compiler/optimizing/parallel_move_resolver.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2014 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 "parallel_move_resolver.h"

 #include "base/stl_util.h"
 #include "nodes.h"

 namespace art {

 void ParallelMoveResolver::BuildInitialMoveList(HParallelMove* parallel_move) {
   // Perform a linear sweep of the moves to add them to the initial list of
   // moves to perform, ignoring any move that is redundant (the source is
   // the same as the destination, the destination is ignored and
   // unallocated, or the move was already eliminated).
   for (size_t i = 0; i < parallel_move->NumMoves(); ++i) {
     MoveOperands* move = parallel_move->MoveOperandsAt(i);
     if (!move->IsRedundant()) {
       moves_.push_back(move);
     }
   }
 }

 void ParallelMoveResolverWithSwap::EmitNativeCode(HParallelMove* parallel_move) {
   DCHECK(moves_.empty());
   // Build up a worklist of moves.
   BuildInitialMoveList(parallel_move);

   // Move stack/stack slot to take advantage of a free register on constrained machines.
   for (size_t i = 0; i < moves_.size(); ++i) {
     const MoveOperands& move = *moves_[i];
     // Ignore constants and moves already eliminated.
     if (move.IsEliminated() || move.GetSource().IsConstant()) {
       continue;
     }

     if ((move.GetSource().IsStackSlot() || move.GetSource().IsDoubleStackSlot()) &&
         (move.GetDestination().IsStackSlot() || move.GetDestination().IsDoubleStackSlot())) {
       PerformMove(i);
     }
   }

   for (size_t i = 0; i < moves_.size(); ++i) {
     const MoveOperands& move = *moves_[i];
     // Skip constants to perform them last.  They don't block other moves
     // and skipping such moves with register destinations keeps those
     // registers free for the whole algorithm.
     if (!move.IsEliminated() && !move.GetSource().IsConstant()) {
       PerformMove(i);
     }
   }

   // Perform the moves with constant sources.
   for (size_t i = 0; i < moves_.size(); ++i) {
     MoveOperands* move = moves_[i];
     if (!move->IsEliminated()) {
       DCHECK(move->GetSource().IsConstant());
       EmitMove(i);
       // Eliminate the move, in case following moves need a scratch register.
       move->Eliminate();
     }
   }

   moves_.clear();
 }

 Location LowOf(Location location) {
   if (location.IsRegisterPair()) {
     return Location::RegisterLocation(location.low());
   } else if (location.IsFpuRegisterPair()) {
     return Location::FpuRegisterLocation(location.low());
   } else if (location.IsDoubleStackSlot()) {
     return Location::StackSlot(location.GetStackIndex());
   } else {
     return Location::NoLocation();
   }
 }

 Location HighOf(Location location) {
   if (location.IsRegisterPair()) {
     return Location::RegisterLocation(location.high());
   } else if (location.IsFpuRegisterPair()) {
     return Location::FpuRegisterLocation(location.high());
   } else if (location.IsDoubleStackSlot()) {
     return Location::StackSlot(location.GetHighStackIndex(4));
   } else {
     return Location::NoLocation();
   }
 }

 // Update the source of `move`, knowing that `updated_location` has been swapped
 // with `new_source`. Note that `updated_location` can be a pair, therefore if
 // `move` is non-pair, we need to extract which register to use.
 static void UpdateSourceOf(MoveOperands* move, Location updated_location, Location new_source) {
   Location source = move->GetSource();
   if (LowOf(updated_location).Equals(source)) {
     move->SetSource(LowOf(new_source));
   } else if (HighOf(updated_location).Equals(source)) {
     move->SetSource(HighOf(new_source));
   } else {
     DCHECK(updated_location.Equals(source)) << updated_location << " " << source;
     move->SetSource(new_source);
   }
 }

 MoveOperands* ParallelMoveResolverWithSwap::PerformMove(size_t index) {
   // Each call to this function performs a move and deletes it from the move
   // graph.  We first recursively perform any move blocking this one.  We
   // mark a move as "pending" on entry to PerformMove in order to detect
   // cycles in the move graph.  We use operand swaps to resolve cycles,
   // which means that a call to PerformMove could change any source operand
   // in the move graph.

   MoveOperands* move = moves_[index];
   DCHECK(!move->IsPending());
   if (move->IsRedundant()) {
     // Because we swap register pairs first, following, un-pending
     // moves may become redundant.
     move->Eliminate();
     return nullptr;
   }

   // Clear this move's destination to indicate a pending move.  The actual
   // destination is saved in a stack-allocated local.  Recursion may allow
   // multiple moves to be pending.
   DCHECK(!move->GetSource().IsInvalid());
   Location destination = move->MarkPending();

