compiler/optimizing/select_generator.cc - LeafOS-Project/android_art - Gitiles

 /*
  * Copyright (C) 2016 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 "select_generator.h"

 #include "optimizing/nodes.h"
 #include "reference_type_propagation.h"

 namespace art HIDDEN {

 static constexpr size_t kMaxInstructionsInBranch = 1u;

 HSelectGenerator::HSelectGenerator(HGraph* graph,
                                    OptimizingCompilerStats* stats,
                                    const char* name)
     : HOptimization(graph, name, stats) {
 }

 // Returns true if `block` has only one predecessor, ends with a Goto
 // or a Return and contains at most `kMaxInstructionsInBranch` other
 // movable instruction with no side-effects.
 static bool IsSimpleBlock(HBasicBlock* block) {
   if (block->GetPredecessors().size() != 1u) {
     return false;
   }
   DCHECK(block->GetPhis().IsEmpty());

   size_t num_instructions = 0u;
   for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
     HInstruction* instruction = it.Current();
     if (instruction->IsControlFlow()) {
       return instruction->IsGoto() || instruction->IsReturn();
     } else if (instruction->CanBeMoved() &&
                !instruction->HasSideEffects() &&
                !instruction->CanThrow()) {
       if (instruction->IsSelect() &&
           instruction->AsSelect()->GetCondition()->GetBlock() == block) {
         // Count one HCondition and HSelect in the same block as a single instruction.
         // This enables finding nested selects.
         continue;
       } else if (++num_instructions > kMaxInstructionsInBranch) {
         return false;  // bail as soon as we exceed number of allowed instructions
       }
     } else {
       return false;
     }
   }

   LOG(FATAL) << "Unreachable";
   UNREACHABLE();
 }

 // Returns true if 'block1' and 'block2' are empty and merge into the
 // same single successor.
 static bool BlocksMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
   return block1->GetSingleSuccessor() == block2->GetSingleSuccessor();
 }

 // Returns nullptr if `block` has either no phis or there is more than one phi. Otherwise returns
 // that phi.
 static HPhi* GetSinglePhi(HBasicBlock* block, size_t index1, size_t index2) {
   DCHECK_NE(index1, index2);

   HPhi* select_phi = nullptr;
   for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
     HPhi* phi = it.Current()->AsPhi();
     if (select_phi == nullptr) {
       // First phi found.
       select_phi = phi;
     } else {
       // More than one phi found, return null.
       return nullptr;
     }
   }
   return select_phi;
 }

 bool HSelectGenerator::TryGenerateSelectSimpleDiamondPattern(
     HBasicBlock* block, ScopedArenaSafeMap<HInstruction*, HSelect*>* cache) {
   DCHECK(block->GetLastInstruction()->IsIf());
   HIf* if_instruction = block->GetLastInstruction()->AsIf();
   HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
   HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
   DCHECK_NE(true_block, false_block);

   if (!IsSimpleBlock(true_block) ||
       !IsSimpleBlock(false_block) ||
       !BlocksMergeTogether(true_block, false_block)) {
     return false;
   }
   HBasicBlock* merge_block = true_block->GetSingleSuccessor();

   // If the branches are not empty, move instructions in front of the If.
   // TODO(dbrazdil): This puts an instruction between If and its condition.
   //                 Implement moving of conditions to first users if possible.
   while (!true_block->IsSingleGoto() && !true_block->IsSingleReturn()) {
     HInstruction* instr = true_block->GetFirstInstruction();
     DCHECK(!instr->CanThrow());
     instr->MoveBefore(if_instruction);
   }
   while (!false_block->IsSingleGoto() && !false_block->IsSingleReturn()) {
     HInstruction* instr = false_block->GetFirstInstruction();
     DCHECK(!instr->CanThrow());
     instr->MoveBefore(if_instruction);
   }
   DCHECK(true_block->IsSingleGoto() || true_block->IsSingleReturn());
   DCHECK(false_block->IsSingleGoto() || false_block->IsSingleReturn());

