source/opt/cfg.cpp - external/github.com/KhronosGroup/SPIRV-Tools - Git at Google

 // Copyright (c) 2017 Google Inc.
 //
 // 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 "source/opt/cfg.h"

 #include <memory>
 #include <utility>

 #include "source/cfa.h"
 #include "source/opt/ir_builder.h"
 #include "source/opt/ir_context.h"
 #include "source/opt/module.h"

 namespace spvtools {
 namespace opt {
 namespace {

 using cbb_ptr = const opt::BasicBlock*;

 // Universal Limit of ResultID + 1
 const int kMaxResultId = 0x400000;

 }  // namespace

 CFG::CFG(Module* module)
     : module_(module),
       pseudo_entry_block_(std::unique_ptr<Instruction>(
           new Instruction(module->context(), SpvOpLabel, 0, 0, {}))),
       pseudo_exit_block_(std::unique_ptr<Instruction>(new Instruction(
           module->context(), SpvOpLabel, 0, kMaxResultId, {}))) {
   for (auto& fn : *module) {
     for (auto& blk : fn) {
       RegisterBlock(&blk);
     }
   }
 }

 void CFG::AddEdges(BasicBlock* blk) {
   uint32_t blk_id = blk->id();
   // Force the creation of an entry, not all basic block have predecessors
   // (such as the entry blocks and some unreachables).
   label2preds_[blk_id];
   const auto* const_blk = blk;
   const_blk->ForEachSuccessorLabel(
       [blk_id, this](const uint32_t succ_id) { AddEdge(blk_id, succ_id); });
 }

 void CFG::RemoveNonExistingEdges(uint32_t blk_id) {
   std::vector<uint32_t> updated_pred_list;
   for (uint32_t id : preds(blk_id)) {
     const BasicBlock* pred_blk = block(id);
     bool has_branch = false;
     pred_blk->ForEachSuccessorLabel([&has_branch, blk_id](uint32_t succ) {
       if (succ == blk_id) {
         has_branch = true;
       }
     });
     if (has_branch) updated_pred_list.push_back(id);
   }

   label2preds_.at(blk_id) = std::move(updated_pred_list);
 }

 void CFG::ComputeStructuredOrder(Function* func, BasicBlock* root,
                                  std::list<BasicBlock*>* order) {
   assert(module_->context()->get_feature_mgr()->HasCapability(
              SpvCapabilityShader) &&
          "This only works on structured control flow");

   // Compute structured successors and do DFS.
   ComputeStructuredSuccessors(func);
   auto ignore_block = [](cbb_ptr) {};
   auto ignore_edge = [](cbb_ptr, cbb_ptr) {};
   auto get_structured_successors = [this](const BasicBlock* b) {
     return &(block2structured_succs_[b]);
   };

   // TODO(greg-lunarg): Get rid of const_cast by making moving const
   // out of the cfa.h prototypes and into the invoking code.
   auto post_order = [&](cbb_ptr b) {
     order->push_front(const_cast<BasicBlock*>(b));
   };
   CFA<BasicBlock>::DepthFirstTraversal(root, get_structured_successors,
                                        ignore_block, post_order, ignore_edge);
 }

 void CFG::ForEachBlockInPostOrder(BasicBlock* bb,
                                   const std::function<void(BasicBlock*)>& f) {
   std::vector<BasicBlock*> po;
   std::unordered_set<BasicBlock*> seen;
   ComputePostOrderTraversal(bb, &po, &seen);

   for (BasicBlock* current_bb : po) {
     if (!IsPseudoExitBlock(current_bb) && !IsPseudoEntryBlock(current_bb)) {
       f(current_bb);
     }
   }
 }

 void CFG::ForEachBlockInReversePostOrder(
     BasicBlock* bb, const std::function<void(BasicBlock*)>& f) {
   WhileEachBlockInReversePostOrder(bb, [f](BasicBlock* b) {
     f(b);
     return true;
   });
 }

 bool CFG::WhileEachBlockInReversePostOrder(
     BasicBlock* bb, const std::function<bool(BasicBlock*)>& f) {
   std::vector<BasicBlock*> po;
   std::unordered_set<BasicBlock*> seen;
   ComputePostOrderTraversal(bb, &po, &seen);

   for (auto current_bb = po.rbegin(); current_bb != po.rend(); ++current_bb) {
     if (!IsPseudoExitBlock(*current_bb) && !IsPseudoEntryBlock(*current_bb)) {
       if (!f(*current_bb)) {
         return false;
       }
     }
   }
   return true;
 }

 void CFG::ComputeStructuredSuccessors(Function* func) {
   block2structured_succs_.clear();
   for (auto& blk : *func) {
     // If no predecessors in function, make successor to pseudo entry.
     if (label2preds_[blk.id()].size() == 0)
       block2structured_succs_[&pseudo_entry_block_].push_back(&blk);