   // Perform a depth-first traversal of the move graph to resolve
   // dependencies.  Any unperformed, unpending move with a source the same
   // as this one's destination blocks this one so recursively perform all
   // such moves.
   MoveOperands* required_swap = nullptr;
   for (size_t i = 0; i < moves_.size(); ++i) {
     const MoveOperands& other_move = *moves_[i];
     if (other_move.Blocks(destination) && !other_move.IsPending()) {
       // Though PerformMove can change any source operand in the move graph,
       // calling `PerformMove` cannot create a blocking move via a swap
       // (this loop does not miss any).
       // For example, assume there is a non-blocking move with source A
       // and this move is blocked on source B and there is a swap of A and
       // B.  Then A and B must be involved in the same cycle (or they would
       // not be swapped).  Since this move's destination is B and there is
       // only a single incoming edge to an operand, this move must also be
       // involved in the same cycle.  In that case, the blocking move will
       // be created but will be "pending" when we return from PerformMove.
       required_swap = PerformMove(i);

       if (required_swap == move) {
         // If this move is required to swap, we do so without looking
         // at the next moves. Swapping is not blocked by anything, it just
         // updates other moves's source.
         break;
       } else if (required_swap == moves_[i]) {
         // If `other_move` was swapped, we iterate again to find a new
         // potential cycle.
         required_swap = nullptr;
         i = -1;
       } else if (required_swap != nullptr) {
         // A move is required to swap. We walk back the cycle to find the
         // move by just returning from this `PerformMove`.
         moves_[index]->ClearPending(destination);
         return required_swap;
       }
     }
   }

   // We are about to resolve this move and don't need it marked as
   // pending, so restore its destination.
   move->ClearPending(destination);

   // This move's source may have changed due to swaps to resolve cycles and
   // so it may now be the last move in the cycle.  If so remove it.
   if (move->GetSource().Equals(destination)) {
     move->Eliminate();
     DCHECK(required_swap == nullptr);
     return nullptr;
   }

   // The move may be blocked on a (at most one) pending move, in which case
   // we have a cycle.  Search for such a blocking move and perform a swap to
   // resolve it.
   bool do_swap = false;
   if (required_swap != nullptr) {
     DCHECK_EQ(required_swap, move);
     do_swap = true;
   } else {
     for (MoveOperands* other_move : moves_) {
       if (other_move->Blocks(destination)) {
         DCHECK(other_move->IsPending()) << "move=" << *move << " other_move=" << *other_move;
         if (!move->Is64BitMove() && other_move->Is64BitMove()) {
           // We swap 64bits moves before swapping 32bits moves. Go back from the
           // cycle by returning the move that must be swapped.
           return other_move;
         }
         do_swap = true;
         break;
       }
     }
   }

   if (do_swap) {
     EmitSwap(index);
     // Any unperformed (including pending) move with a source of either
     // this move's source or destination needs to have their source
     // changed to reflect the state of affairs after the swap.
     Location source = move->GetSource();
     Location swap_destination = move->GetDestination();
     move->Eliminate();
     for (MoveOperands* other_move : moves_) {
       if (other_move->Blocks(source)) {
         UpdateSourceOf(other_move, source, swap_destination);
       } else if (other_move->Blocks(swap_destination)) {
         UpdateSourceOf(other_move, swap_destination, source);
       }
     }
     // If the swap was required because of a 64bits move in the middle of a cycle,
     // we return the swapped move, so that the caller knows it needs to re-iterate
     // its dependency loop.
     return required_swap;
   } else {
     // This move is not blocked.
     EmitMove(index);
     move->Eliminate();
     DCHECK(required_swap == nullptr);
     return nullptr;
   }
 }

 bool ParallelMoveResolverWithSwap::IsScratchLocation(Location loc) {
   for (MoveOperands* move : moves_) {
     if (move->Blocks(loc)) {
       return false;
     }
   }

   for (MoveOperands* move : moves_) {
     if (move->GetDestination().Equals(loc)) {
       return true;
     }
   }

   return false;
 }

 int ParallelMoveResolverWithSwap::AllocateScratchRegister(int blocked,
                                                           int register_count,
                                                           int if_scratch,
                                                           bool* spilled) {
   DCHECK_NE(blocked, if_scratch);
   int scratch = -1;
   for (int reg = 0; reg < register_count; ++reg) {
     if ((blocked != reg) && IsScratchLocation(Location::RegisterLocation(reg))) {
       scratch = reg;
       break;
     }
   }

   if (scratch == -1) {
     *spilled = true;
     scratch = if_scratch;
   } else {
     *spilled = false;
   }

   return scratch;
 }


 ParallelMoveResolverWithSwap::ScratchRegisterScope::ScratchRegisterScope(
     ParallelMoveResolverWithSwap* resolver, int blocked, int if_scratch, int number_of_registers)
     : resolver_(resolver),
       reg_(kNoRegister),
       spilled_(false) {
   reg_ = resolver_->AllocateScratchRegister(blocked, number_of_registers, if_scratch, &spilled_);

   if (spilled_) {
     resolver->SpillScratch(reg_);
   }
 }


 ParallelMoveResolverWithSwap::ScratchRegisterScope::~ScratchRegisterScope() {
   if (spilled_) {
     resolver_->RestoreScratch(reg_);
   }
 }

 void ParallelMoveResolverNoSwap::EmitNativeCode(HParallelMove* parallel_move) {
   DCHECK_EQ(GetNumberOfPendingMoves(), 0u);
   DCHECK(moves_.empty());
   DCHECK(scratches_.empty());