   // Find the resulting true/false values.
   size_t predecessor_index_true = merge_block->GetPredecessorIndexOf(true_block);
   size_t predecessor_index_false = merge_block->GetPredecessorIndexOf(false_block);
   DCHECK_NE(predecessor_index_true, predecessor_index_false);

   bool both_successors_return = true_block->IsSingleReturn() && false_block->IsSingleReturn();
   // TODO(solanes): Extend to support multiple phis? e.g.
   //   int a, b;
   //   if (bool) {
   //     a = 0; b = 1;
   //   } else {
   //     a = 1; b = 2;
   //   }
   //   // use a and b
   HPhi* phi = GetSinglePhi(merge_block, predecessor_index_true, predecessor_index_false);

   HInstruction* true_value = nullptr;
   HInstruction* false_value = nullptr;
   if (both_successors_return) {
     true_value = true_block->GetFirstInstruction()->InputAt(0);
     false_value = false_block->GetFirstInstruction()->InputAt(0);
   } else if (phi != nullptr) {
     true_value = phi->InputAt(predecessor_index_true);
     false_value = phi->InputAt(predecessor_index_false);
   } else {
     return false;
   }
   DCHECK(both_successors_return || phi != nullptr);

   // Create the Select instruction and insert it in front of the If.
   HInstruction* condition = if_instruction->InputAt(0);
   HSelect* select = new (graph_->GetAllocator()) HSelect(condition,
                                                           true_value,
                                                           false_value,
                                                           if_instruction->GetDexPc());
   if (both_successors_return) {
     if (true_value->GetType() == DataType::Type::kReference) {
       DCHECK(false_value->GetType() == DataType::Type::kReference);
       ReferenceTypePropagation::FixUpInstructionType(select, graph_->GetHandleCache());
     }
   } else if (phi->GetType() == DataType::Type::kReference) {
     select->SetReferenceTypeInfoIfValid(phi->GetReferenceTypeInfo());
   }
   block->InsertInstructionBefore(select, if_instruction);

   // Remove the true branch which removes the corresponding Phi
   // input if needed. If left only with the false branch, the Phi is
   // automatically removed.
   if (both_successors_return) {
     false_block->GetFirstInstruction()->ReplaceInput(select, 0);
   } else {
     phi->ReplaceInput(select, predecessor_index_false);
   }

   bool only_two_predecessors = (merge_block->GetPredecessors().size() == 2u);
   true_block->DisconnectAndDelete();

   // Merge remaining blocks which are now connected with Goto.
   DCHECK_EQ(block->GetSingleSuccessor(), false_block);
   block->MergeWith(false_block);
   if (!both_successors_return && only_two_predecessors) {
     DCHECK_EQ(only_two_predecessors, phi->GetBlock() == nullptr);
     DCHECK_EQ(block->GetSingleSuccessor(), merge_block);
     block->MergeWith(merge_block);
   }

   MaybeRecordStat(stats_, MethodCompilationStat::kSelectGenerated);

   // Very simple way of finding common subexpressions in the generated HSelect statements
   // (since this runs after GVN). Lookup by condition, and reuse latest one if possible
   // (due to post order, latest select is most likely replacement). If needed, we could
   // improve this by e.g. using the operands in the map as well.
   auto it = cache->find(condition);
   if (it == cache->end()) {
     cache->Put(condition, select);
   } else {
     // Found cached value. See if latest can replace cached in the HIR.
     HSelect* cached_select = it->second;
     DCHECK_EQ(cached_select->GetCondition(), select->GetCondition());
     if (cached_select->GetTrueValue() == select->GetTrueValue() &&
         cached_select->GetFalseValue() == select->GetFalseValue() &&
         select->StrictlyDominates(cached_select)) {
       cached_select->ReplaceWith(select);
       cached_select->GetBlock()->RemoveInstruction(cached_select);
     }
     it->second = select;  // always cache latest
   }

