remainder2 - abs_d;
    }
    delta = abs_d - remainder2;
  } while (quotient1 < delta || (quotient1 == delta && remainder1 == 0));

  *magic = (divisor > 0) ? (quotient2 + 1) : (-quotient2 - 1);

  if (!is_long) {
    *magic = static_cast<int>(*magic);
  }

  *shift = is_long ? p - 64 : p - 32;
}

bool IsBooleanValueOrMaterializedCondition(HInstruction* cond_input) {
  return !cond_input->IsCondition() || !cond_input->IsEmittedAtUseSite();
}

// A helper class to group functions analyzing if values are non-negative
// at the point of use. The class keeps some context used by the functions.
// The class is not supposed to be used directly or its instances to be kept.
// The main function using it is HasNonNegativeInputAt.
// If you want to use the class methods you need to become a friend of the class.
class UnsignedUseAnalyzer {
 private:
  explicit UnsignedUseAnalyzer(ArenaAllocator* allocator)
      : seen_values_(allocator->Adapter(kArenaAllocCodeGenerator)) {
  }

  bool IsNonNegativeUse(HInstruction* target_user, HInstruction* value);
  bool IsComparedValueNonNegativeInBlock(HInstruction* value,
                                         HCondition* cond,
                                         HBasicBlock* target_block);

  ArenaSet<HInstruction*> seen_values_;

  friend bool HasNonNegativeInputAt(HInstruction* instr, size_t i);
};

// Check that the value compared with a non-negavite value is
// non-negative in the specified basic block.
bool UnsignedUseAnalyzer::IsComparedValueNonNegativeInBlock(HInstruction* value,
                                                            HCondition* cond,
                                                            HBasicBlock* target_block) {
  DCHECK(cond->HasInput(value));

  // To simplify analysis, we require:
  // 1. The condition basic block and target_block to be different.
  // 2. The condition basic block to end with HIf.
  // 3. HIf to use the condition.
  if (cond->GetBlock() == target_block ||
      !cond->GetBlock()->EndsWithIf() ||
      cond->GetBlock()->GetLastInstruction()->InputAt(0) != cond) {
    return false;
  }

  // We need to find a successor basic block of HIf for the case when instr is non-negative.
  // If the successor dominates target_block, instructions in target_block see a non-negative value.
  HIf* if_instr = cond->GetBlock()->GetLastInstruction()->AsIf();
  HBasicBlock* successor = nullptr;
  switch (cond->GetCondition()) {
    case kCondGT:
    case kCondGE: {
      if (cond->GetLeft() == value) {
        // The expression is v > A or v >= A.
        // If A is non-negative, we need the true successor.
        if (IsNonNegativeUse(cond, cond->GetRight())) {
          successor = if_instr->IfTrueSuccessor();
        } else {
          return false;
        }
      } else {
        DCHECK_EQ(cond->GetRight(), value);
        // The expression is A > v or A >= v.
        // If A is non-negative, we need the false successor.
        if (IsNonNegativeUse(cond, cond->GetLeft())) {
          successor = if_instr->IfFalseSuccessor();
        } else {
          return false;
        }
      }
      break;
    }

    case kCondLT:
    case kCondLE: {
      if (cond->GetLeft() == value) {
        // The expression is v < A or v <= A.
        // If A is non-negative, we need the false successor.
        if (IsNonNegativeUse(cond, cond->GetRight())) {
          successor = if_instr->IfFalseSuccessor();
        } else {
          return false;
        }
      } else {
        DCHECK_EQ(cond->GetRight(), value);
        // The expression is A < v or A <= v.
        // If A is non-negative, we need the true successor.
        if (IsNonNegativeUse(cond, cond->GetLeft())) {
          successor = if_instr->IfTrueSuccessor();
        } else {
          return false;
        }
      }
      break;
    }

    default:
      return false;
  }
  DCHECK_NE(successor, nullptr);

  return successor->Dominates(target_block);
}

// Check the value used by target_user is non-negative.
bool UnsignedUseAnalyzer::IsNonNegativeUse(HInstruction* target_user, HInstruction* value) {
  DCHECK(target_user->HasInput(value));

  // Prevent infinitive recursion which can happen when the value is an induction variable.
  if (!seen_values_.insert(value).second) {
    return false;
  }

  // Check if the value is always non-negative.
  if (IsGEZero(value)) {
    return true;
  }

  for (const HUseListNode<HInstruction*>& use : value->GetUses()) {
    HInstruction* user = use.GetUser();
    if (user == target_user) {
      continue;
    }

    // If the value is compared with some non-negative value, this can guarantee the value to be
    // non-negative at its use.
    // JFYI: We're not using HTypeConversion to bind the new information because it would
    // increase the complexity of optimizations: HTypeConversion can create a dependency
    // which does not exist in the input program, for example:
    // between two uses, 1st - cmp, 2nd - target_user.
    if (user->IsCondition()) {
      // The condition must dominate target_user to guarantee that the value is always checked
      // before it is used by target_user.
      if (user->GetBlock()->Dominates(target_user->GetBlock()) &&
          IsComparedValueNonNegativeInBlock(value, user->AsCondition(), target_user->GetBlock())) {
        return true;
      }
    }

    // TODO The value is non-negative if it is used as an array index before.
    // TODO The value is non-negative if it is initialized by a positive number and all of its
    //      modifications keep the value non-negative, for example the division operation.
  }

  return false;
}

bool HasNonNegativeInputAt(HInstruction* instr, size_t i) {
  UnsignedUseAnalyzer analyzer(instr->GetBlock()->GetGraph()->GetAllocator());
  return analyzer.IsNonNegativeUse(instr, instr->InputAt(i));
}

bool HasNonNegativeOrMinIntInputAt(HInstruction* instr, size_t i) {
  HInstruction* input = instr->InputAt(i);
  return input->IsAbs() ||
         IsInt64Value(input, DataType::MinValueOfIntegralType(input->GetType())) ||
         HasNonNegativeInputAt(instr, i);
}

}  // namespace art