   // Backend dependent initialization.
   PrepareForEmitNativeCode();

   // Build up a worklist of moves.
   BuildInitialMoveList(parallel_move);

   for (size_t i = 0; i < moves_.size(); ++i) {
     const MoveOperands& move = *moves_[i];
     // Skip constants to perform them last. They don't block other moves and
     // skipping such moves with register destinations keeps those registers
     // free for the whole algorithm.
     if (!move.IsEliminated() && !move.GetSource().IsConstant()) {
       PerformMove(i);
     }
   }

   // Perform the moves with constant sources and register destinations with UpdateMoveSource()
   // to reduce the number of literal loads. Stack destinations are skipped since we won't be benefit
   // from changing the constant sources to stack locations.
   for (size_t i = 0; i < moves_.size(); ++i) {
     MoveOperands* move = moves_[i];
     Location destination = move->GetDestination();
     if (!move->IsEliminated() && !destination.IsStackSlot() && !destination.IsDoubleStackSlot()) {
       Location source = move->GetSource();
       EmitMove(i);
       move->Eliminate();
       // This may introduce additional instruction dependency, but reduce number
       // of moves and possible literal loads. For example,
       // Original moves:
       //   1234.5678 -> D0
       //   1234.5678 -> D1
       // Updated moves:
       //   1234.5678 -> D0
       //   D0 -> D1
       UpdateMoveSource(source, destination);
     }
   }

   // Perform the rest of the moves.
   for (size_t i = 0; i < moves_.size(); ++i) {
     MoveOperands* move = moves_[i];
     if (!move->IsEliminated()) {
       EmitMove(i);
       move->Eliminate();
     }
   }

   // All pending moves that we have added for resolve cycles should be performed.
   DCHECK_EQ(GetNumberOfPendingMoves(), 0u);

   // Backend dependent cleanup.
   FinishEmitNativeCode();

   moves_.clear();
   scratches_.clear();
 }

 Location ParallelMoveResolverNoSwap::GetScratchLocation(Location::Kind kind) {
   for (Location loc : scratches_) {
     if (loc.GetKind() == kind && !IsBlockedByMoves(loc)) {
       return loc;
     }
   }
   for (MoveOperands* move : moves_) {
     Location loc = move->GetDestination();
     if (loc.GetKind() == kind && !IsBlockedByMoves(loc)) {
       return loc;
     }
   }
   return Location::NoLocation();
 }

 void ParallelMoveResolverNoSwap::AddScratchLocation(Location loc) {
   if (kIsDebugBuild) {
     for (Location scratch : scratches_) {
       CHECK(!loc.Equals(scratch));
     }
   }
   scratches_.push_back(loc);
 }

 void ParallelMoveResolverNoSwap::RemoveScratchLocation(Location loc) {
   DCHECK(!IsBlockedByMoves(loc));
   for (auto it = scratches_.begin(), end = scratches_.end(); it != end; ++it) {
     if (loc.Equals(*it)) {
       scratches_.erase(it);
       break;
     }
   }
 }

 void ParallelMoveResolverNoSwap::PerformMove(size_t index) {
   // Each call to this function performs a move and deletes it from the move
   // graph. We first recursively perform any move blocking this one. We mark
   // a move as "pending" on entry to PerformMove in order to detect cycles
   // in the move graph. We use scratch location to resolve cycles, also
   // additional pending moves might be added. After move has been performed,
   // we will update source operand in the move graph to reduce dependencies in
   // the graph.

   MoveOperands* move = moves_[index];
   DCHECK(!move->IsPending());
   DCHECK(!move->IsEliminated());
   if (move->IsRedundant()) {
     // Previous operations on the list of moves have caused this particular move
     // to become a no-op, so we can safely eliminate it. Consider for example
     // (0 -> 1) (1 -> 0) (1 -> 2). There is a cycle (0 -> 1) (1 -> 0), that we will
     // resolve as (1 -> scratch) (0 -> 1) (scratch -> 0). If, by chance, '2' is
     // used as the scratch location, the move (1 -> 2) will occur while resolving
     // the cycle. When that move is emitted, the code will update moves with a '1'
     // as their source to use '2' instead (see `UpdateMoveSource()`. In our example
     // the initial move (1 -> 2) would then become the no-op (2 -> 2) that can be
     // eliminated here.
     move->Eliminate();
     return;
   }

   // Clear this move's destination to indicate a pending move. The actual
   // destination is saved in a stack-allocated local. Recursion may allow
   // multiple moves to be pending.
   DCHECK(!move->GetSource().IsInvalid());
   Location destination = move->MarkPending();

   // Perform a depth-first traversal of the move graph to resolve
   // dependencies. Any unperformed, unpending move with a source the same
   // as this one's destination blocks this one so recursively perform all
   // such moves.
   for (size_t i = 0; i < moves_.size(); ++i) {
     const MoveOperands& other_move = *moves_[i];
     if (other_move.Blocks(destination) && !other_move.IsPending()) {
       PerformMove(i);
     }
   }