   // No need to update dominance information, as we are simplifying
   // a simple diamond shape, where the join block is merged with the
   // entry block. Any following blocks would have had the join block
   // as a dominator, and `MergeWith` handles changing that to the
   // entry block
   return true;
 }

 HBasicBlock* HSelectGenerator::TryFixupDoubleDiamondPattern(HBasicBlock* block) {
   DCHECK(block->GetLastInstruction()->IsIf());
   HIf* if_instruction = block->GetLastInstruction()->AsIf();
   HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
   HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
   DCHECK_NE(true_block, false_block);

   // One branch must be a single goto, and the other one the inner if.
   if (true_block->IsSingleGoto() == false_block->IsSingleGoto()) {
     return nullptr;
   }

   HBasicBlock* single_goto = true_block->IsSingleGoto() ? true_block : false_block;
   HBasicBlock* inner_if_block = true_block->IsSingleGoto() ? false_block : true_block;

   // The innner if branch has to be a block with just a comparison and an if.
   if (!inner_if_block->EndsWithIf() ||
       inner_if_block->GetLastInstruction()->AsIf()->InputAt(0) !=
           inner_if_block->GetFirstInstruction() ||
       inner_if_block->GetLastInstruction()->GetPrevious() !=
           inner_if_block->GetFirstInstruction() ||
       !inner_if_block->GetFirstInstruction()->IsCondition()) {
     return nullptr;
   }

   HIf* inner_if_instruction = inner_if_block->GetLastInstruction()->AsIf();
   HBasicBlock* inner_if_true_block = inner_if_instruction->IfTrueSuccessor();
   HBasicBlock* inner_if_false_block = inner_if_instruction->IfFalseSuccessor();
   if (!inner_if_true_block->IsSingleGoto() || !inner_if_false_block->IsSingleGoto()) {
     return nullptr;
   }

   // One must merge into the outer condition and the other must not.
   if (BlocksMergeTogether(single_goto, inner_if_true_block) ==
       BlocksMergeTogether(single_goto, inner_if_false_block)) {
     return nullptr;
   }

   // First merge merges the outer if with one of the inner if branches. The block must be a Phi and
   // a Goto.
   HBasicBlock* first_merge = single_goto->GetSingleSuccessor();
   if (first_merge->GetNumberOfPredecessors() != 2 ||
       first_merge->GetPhis().CountSize() != 1 ||
       !first_merge->GetLastInstruction()->IsGoto() ||
       first_merge->GetFirstInstruction() != first_merge->GetLastInstruction()) {
     return nullptr;
   }

   HPhi* first_phi = first_merge->GetFirstPhi()->AsPhi();

   // Second merge is first_merge and the remainder branch merging. It must be phi + goto, or phi +
   // return. Depending on the first merge, we define the second merge.
   HBasicBlock* merges_into_second_merge =
     BlocksMergeTogether(single_goto, inner_if_true_block)
       ? inner_if_false_block
       : inner_if_true_block;
   if (!BlocksMergeTogether(first_merge, merges_into_second_merge)) {
     return nullptr;
   }

   HBasicBlock* second_merge = merges_into_second_merge->GetSingleSuccessor();
   if (second_merge->GetNumberOfPredecessors() != 2 ||
       second_merge->GetPhis().CountSize() != 1 ||
       !(second_merge->GetLastInstruction()->IsGoto() ||
         second_merge->GetLastInstruction()->IsReturn()) ||
       second_merge->GetFirstInstruction() != second_merge->GetLastInstruction()) {
     return nullptr;
   }

   size_t index = second_merge->GetPredecessorIndexOf(merges_into_second_merge);
   HPhi* second_phi = second_merge->GetFirstPhi()->AsPhi();

   // Merge the phis.
   first_phi->AddInput(second_phi->InputAt(index));
   merges_into_second_merge->ReplaceSuccessor(second_merge, first_merge);
   second_phi->ReplaceWith(first_phi);
   second_merge->RemovePhi(second_phi);

