src/hydrogen-redundant-phi.cc

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#include "hydrogen-redundant-phi.h"

namespace v8 {
namespace internal {

void HRedundantPhiEliminationPhase::Run() {
  // We do a simple fixed point iteration without any work list, because
  // machine-generated JavaScript can lead to a very dense Hydrogen graph with
  // an enormous work list and will consequently result in OOM. Experiments
  // showed that this simple algorithm is good enough, and even e.g. tracking
  // the set or range of blocks to consider is not a real improvement.
  bool need_another_iteration;
  const ZoneList<HBasicBlock*>* blocks(graph()->blocks());
  ZoneList<HPhi*> redundant_phis(blocks->length(), zone());
  do {
    need_another_iteration = false;
    for (int i = 0; i < blocks->length(); ++i) {
      HBasicBlock* block = blocks->at(i);
      for (int j = 0; j < block->phis()->length(); j++) {
        HPhi* phi = block->phis()->at(j);
        HValue* replacement = phi->GetRedundantReplacement();
        if (replacement != NULL) {
          // Remember phi to avoid concurrent modification of the block's phis.
          redundant_phis.Add(phi, zone());
          for (HUseIterator it(phi->uses()); !it.Done(); it.Advance()) {
            HValue* value = it.value();
            value->SetOperandAt(it.index(), replacement);
            need_another_iteration |= value->IsPhi();
          }
        }
      }
      for (int i = 0; i < redundant_phis.length(); i++) {
        block->RemovePhi(redundant_phis[i]);
      }
      redundant_phis.Clear();
    }
  } while (need_another_iteration);

#if DEBUG
  // Make sure that we *really* removed all redundant phis.
  for (int i = 0; i < blocks->length(); ++i) {
    for (int j = 0; j < blocks->at(i)->phis()->length(); j++) {
      ASSERT(blocks->at(i)->phis()->at(j)->GetRedundantReplacement() == NULL);
    }
  }
#endif
}

} }  // namespace v8::internal