   // We are about to resolve this move and don't need it marked as
   // pending, so restore its destination.
   move->ClearPending(destination);

   // No one else should write to the move destination when the it is pending.
   DCHECK(!move->IsRedundant());

   Location source = move->GetSource();
   // The move may be blocked on several pending moves, in case we have a cycle.
   if (IsBlockedByMoves(destination)) {
     // For a cycle like: (A -> B) (B -> C) (C -> A), we change it to following
     // sequence:
     // (C -> scratch)     # Emit right now.
     // (A -> B) (B -> C)  # Unblocked.
     // (scratch -> A)     # Add to pending_moves_, blocked by (A -> B).
     Location::Kind kind = source.GetKind();
     DCHECK_NE(kind, Location::kConstant);
     Location scratch = AllocateScratchLocationFor(kind);
     // We only care about the move size.
     Primitive::Type type = move->Is64BitMove() ? Primitive::kPrimLong : Primitive::kPrimInt;
     // Perform (C -> scratch)
     move->SetDestination(scratch);
     EmitMove(index);
     move->Eliminate();
     UpdateMoveSource(source, scratch);
     // Add (scratch -> A).
     AddPendingMove(scratch, destination, type);
   } else {
     // This move is not blocked.
     EmitMove(index);
     move->Eliminate();
     UpdateMoveSource(source, destination);
   }

   // Moves in the pending list should not block any other moves. But performing
   // unblocked moves in the pending list can free scratch registers, so we do this
   // as early as possible.
   MoveOperands* pending_move;
   while ((pending_move = GetUnblockedPendingMove(source)) != nullptr) {
     Location pending_source = pending_move->GetSource();
     Location pending_destination = pending_move->GetDestination();
     // We do not depend on the pending move index. So just delete the move instead
     // of eliminating it to make the pending list cleaner.
     DeletePendingMove(pending_move);
     move->SetSource(pending_source);
     move->SetDestination(pending_destination);
     EmitMove(index);
     move->Eliminate();
     UpdateMoveSource(pending_source, pending_destination);
     // Free any unblocked locations in the scratch location list.
     // Note: Fetch size() on each iteration because scratches_ can be modified inside the loop.
     // FIXME: If FreeScratchLocation() removes the location from scratches_,
     // we skip the next location. This happens for arm64.
     for (size_t i = 0; i < scratches_.size(); ++i) {
       Location scratch = scratches_[i];
       // Only scratch overlapping with performed move source can be unblocked.
       if (scratch.OverlapsWith(pending_source) && !IsBlockedByMoves(scratch)) {
         FreeScratchLocation(pending_source);
       }
     }
   }
 }

 void ParallelMoveResolverNoSwap::UpdateMoveSource(Location from, Location to) {
   // This function is used to reduce the dependencies in the graph after
   // (from -> to) has been performed. Since we ensure there is no move with the same
   // destination, (to -> X) cannot be blocked while (from -> X) might still be
   // blocked. Consider for example the moves (0 -> 1) (1 -> 2) (1 -> 3). After
   // (1 -> 2) has been performed, the moves left are (0 -> 1) and (1 -> 3). There is
   // a dependency between the two. If we update the source location from 1 to 2, we
   // will get (0 -> 1) and (2 -> 3). There is no dependency between the two.
   //
   // This is not something we must do, but we can use fewer scratch locations with
   // this trick. For example, we can avoid using additional scratch locations for
   // moves (0 -> 1), (1 -> 2), (1 -> 0).
   for (MoveOperands* move : moves_) {
     if (move->GetSource().Equals(from)) {
       move->SetSource(to);
     }
   }
 }

 void ParallelMoveResolverNoSwap::AddPendingMove(Location source,
     Location destination, Primitive::Type type) {
   pending_moves_.push_back(new (allocator_) MoveOperands(source, destination, type, nullptr));
 }

 void ParallelMoveResolverNoSwap::DeletePendingMove(MoveOperands* move) {
   RemoveElement(pending_moves_, move);
 }

 MoveOperands* ParallelMoveResolverNoSwap::GetUnblockedPendingMove(Location loc) {
   for (MoveOperands* move : pending_moves_) {
     Location destination = move->GetDestination();
     // Only moves with destination overlapping with input loc can be unblocked.
     if (destination.OverlapsWith(loc) && !IsBlockedByMoves(destination)) {
       return move;
     }
   }
   return nullptr;
 }

 bool ParallelMoveResolverNoSwap::IsBlockedByMoves(Location loc) {
   for (MoveOperands* move : pending_moves_) {
     if (move->Blocks(loc)) {
       return true;
     }
   }
   for (MoveOperands* move : moves_) {
     if (move->Blocks(loc)) {
       return true;
     }
   }
   return false;
 }

 // So far it is only used for debugging purposes to make sure all pending moves
 // have been performed.
 size_t ParallelMoveResolverNoSwap::GetNumberOfPendingMoves() {
   return pending_moves_.size();
 }