   // Sort out the new domination before merging the blocks
   DCHECK_EQ(second_merge->GetSinglePredecessor(), first_merge);
   second_merge->GetDominator()->RemoveDominatedBlock(second_merge);
   second_merge->SetDominator(first_merge);
   first_merge->AddDominatedBlock(second_merge);
   first_merge->MergeWith(second_merge);

   // No need to update dominance information. There's a chance that `merges_into_second_merge`
   // doesn't come before `first_merge` but we don't need to fix it since `merges_into_second_merge`
   // will disappear from the graph altogether when doing the follow-up
   // TryGenerateSelectSimpleDiamondPattern.

   return inner_if_block;
 }

 bool HSelectGenerator::Run() {
   bool did_select = false;
   // Select cache with local allocator.
   ScopedArenaAllocator allocator(graph_->GetArenaStack());
   ScopedArenaSafeMap<HInstruction*, HSelect*> cache(std::less<HInstruction*>(),
                                                     allocator.Adapter(kArenaAllocSelectGenerator));

   // Iterate in post order in the unlikely case that removing one occurrence of
   // the selection pattern empties a branch block of another occurrence.
   for (HBasicBlock* block : graph_->GetPostOrder()) {
     if (!block->EndsWithIf()) {
       continue;
     }

     if (TryGenerateSelectSimpleDiamondPattern(block, &cache)) {
       did_select = true;
     } else {
       // Try to fix up the odd version of the double diamond pattern. If we could do it, it means
       // that we can generate two selects.
       HBasicBlock* inner_if_block = TryFixupDoubleDiamondPattern(block);
       if (inner_if_block != nullptr) {
         // Generate the selects now since `inner_if_block` should be after `block` in PostOrder.
         bool result = TryGenerateSelectSimpleDiamondPattern(inner_if_block, &cache);
         DCHECK(result);
         result = TryGenerateSelectSimpleDiamondPattern(block, &cache);
         DCHECK(result);
         did_select = true;
       }
     }
   }

   return did_select;
 }

 }  // namespace art
	/*
	* Copyright (C) 2016 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 "select_generator.h"

	#include "optimizing/nodes.h"
	#include "reference_type_propagation.h"

	namespace art HIDDEN {

	static constexpr size_t kMaxInstructionsInBranch = 1u;

	HSelectGenerator::HSelectGenerator(HGraph* graph,
	OptimizingCompilerStats* stats,
	const char* name)
	: HOptimization(graph, name, stats) {
	}

	// Returns true if `block` has only one predecessor, ends with a Goto
	// or a Return and contains at most `kMaxInstructionsInBranch` other
	// movable instruction with no side-effects.
	static bool IsSimpleBlock(HBasicBlock* block) {
	if (block->GetPredecessors().size() != 1u) {
	return false;
	}
	DCHECK(block->GetPhis().IsEmpty());

	size_t num_instructions = 0u;
	for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
	HInstruction* instruction = it.Current();
	if (instruction->IsControlFlow()) {
	return instruction->IsGoto() \|\| instruction->IsReturn();
	} else if (instruction->CanBeMoved() &&
	!instruction->HasSideEffects() &&
	!instruction->CanThrow()) {
	if (instruction->IsSelect() &&
	instruction->AsSelect()->GetCondition()->GetBlock() == block) {
	// Count one HCondition and HSelect in the same block as a single instruction.
	// This enables finding nested selects.
	continue;
	} else if (++num_instructions > kMaxInstructionsInBranch) {
	return false; // bail as soon as we exceed number of allowed instructions
	}
	} else {
	return false;
	}
	}

	LOG(FATAL) << "Unreachable";
	UNREACHABLE();
	}

	// Returns true if 'block1' and 'block2' are empty and merge into the
	// same single successor.
	static bool BlocksMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
	return block1->GetSingleSuccessor() == block2->GetSingleSuccessor();
	}

