CSE2410 Fall 2014 Bounce

// Copyright 2013 the V8 project authors. All rights reserved.

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#include "hydrogen-uint32-analysis.h"

namespace v8 {

namespace internal {

bool HUint32AnalysisPhase::IsSafeUint32Use(HValue* val, HValue* use) {

  // Operations that operate on bits are safe.

  if (use->IsBitwise() || use->IsShl() || use->IsSar() || use->IsShr()) {

    return true;

  } else if (use->IsSimulate()) {

    // Deoptimization has special support for uint32.

    return true;

  } else if (use->IsChange()) {

    // Conversions have special support for uint32.

    // This ASSERT guards that the conversion in question is actually

    // implemented. Do not extend the whitelist without adding

    // support to LChunkBuilder::DoChange().

    ASSERT(HChange::cast(use)->to().IsDouble() ||

           HChange::cast(use)->to().IsSmi() ||

           HChange::cast(use)->to().IsTagged());

    return true;

  } else if (use->IsStoreKeyed()) {

    HStoreKeyed* store = HStoreKeyed::cast(use);

    if (store->is_external()) {

      // Storing a value into an external integer array is a bit level

      // operation.

      if (store->value() == val) {

        // Clamping or a conversion to double should have beed inserted.

        ASSERT(store->elements_kind() != EXTERNAL_PIXEL_ELEMENTS);

        ASSERT(store->elements_kind() != EXTERNAL_FLOAT_ELEMENTS);

        ASSERT(store->elements_kind() != EXTERNAL_DOUBLE_ELEMENTS);

        return true;

      }

    }

  }

  return false;

}

// Iterate over all uses and verify that they are uint32 safe: either don't

// distinguish between int32 and uint32 due to their bitwise nature or

// have special support for uint32 values.

// Encountered phis are optimistically treated as safe uint32 uses,

// marked with kUint32 flag and collected in the phis_ list. A separate

// pass will be performed later by UnmarkUnsafePhis to clear kUint32 from

// phis that are not actually uint32-safe (it requires fix point iteration).

bool HUint32AnalysisPhase::Uint32UsesAreSafe(HValue* uint32val) {

  bool collect_phi_uses = false;

  for (HUseIterator it(uint32val->uses()); !it.Done(); it.Advance()) {

    HValue* use = it.value();

    if (use->IsPhi()) {

      if (!use->CheckFlag(HInstruction::kUint32)) {

        // There is a phi use of this value from a phi that is not yet

        // collected in phis_ array. Separate pass is required.

        collect_phi_uses = true;

      }

      // Optimistically treat phis as uint32 safe.

      continue;

    }

    if (!IsSafeUint32Use(uint32val, use)) {

      return false;

    }

  }

  if (collect_phi_uses) {

    for (HUseIterator it(uint32val->uses()); !it.Done(); it.Advance()) {

      HValue* use = it.value();

      // There is a phi use of this value from a phi that is not yet

      // collected in phis_ array. Separate pass is required.

      if (use->IsPhi() && !use->CheckFlag(HInstruction::kUint32)) {

        use->SetFlag(HInstruction::kUint32);

        phis_.Add(HPhi::cast(use), zone());

      }

    }

  }

  return true;

}

// Check if all operands to the given phi are marked with kUint32 flag.

bool HUint32AnalysisPhase::CheckPhiOperands(HPhi* phi) {

  if (!phi->CheckFlag(HInstruction::kUint32)) {

    // This phi is not uint32 safe. No need to check operands.

    return false;

  }

  for (int j = 0; j < phi->OperandCount(); j++) {

    HValue* operand = phi->OperandAt(j);

    if (!operand->CheckFlag(HInstruction::kUint32)) {

      // Lazily mark constants that fit into uint32 range with kUint32 flag.

      if (operand->IsInteger32Constant() &&

          operand->GetInteger32Constant() >= 0) {

        operand->SetFlag(HInstruction::kUint32);

        continue;

      }

      // This phi is not safe, some operands are not uint32 values.

      return false;

    }

  }

  return true;

}

// Remove kUint32 flag from the phi itself and its operands. If any operand

// was a phi marked with kUint32 place it into a worklist for

// transitive clearing of kUint32 flag.

void HUint32AnalysisPhase::UnmarkPhi(HPhi* phi, ZoneList<HPhi*>* worklist) {

  phi->ClearFlag(HInstruction::kUint32);

  for (int j = 0; j < phi->OperandCount(); j++) {

    HValue* operand = phi->OperandAt(j);

    if (operand->CheckFlag(HInstruction::kUint32)) {

      operand->ClearFlag(HInstruction::kUint32);

      if (operand->IsPhi()) {

        worklist->Add(HPhi::cast(operand), zone());

      }

    }

  }

}

void HUint32AnalysisPhase::UnmarkUnsafePhis() {

  // No phis were collected. Nothing to do.

  if (phis_.length() == 0) return;

  // Worklist used to transitively clear kUint32 from phis that

  // are used as arguments to other phis.

  ZoneList<HPhi*> worklist(phis_.length(), zone());

  // Phi can be used as a uint32 value if and only if

  // all its operands are uint32 values and all its

  // uses are uint32 safe.

  // Iterate over collected phis and unmark those that

  // are unsafe. When unmarking phi unmark its operands

  // and add it to the worklist if it is a phi as well.

  // Phis that are still marked as safe are shifted down

  // so that all safe phis form a prefix of the phis_ array.

  int phi_count = 0;

  for (int i = 0; i < phis_.length(); i++) {

    HPhi* phi = phis_[i];

    if (CheckPhiOperands(phi) && Uint32UsesAreSafe(phi)) {

      phis_[phi_count++] = phi;

    } else {

      UnmarkPhi(phi, &worklist);

    }

  }

  // Now phis array contains only those phis that have safe

  // non-phi uses. Start transitively clearing kUint32 flag

  // from phi operands of discovered non-safe phis until

  // only safe phis are left.

  while (!worklist.is_empty())  {

    while (!worklist.is_empty()) {

      HPhi* phi = worklist.RemoveLast();

      UnmarkPhi(phi, &worklist);

    }

    // Check if any operands to safe phis were unmarked

    // turning a safe phi into unsafe. The same value

    // can flow into several phis.

    int new_phi_count = 0;

    for (int i = 0; i < phi_count; i++) {

      HPhi* phi = phis_[i];

      if (CheckPhiOperands(phi)) {

        phis_[new_phi_count++] = phi;

      } else {

        UnmarkPhi(phi, &worklist);

      }

    }

    phi_count = new_phi_count;

  }

}

void HUint32AnalysisPhase::Run() {

  if (!graph()->has_uint32_instructions()) return;

  ZoneList<HInstruction*>* uint32_instructions = graph()->uint32_instructions();

  for (int i = 0; i < uint32_instructions->length(); ++i) {

    // Analyze instruction and mark it with kUint32 if all

    // its uses are uint32 safe.

    HInstruction* current = uint32_instructions->at(i);

    if (current->IsLinked() &&

        current->representation().IsInteger32() &&

        Uint32UsesAreSafe(current)) {

      current->SetFlag(HInstruction::kUint32);

    }

  }

  // Some phis might have been optimistically marked with kUint32 flag.

  // Remove this flag from those phis that are unsafe and propagate

  // this information transitively potentially clearing kUint32 flag

  // from some non-phi operations that are used as operands to unsafe phis.

  UnmarkUnsafePhis();

}

} }  // namespace v8::internal