 }  // namespace art
	/*
	* Copyright (C) 2014 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 "parallel_move_resolver.h"

	#include "base/stl_util.h"
	#include "nodes.h"

	namespace art {

	void ParallelMoveResolver::BuildInitialMoveList(HParallelMove* parallel_move) {
	// Perform a linear sweep of the moves to add them to the initial list of
	// moves to perform, ignoring any move that is redundant (the source is
	// the same as the destination, the destination is ignored and
	// unallocated, or the move was already eliminated).
	for (size_t i = 0; i < parallel_move->NumMoves(); ++i) {
	MoveOperands* move = parallel_move->MoveOperandsAt(i);
	if (!move->IsRedundant()) {
	moves_.push_back(move);
	}
	}
	}

	void ParallelMoveResolverWithSwap::EmitNativeCode(HParallelMove* parallel_move) {
	DCHECK(moves_.empty());
	// Build up a worklist of moves.
	BuildInitialMoveList(parallel_move);

	// Move stack/stack slot to take advantage of a free register on constrained machines.
	for (size_t i = 0; i < moves_.size(); ++i) {
	const MoveOperands& move = *moves_[i];
	// Ignore constants and moves already eliminated.
	if (move.IsEliminated() \|\| move.GetSource().IsConstant()) {
	continue;
	}

	if ((move.GetSource().IsStackSlot() \|\| move.GetSource().IsDoubleStackSlot()) &&
	(move.GetDestination().IsStackSlot() \|\| move.GetDestination().IsDoubleStackSlot())) {
	PerformMove(i);
	}
	}

	for (size_t i = 0; i < moves_.size(); ++i) {
	const MoveOperands& move = *moves_[i];
	// Skip constants to perform them last. They don't block other moves
	// and skipping such moves with register destinations keeps those
	// registers free for the whole algorithm.
	if (!move.IsEliminated() && !move.GetSource().IsConstant()) {
	PerformMove(i);
	}
	}

	// Perform the moves with constant sources.
	for (size_t i = 0; i < moves_.size(); ++i) {
	MoveOperands* move = moves_[i];
	if (!move->IsEliminated()) {
	DCHECK(move->GetSource().IsConstant());
	EmitMove(i);
	// Eliminate the move, in case following moves need a scratch register.
	move->Eliminate();
	}
	}

	moves_.clear();
	}

	Location LowOf(Location location) {
	if (location.IsRegisterPair()) {
	return Location::RegisterLocation(location.low());
	} else if (location.IsFpuRegisterPair()) {
	return Location::FpuRegisterLocation(location.low());
	} else if (location.IsDoubleStackSlot()) {
	return Location::StackSlot(location.GetStackIndex());
	} else {
	return Location::NoLocation();
	}
	}

	Location HighOf(Location location) {
	if (location.IsRegisterPair()) {
	return Location::RegisterLocation(location.high());
	} else if (location.IsFpuRegisterPair()) {
	return Location::FpuRegisterLocation(location.high());
	} else if (location.IsDoubleStackSlot()) {
	return Location::StackSlot(location.GetHighStackIndex(4));
	} else {
	return Location::NoLocation();
	}
	}

	// Update the source of `move`, knowing that `updated_location` has been swapped
	// with `new_source`. Note that `updated_location` can be a pair, therefore if
	// `move` is non-pair, we need to extract which register to use.
	static void UpdateSourceOf(MoveOperands* move, Location updated_location, Location new_source) {
	Location source = move->GetSource();
	if (LowOf(updated_location).Equals(source)) {
	move->SetSource(LowOf(new_source));
	} else if (HighOf(updated_location).Equals(source)) {
	move->SetSource(HighOf(new_source));
	} else {
	DCHECK(updated_location.Equals(source)) << updated_location << " " << source;
	move->SetSource(new_source);
	}
	}

	MoveOperands* ParallelMoveResolverWithSwap::PerformMove(size_t index) {
	// Each call to this function performs a move and deletes it from the move
	// graph. We first recursively perform any move blocking this one. We
	// mark a move as "pending" on entry to PerformMove in order to detect
	// cycles in the move graph. We use operand swaps to resolve cycles,
	// which means that a call to PerformMove could change any source operand
	// in the move graph.

	MoveOperands* move = moves_[index];
	DCHECK(!move->IsPending());
	if (move->IsRedundant()) {
	// Because we swap register pairs first, following, un-pending
	// moves may become redundant.
	move->Eliminate();
	return nullptr;
	}

	// Clear this move's destination to indicate a pending move. The actual
	// destination is saved in a stack-allocated local. Recursion may allow
	// multiple moves to be pending.
	DCHECK(!move->GetSource().IsInvalid());
	Location destination = move->MarkPending();