	// Returns nullptr if `block` has either no phis or there is more than one phi. Otherwise returns
	// that phi.
	static HPhi* GetSinglePhi(HBasicBlock* block, size_t index1, size_t index2) {
	DCHECK_NE(index1, index2);

	HPhi* select_phi = nullptr;
	for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
	HPhi* phi = it.Current()->AsPhi();
	if (select_phi == nullptr) {
	// First phi found.
	select_phi = phi;
	} else {
	// More than one phi found, return null.
	return nullptr;
	}
	}
	return select_phi;
	}

	bool HSelectGenerator::TryGenerateSelectSimpleDiamondPattern(
	HBasicBlock* block, ScopedArenaSafeMap<HInstruction, HSelect>* cache) {
	DCHECK(block->GetLastInstruction()->IsIf());
	HIf* if_instruction = block->GetLastInstruction()->AsIf();
	HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
	HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
	DCHECK_NE(true_block, false_block);

	if (!IsSimpleBlock(true_block) \|\|
	!IsSimpleBlock(false_block) \|\|
	!BlocksMergeTogether(true_block, false_block)) {
	return false;
	}
	HBasicBlock* merge_block = true_block->GetSingleSuccessor();

	// If the branches are not empty, move instructions in front of the If.
	// TODO(dbrazdil): This puts an instruction between If and its condition.
	// Implement moving of conditions to first users if possible.
	while (!true_block->IsSingleGoto() && !true_block->IsSingleReturn()) {
	HInstruction* instr = true_block->GetFirstInstruction();
	DCHECK(!instr->CanThrow());
	instr->MoveBefore(if_instruction);
	}
	while (!false_block->IsSingleGoto() && !false_block->IsSingleReturn()) {
	HInstruction* instr = false_block->GetFirstInstruction();
	DCHECK(!instr->CanThrow());
	instr->MoveBefore(if_instruction);
	}
	DCHECK(true_block->IsSingleGoto() \|\| true_block->IsSingleReturn());
	DCHECK(false_block->IsSingleGoto() \|\| false_block->IsSingleReturn());

	// Find the resulting true/false values.
	size_t predecessor_index_true = merge_block->GetPredecessorIndexOf(true_block);
	size_t predecessor_index_false = merge_block->GetPredecessorIndexOf(false_block);
	DCHECK_NE(predecessor_index_true, predecessor_index_false);

	bool both_successors_return = true_block->IsSingleReturn() && false_block->IsSingleReturn();
	// TODO(solanes): Extend to support multiple phis? e.g.
	// int a, b;
	// if (bool) {
	// a = 0; b = 1;
	// } else {
	// a = 1; b = 2;
	// }
	// // use a and b
	HPhi* phi = GetSinglePhi(merge_block, predecessor_index_true, predecessor_index_false);

	HInstruction* true_value = nullptr;
	HInstruction* false_value = nullptr;
	if (both_successors_return) {
	true_value = true_block->GetFirstInstruction()->InputAt(0);
	false_value = false_block->GetFirstInstruction()->InputAt(0);
	} else if (phi != nullptr) {
	true_value = phi->InputAt(predecessor_index_true);
	false_value = phi->InputAt(predecessor_index_false);
	} else {
	return false;
	}
	DCHECK(both_successors_return \|\| phi != nullptr);

	// Create the Select instruction and insert it in front of the If.
	HInstruction* condition = if_instruction->InputAt(0);
	HSelect* select = new (graph_->GetAllocator()) HSelect(condition,
	true_value,
	false_value,
	if_instruction->GetDexPc());
	if (both_successors_return) {
	if (true_value->GetType() == DataType::Type::kReference) {
	DCHECK(false_value->GetType() == DataType::Type::kReference);
	ReferenceTypePropagation::FixUpInstructionType(select, graph_->GetHandleCache());
	}
	} else if (phi->GetType() == DataType::Type::kReference) {
	select->SetReferenceTypeInfoIfValid(phi->GetReferenceTypeInfo());
	}
	block->InsertInstructionBefore(select, if_instruction);

