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

 /*
  * 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 "base/scoped_arena_containers.h"
 #include "reference_type_propagation.h"

 namespace art {

 static constexpr size_t kMaxInstructionsInBranch = 1u;

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

 // 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
 // with different inputs at `index1` and `index2`. Otherwise returns that phi.
 static HPhi* GetSingleChangedPhi(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 (phi->InputAt(index1) != phi->InputAt(index2)) {
       if (select_phi == nullptr) {
         // First phi with different inputs for the two indices found.
         select_phi = phi;
       } else {
         // More than one phis has different inputs for the two indices.
         return nullptr;
       }
     }
   }
   return select_phi;
 }

 bool HSelectGenerator::Run() {
   bool didSelect = 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;

     // Find elements of the diamond pattern.
     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)) {
       continue;
     }
     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();
     HPhi* phi = GetSingleChangedPhi(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 {
       continue;
     }
     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, handle_scope_);
       }
     } else if (phi->GetType() == DataType::Type::kReference) {
       select->SetReferenceTypeInfo(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 = it->second;
       DCHECK_EQ(cached->GetCondition(), select->GetCondition());
       if (cached->GetTrueValue() == select->GetTrueValue() &&
           cached->GetFalseValue() == select->GetFalseValue() &&
           select->StrictlyDominates(cached)) {
        cached->ReplaceWith(select);
        cached->GetBlock()->RemoveInstruction(cached);
       }
       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.
     didSelect = true;
   }
   return didSelect;
 }

 }  // 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 "base/scoped_arena_containers.h"
	#include "reference_type_propagation.h"

	namespace art {

	static constexpr size_t kMaxInstructionsInBranch = 1u;

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

	// 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
	// with different inputs at `index1` and `index2`. Otherwise returns that phi.
	static HPhi* GetSingleChangedPhi(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 (phi->InputAt(index1) != phi->InputAt(index2)) {
	if (select_phi == nullptr) {
	// First phi with different inputs for the two indices found.
	select_phi = phi;
	} else {
	// More than one phis has different inputs for the two indices.
	return nullptr;
	}
	}
	}
	return select_phi;
	}

	bool HSelectGenerator::Run() {
	bool didSelect = 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;

	// Find elements of the diamond pattern.
	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)) {
	continue;
	}
	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();
	HPhi* phi = GetSingleChangedPhi(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 {
	continue;
	}
	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, handle_scope_);
	}
	} else if (phi->GetType() == DataType::Type::kReference) {
	select->SetReferenceTypeInfo(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 = it->second;
	DCHECK_EQ(cached->GetCondition(), select->GetCondition());
	if (cached->GetTrueValue() == select->GetTrueValue() &&
	cached->GetFalseValue() == select->GetFalseValue() &&
	select->StrictlyDominates(cached)) {
	cached->ReplaceWith(select);
	cached->GetBlock()->RemoveInstruction(cached);
	}
	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.
	didSelect = true;
	}
	return didSelect;
	}

	} // namespace art