	// Perform a depth-first traversal of the move graph to resolve
	// dependencies. Any unperformed, unpending move with a source the same
	// as this one's destination blocks this one so recursively perform all
	// such moves.
	MoveOperands* required_swap = nullptr;
	for (size_t i = 0; i < moves_.size(); ++i) {
	const MoveOperands& other_move = *moves_[i];
	if (other_move.Blocks(destination) && !other_move.IsPending()) {
	// Though PerformMove can change any source operand in the move graph,
	// calling `PerformMove` cannot create a blocking move via a swap
	// (this loop does not miss any).
	// For example, assume there is a non-blocking move with source A
	// and this move is blocked on source B and there is a swap of A and
	// B. Then A and B must be involved in the same cycle (or they would
	// not be swapped). Since this move's destination is B and there is
	// only a single incoming edge to an operand, this move must also be
	// involved in the same cycle. In that case, the blocking move will
	// be created but will be "pending" when we return from PerformMove.
	required_swap = PerformMove(i);

	if (required_swap == move) {
	// If this move is required to swap, we do so without looking
	// at the next moves. Swapping is not blocked by anything, it just
	// updates other moves's source.
	break;
	} else if (required_swap == moves_[i]) {
	// If `other_move` was swapped, we iterate again to find a new
	// potential cycle.
	required_swap = nullptr;
	i = -1;
	} else if (required_swap != nullptr) {
	// A move is required to swap. We walk back the cycle to find the
	// move by just returning from this `PerformMove`.
	moves_[index]->ClearPending(destination);
	return required_swap;
	}
	}
	}

	// We are about to resolve this move and don't need it marked as
	// pending, so restore its destination.
	move->ClearPending(destination);

	// This move's source may have changed due to swaps to resolve cycles and
	// so it may now be the last move in the cycle. If so remove it.
	if (move->GetSource().Equals(destination)) {
	move->Eliminate();
	DCHECK(required_swap == nullptr);
	return nullptr;
	}

	// The move may be blocked on a (at most one) pending move, in which case
	// we have a cycle. Search for such a blocking move and perform a swap to
	// resolve it.
	bool do_swap = false;
	if (required_swap != nullptr) {
	DCHECK_EQ(required_swap, move);
	do_swap = true;
	} else {
	for (MoveOperands* other_move : moves_) {
	if (other_move->Blocks(destination)) {
	DCHECK(other_move->IsPending()) << "move=" << move << " other_move=" << other_move;
	if (!move->Is64BitMove() && other_move->Is64BitMove()) {
	// We swap 64bits moves before swapping 32bits moves. Go back from the
	// cycle by returning the move that must be swapped.
	return other_move;
	}
	do_swap = true;
	break;
	}
	}
	}

	if (do_swap) {
	EmitSwap(index);
	// Any unperformed (including pending) move with a source of either
	// this move's source or destination needs to have their source
	// changed to reflect the state of affairs after the swap.
	Location source = move->GetSource();
	Location swap_destination = move->GetDestination();
	move->Eliminate();
	for (MoveOperands* other_move : moves_) {
	if (other_move->Blocks(source)) {
	UpdateSourceOf(other_move, source, swap_destination);
	} else if (other_move->Blocks(swap_destination)) {
	UpdateSourceOf(other_move, swap_destination, source);
	}
	}
	// If the swap was required because of a 64bits move in the middle of a cycle,
	// we return the swapped move, so that the caller knows it needs to re-iterate
	// its dependency loop.
	return required_swap;
	} else {
	// This move is not blocked.
	EmitMove(index);
	move->Eliminate();
	DCHECK(required_swap == nullptr);
	return nullptr;
	}
	}

	bool ParallelMoveResolverWithSwap::IsScratchLocation(Location loc) {
	for (MoveOperands* move : moves_) {
	if (move->Blocks(loc)) {
	return false;
	}
	}

	for (MoveOperands* move : moves_) {
	if (move->GetDestination().Equals(loc)) {
	return true;
	}
	}

	return false;
	}

	int ParallelMoveResolverWithSwap::AllocateScratchRegister(int blocked,
	int register_count,
	int if_scratch,
	bool* spilled) {
	DCHECK_NE(blocked, if_scratch);
	int scratch = -1;
	for (int reg = 0; reg < register_count; ++reg) {
	if ((blocked != reg) && IsScratchLocation(Location::RegisterLocation(reg))) {
	scratch = reg;
	break;
	}
	}

	if (scratch == -1) {
	*spilled = true;
	scratch = if_scratch;
	} else {
	*spilled = false;
	}

	return scratch;
	}


	ParallelMoveResolverWithSwap::ScratchRegisterScope::ScratchRegisterScope(
	ParallelMoveResolverWithSwap* resolver, int blocked, int if_scratch, int number_of_registers)
	: resolver_(resolver),
	reg_(kNoRegister),
	spilled_(false) {
	reg_ = resolver_->AllocateScratchRegister(blocked, number_of_registers, if_scratch, &spilled_);

	if (spilled_) {
	resolver->SpillScratch(reg_);
	}
	}


	ParallelMoveResolverWithSwap::ScratchRegisterScope::~ScratchRegisterScope() {
	if (spilled_) {
	resolver_->RestoreScratch(reg_);
	}
	}

	void ParallelMoveResolverNoSwap::EmitNativeCode(HParallelMove* parallel_move) {
	DCHECK_EQ(GetNumberOfPendingMoves(), 0u);
	DCHECK(moves_.empty());
	DCHECK(scratches_.empty());