	// Remove the true branch which removes the corresponding Phi
	// input if needed. If left only with the false branch, the Phi is
	// automatically removed.
	if (both_successors_return) {
	false_block->GetFirstInstruction()->ReplaceInput(select, 0);
	} else {
	phi->ReplaceInput(select, predecessor_index_false);
	}

	bool only_two_predecessors = (merge_block->GetPredecessors().size() == 2u);
	true_block->DisconnectAndDelete();

	// Merge remaining blocks which are now connected with Goto.
	DCHECK_EQ(block->GetSingleSuccessor(), false_block);
	block->MergeWith(false_block);
	if (!both_successors_return && only_two_predecessors) {
	DCHECK_EQ(only_two_predecessors, phi->GetBlock() == nullptr);
	DCHECK_EQ(block->GetSingleSuccessor(), merge_block);
	block->MergeWith(merge_block);
	}

	MaybeRecordStat(stats_, MethodCompilationStat::kSelectGenerated);

	// Very simple way of finding common subexpressions in the generated HSelect statements
	// (since this runs after GVN). Lookup by condition, and reuse latest one if possible
	// (due to post order, latest select is most likely replacement). If needed, we could
	// improve this by e.g. using the operands in the map as well.
	auto it = cache->find(condition);
	if (it == cache->end()) {
	cache->Put(condition, select);
	} else {
	// Found cached value. See if latest can replace cached in the HIR.
	HSelect* cached_select = it->second;
	DCHECK_EQ(cached_select->GetCondition(), select->GetCondition());
	if (cached_select->GetTrueValue() == select->GetTrueValue() &&
	cached_select->GetFalseValue() == select->GetFalseValue() &&
	select->StrictlyDominates(cached_select)) {
	cached_select->ReplaceWith(select);
	cached_select->GetBlock()->RemoveInstruction(cached_select);
	}
	it->second = select; // always cache latest
	}

	// No need to update dominance information, as we are simplifying
	// a simple diamond shape, where the join block is merged with the
	// entry block. Any following blocks would have had the join block
	// as a dominator, and `MergeWith` handles changing that to the
	// entry block
	return true;
	}

	HBasicBlock* HSelectGenerator::TryFixupDoubleDiamondPattern(HBasicBlock* block) {
	DCHECK(block->GetLastInstruction()->IsIf());
	HIf* if_instruction = block->GetLastInstruction()->AsIf();
	HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
	HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
	DCHECK_NE(true_block, false_block);

	// One branch must be a single goto, and the other one the inner if.
	if (true_block->IsSingleGoto() == false_block->IsSingleGoto()) {
	return nullptr;
	}

	HBasicBlock* single_goto = true_block->IsSingleGoto() ? true_block : false_block;
	HBasicBlock* inner_if_block = true_block->IsSingleGoto() ? false_block : true_block;

	// The innner if branch has to be a block with just a comparison and an if.
	if (!inner_if_block->EndsWithIf() \|\|
	inner_if_block->GetLastInstruction()->AsIf()->InputAt(0) !=
	inner_if_block->GetFirstInstruction() \|\|
	inner_if_block->GetLastInstruction()->GetPrevious() !=
	inner_if_block->GetFirstInstruction() \|\|
	!inner_if_block->GetFirstInstruction()->IsCondition()) {
	return nullptr;
	}

	HIf* inner_if_instruction = inner_if_block->GetLastInstruction()->AsIf();
	HBasicBlock* inner_if_true_block = inner_if_instruction->IfTrueSuccessor();
	HBasicBlock* inner_if_false_block = inner_if_instruction->IfFalseSuccessor();
	if (!inner_if_true_block->IsSingleGoto() \|\| !inner_if_false_block->IsSingleGoto()) {
	return nullptr;
	}

	// One must merge into the outer condition and the other must not.
	if (BlocksMergeTogether(single_goto, inner_if_true_block) ==
	BlocksMergeTogether(single_goto, inner_if_false_block)) {
	return nullptr;
	}