	// Backend dependent initialization.
	PrepareForEmitNativeCode();

	// Build up a worklist of moves.
	BuildInitialMoveList(parallel_move);

	for (size_t i = 0; i < moves_.size(); ++i) {
	const MoveOperands& move = *moves_[i];
	// Skip constants to perform them last. They don't block other moves and
	// skipping such moves with register destinations keeps those registers
	// free for the whole algorithm.
	if (!move.IsEliminated() && !move.GetSource().IsConstant()) {
	PerformMove(i);
	}
	}

	// Perform the moves with constant sources and register destinations with UpdateMoveSource()
	// to reduce the number of literal loads. Stack destinations are skipped since we won't be benefit
	// from changing the constant sources to stack locations.
	for (size_t i = 0; i < moves_.size(); ++i) {
	MoveOperands* move = moves_[i];
	Location destination = move->GetDestination();
	if (!move->IsEliminated() && !destination.IsStackSlot() && !destination.IsDoubleStackSlot()) {
	Location source = move->GetSource();
	EmitMove(i);
	move->Eliminate();
	// This may introduce additional instruction dependency, but reduce number
	// of moves and possible literal loads. For example,
	// Original moves:
	// 1234.5678 -> D0
	// 1234.5678 -> D1
	// Updated moves:
	// 1234.5678 -> D0
	// D0 -> D1
	UpdateMoveSource(source, destination);
	}
	}

	// Perform the rest of the moves.
	for (size_t i = 0; i < moves_.size(); ++i) {
	MoveOperands* move = moves_[i];
	if (!move->IsEliminated()) {
	EmitMove(i);
	move->Eliminate();
	}
	}

	// All pending moves that we have added for resolve cycles should be performed.
	DCHECK_EQ(GetNumberOfPendingMoves(), 0u);

	// Backend dependent cleanup.
	FinishEmitNativeCode();

	moves_.clear();
	scratches_.clear();
	}

	Location ParallelMoveResolverNoSwap::GetScratchLocation(Location::Kind kind) {
	for (Location loc : scratches_) {
	if (loc.GetKind() == kind && !IsBlockedByMoves(loc)) {
	return loc;
	}
	}
	for (MoveOperands* move : moves_) {
	Location loc = move->GetDestination();
	if (loc.GetKind() == kind && !IsBlockedByMoves(loc)) {
	return loc;
	}
	}
	return Location::NoLocation();
	}

	void ParallelMoveResolverNoSwap::AddScratchLocation(Location loc) {
	if (kIsDebugBuild) {
	for (Location scratch : scratches_) {
	CHECK(!loc.Equals(scratch));
	}
	}
	scratches_.push_back(loc);
	}

	void ParallelMoveResolverNoSwap::RemoveScratchLocation(Location loc) {
	DCHECK(!IsBlockedByMoves(loc));
	for (auto it = scratches_.begin(), end = scratches_.end(); it != end; ++it) {
	if (loc.Equals(*it)) {
	scratches_.erase(it);
	break;
	}
	}
	}

	void ParallelMoveResolverNoSwap::PerformMove(size_t index) {
	// Each call to this function performs a move and deletes it from the move
	// graph. We first recursively perform any move blocking this one. We mark
	// a move as "pending" on entry to PerformMove in order to detect cycles
	// in the move graph. We use scratch location to resolve cycles, also
	// additional pending moves might be added. After move has been performed,
	// we will update source operand in the move graph to reduce dependencies in
	// the graph.

	MoveOperands* move = moves_[index];
	DCHECK(!move->IsPending());
	DCHECK(!move->IsEliminated());
	if (move->IsRedundant()) {
	// Previous operations on the list of moves have caused this particular move
	// to become a no-op, so we can safely eliminate it. Consider for example
	// (0 -> 1) (1 -> 0) (1 -> 2). There is a cycle (0 -> 1) (1 -> 0), that we will
	// resolve as (1 -> scratch) (0 -> 1) (scratch -> 0). If, by chance, '2' is
	// used as the scratch location, the move (1 -> 2) will occur while resolving
	// the cycle. When that move is emitted, the code will update moves with a '1'
	// as their source to use '2' instead (see `UpdateMoveSource()`. In our example
	// the initial move (1 -> 2) would then become the no-op (2 -> 2) that can be
	// eliminated here.
	move->Eliminate();
	return;
	}

	// Clear this move's destination to indicate a pending move. The actual
	// destination is saved in a stack-allocated local. Recursion may allow
	// multiple moves to be pending.
	DCHECK(!move->GetSource().IsInvalid());
	Location destination = move->MarkPending();

	// Perform a depth-first traversal of the move graph to resolve
	// dependencies. Any unperformed, unpending move with a source the same
	// as this one's destination blocks this one so recursively perform all
	// such moves.
	for (size_t i = 0; i < moves_.size(); ++i) {
	const MoveOperands& other_move = *moves_[i];
	if (other_move.Blocks(destination) && !other_move.IsPending()) {
	PerformMove(i);
	}
	}