	// First merge merges the outer if with one of the inner if branches. The block must be a Phi and
	// a Goto.
	HBasicBlock* first_merge = single_goto->GetSingleSuccessor();
	if (first_merge->GetNumberOfPredecessors() != 2 \|\|
	first_merge->GetPhis().CountSize() != 1 \|\|
	!first_merge->GetLastInstruction()->IsGoto() \|\|
	first_merge->GetFirstInstruction() != first_merge->GetLastInstruction()) {
	return nullptr;
	}

	HPhi* first_phi = first_merge->GetFirstPhi()->AsPhi();

	// Second merge is first_merge and the remainder branch merging. It must be phi + goto, or phi +
	// return. Depending on the first merge, we define the second merge.
	HBasicBlock* merges_into_second_merge =
	BlocksMergeTogether(single_goto, inner_if_true_block)
	? inner_if_false_block
	: inner_if_true_block;
	if (!BlocksMergeTogether(first_merge, merges_into_second_merge)) {
	return nullptr;
	}

	HBasicBlock* second_merge = merges_into_second_merge->GetSingleSuccessor();
	if (second_merge->GetNumberOfPredecessors() != 2 \|\|
	second_merge->GetPhis().CountSize() != 1 \|\|
	!(second_merge->GetLastInstruction()->IsGoto() \|\|
	second_merge->GetLastInstruction()->IsReturn()) \|\|
	second_merge->GetFirstInstruction() != second_merge->GetLastInstruction()) {
	return nullptr;
	}

	size_t index = second_merge->GetPredecessorIndexOf(merges_into_second_merge);
	HPhi* second_phi = second_merge->GetFirstPhi()->AsPhi();

	// Merge the phis.
	first_phi->AddInput(second_phi->InputAt(index));
	merges_into_second_merge->ReplaceSuccessor(second_merge, first_merge);
	second_phi->ReplaceWith(first_phi);
	second_merge->RemovePhi(second_phi);

	// Sort out the new domination before merging the blocks
	DCHECK_EQ(second_merge->GetSinglePredecessor(), first_merge);
	second_merge->GetDominator()->RemoveDominatedBlock(second_merge);
	second_merge->SetDominator(first_merge);
	first_merge->AddDominatedBlock(second_merge);
	first_merge->MergeWith(second_merge);

	// No need to update dominance information. There's a chance that `merges_into_second_merge`
	// doesn't come before `first_merge` but we don't need to fix it since `merges_into_second_merge`
	// will disappear from the graph altogether when doing the follow-up
	// TryGenerateSelectSimpleDiamondPattern.

	return inner_if_block;
	}

	bool HSelectGenerator::Run() {
	bool did_select = false;
	// Select cache with local allocator.
	ScopedArenaAllocator allocator(graph_->GetArenaStack());
	ScopedArenaSafeMap<HInstruction, HSelect> cache(std::less<HInstruction*>(),
	allocator.Adapter(kArenaAllocSelectGenerator));

	// Iterate in post order in the unlikely case that removing one occurrence of
	// the selection pattern empties a branch block of another occurrence.
	for (HBasicBlock* block : graph_->GetPostOrder()) {
	if (!block->EndsWithIf()) {
	continue;
	}

	if (TryGenerateSelectSimpleDiamondPattern(block, &cache)) {
	did_select = true;
	} else {
	// Try to fix up the odd version of the double diamond pattern. If we could do it, it means
	// that we can generate two selects.
	HBasicBlock* inner_if_block = TryFixupDoubleDiamondPattern(block);
	if (inner_if_block != nullptr) {
	// Generate the selects now since `inner_if_block` should be after `block` in PostOrder.
	bool result = TryGenerateSelectSimpleDiamondPattern(inner_if_block, &cache);
	DCHECK(result);
	result = TryGenerateSelectSimpleDiamondPattern(block, &cache);
	DCHECK(result);
	did_select = true;
	}
	}
	}

	return did_select;
	}

	} // namespace art