	// We are about to resolve this move and don't need it marked as
	// pending, so restore its destination.
	move->ClearPending(destination);

	// No one else should write to the move destination when the it is pending.
	DCHECK(!move->IsRedundant());

	Location source = move->GetSource();
	// The move may be blocked on several pending moves, in case we have a cycle.
	if (IsBlockedByMoves(destination)) {
	// For a cycle like: (A -> B) (B -> C) (C -> A), we change it to following
	// sequence:
	// (C -> scratch) # Emit right now.
	// (A -> B) (B -> C) # Unblocked.
	// (scratch -> A) # Add to pending_moves_, blocked by (A -> B).
	Location::Kind kind = source.GetKind();
	DCHECK_NE(kind, Location::kConstant);
	Location scratch = AllocateScratchLocationFor(kind);
	// We only care about the move size.
	Primitive::Type type = move->Is64BitMove() ? Primitive::kPrimLong : Primitive::kPrimInt;
	// Perform (C -> scratch)
	move->SetDestination(scratch);
	EmitMove(index);
	move->Eliminate();
	UpdateMoveSource(source, scratch);
	// Add (scratch -> A).
	AddPendingMove(scratch, destination, type);
	} else {
	// This move is not blocked.
	EmitMove(index);
	move->Eliminate();
	UpdateMoveSource(source, destination);
	}

	// Moves in the pending list should not block any other moves. But performing
	// unblocked moves in the pending list can free scratch registers, so we do this
	// as early as possible.
	MoveOperands* pending_move;
	while ((pending_move = GetUnblockedPendingMove(source)) != nullptr) {
	Location pending_source = pending_move->GetSource();
	Location pending_destination = pending_move->GetDestination();
	// We do not depend on the pending move index. So just delete the move instead
	// of eliminating it to make the pending list cleaner.
	DeletePendingMove(pending_move);
	move->SetSource(pending_source);
	move->SetDestination(pending_destination);
	EmitMove(index);
	move->Eliminate();
	UpdateMoveSource(pending_source, pending_destination);
	// Free any unblocked locations in the scratch location list.
	// Note: Fetch size() on each iteration because scratches_ can be modified inside the loop.
	// FIXME: If FreeScratchLocation() removes the location from scratches_,
	// we skip the next location. This happens for arm64.
	for (size_t i = 0; i < scratches_.size(); ++i) {
	Location scratch = scratches_[i];
	// Only scratch overlapping with performed move source can be unblocked.
	if (scratch.OverlapsWith(pending_source) && !IsBlockedByMoves(scratch)) {
	FreeScratchLocation(pending_source);
	}
	}
	}
	}

	void ParallelMoveResolverNoSwap::UpdateMoveSource(Location from, Location to) {
	// This function is used to reduce the dependencies in the graph after
	// (from -> to) has been performed. Since we ensure there is no move with the same
	// destination, (to -> X) cannot be blocked while (from -> X) might still be
	// blocked. Consider for example the moves (0 -> 1) (1 -> 2) (1 -> 3). After
	// (1 -> 2) has been performed, the moves left are (0 -> 1) and (1 -> 3). There is
	// a dependency between the two. If we update the source location from 1 to 2, we
	// will get (0 -> 1) and (2 -> 3). There is no dependency between the two.
	//
	// This is not something we must do, but we can use fewer scratch locations with
	// this trick. For example, we can avoid using additional scratch locations for
	// moves (0 -> 1), (1 -> 2), (1 -> 0).
	for (MoveOperands* move : moves_) {
	if (move->GetSource().Equals(from)) {
	move->SetSource(to);
	}
	}
	}

	void ParallelMoveResolverNoSwap::AddPendingMove(Location source,
	Location destination, Primitive::Type type) {
	pending_moves_.push_back(new (allocator_) MoveOperands(source, destination, type, nullptr));
	}

	void ParallelMoveResolverNoSwap::DeletePendingMove(MoveOperands* move) {
	RemoveElement(pending_moves_, move);
	}

	MoveOperands* ParallelMoveResolverNoSwap::GetUnblockedPendingMove(Location loc) {
	for (MoveOperands* move : pending_moves_) {
	Location destination = move->GetDestination();
	// Only moves with destination overlapping with input loc can be unblocked.
	if (destination.OverlapsWith(loc) && !IsBlockedByMoves(destination)) {
	return move;
	}
	}
	return nullptr;
	}

	bool ParallelMoveResolverNoSwap::IsBlockedByMoves(Location loc) {
	for (MoveOperands* move : pending_moves_) {
	if (move->Blocks(loc)) {
	return true;
	}
	}
	for (MoveOperands* move : moves_) {
	if (move->Blocks(loc)) {
	return true;
	}
	}
	return false;
	}

	// So far it is only used for debugging purposes to make sure all pending moves
	// have been performed.
	size_t ParallelMoveResolverNoSwap::GetNumberOfPendingMoves() {
	return pending_moves_.size();
	}

	} // namespace art