     // If header, make merge block first successor and continue block second
     // successor if there is one.
     uint32_t mbid = blk.MergeBlockIdIfAny();
     if (mbid != 0) {
       block2structured_succs_[&blk].push_back(block(mbid));
       uint32_t cbid = blk.ContinueBlockIdIfAny();
       if (cbid != 0) {
         block2structured_succs_[&blk].push_back(block(cbid));
       }
     }

     // Add true successors.
     const auto& const_blk = blk;
     const_blk.ForEachSuccessorLabel([&blk, this](const uint32_t sbid) {
       block2structured_succs_[&blk].push_back(block(sbid));
     });
   }
 }

 void CFG::ComputePostOrderTraversal(BasicBlock* bb,
                                     std::vector<BasicBlock*>* order,
                                     std::unordered_set<BasicBlock*>* seen) {
   std::vector<BasicBlock*> stack;
   stack.push_back(bb);
   while (!stack.empty()) {
     bb = stack.back();
     seen->insert(bb);
     static_cast<const BasicBlock*>(bb)->WhileEachSuccessorLabel(
         [&seen, &stack, this](const uint32_t sbid) {
           BasicBlock* succ_bb = id2block_[sbid];
           if (!seen->count(succ_bb)) {
             stack.push_back(succ_bb);
             return false;
           }
           return true;
         });
     if (stack.back() == bb) {
       order->push_back(bb);
       stack.pop_back();
     }
   }
 }

 BasicBlock* CFG::SplitLoopHeader(BasicBlock* bb) {
   assert(bb->GetLoopMergeInst() && "Expecting bb to be the header of a loop.");

   Function* fn = bb->GetParent();
   IRContext* context = module_->context();

   // Get the new header id up front.  If we are out of ids, then we cannot split
   // the loop.
   uint32_t new_header_id = context->TakeNextId();
   if (new_header_id == 0) {
     return nullptr;
   }

   // Find the insertion point for the new bb.
   Function::iterator header_it = std::find_if(
       fn->begin(), fn->end(),
       [bb](BasicBlock& block_in_func) { return &block_in_func == bb; });
   assert(header_it != fn->end());

   const std::vector<uint32_t>& pred = preds(bb->id());
   // Find the back edge
   BasicBlock* latch_block = nullptr;
   Function::iterator latch_block_iter = header_it;
   while (++latch_block_iter != fn->end()) {
     // If blocks are in the proper order, then the only branch that appears
     // after the header is the latch.
     if (std::find(pred.begin(), pred.end(), latch_block_iter->id()) !=
         pred.end()) {
       break;
     }
   }
   assert(latch_block_iter != fn->end() && "Could not find the latch.");
   latch_block = &*latch_block_iter;

   RemoveSuccessorEdges(bb);

   // Create the new header bb basic bb.
   // Leave the phi instructions behind.
   auto iter = bb->begin();
   while (iter->opcode() == SpvOpPhi) {
     ++iter;
   }

   BasicBlock* new_header = bb->SplitBasicBlock(context, new_header_id, iter);
   context->AnalyzeDefUse(new_header->GetLabelInst());

   // Update cfg
   RegisterBlock(new_header);

   // Update bb mappings.
   context->set_instr_block(new_header->GetLabelInst(), new_header);
   new_header->ForEachInst([new_header, context](Instruction* inst) {
     context->set_instr_block(inst, new_header);
   });

   // Adjust the OpPhi instructions as needed.
   bb->ForEachPhiInst([latch_block, bb, new_header, context](Instruction* phi) {
     std::vector<uint32_t> preheader_phi_ops;
     std::vector<Operand> header_phi_ops;

     // Identify where the original inputs to original OpPhi belong: header or
     // preheader.
     for (uint32_t i = 0; i < phi->NumInOperands(); i += 2) {
       uint32_t def_id = phi->GetSingleWordInOperand(i);
       uint32_t branch_id = phi->GetSingleWordInOperand(i + 1);
       if (branch_id == latch_block->id()) {
         header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {def_id}});
         header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {branch_id}});
       } else {
         preheader_phi_ops.push_back(def_id);
         preheader_phi_ops.push_back(branch_id);
       }
     }

     // Create a phi instruction if and only if the preheader_phi_ops has more
     // than one pair.
     if (preheader_phi_ops.size() > 2) {
       InstructionBuilder builder(
           context, &*bb->begin(),
           IRContext::kAnalysisDefUse | IRContext::kAnalysisInstrToBlockMapping);

       Instruction* new_phi = builder.AddPhi(phi->type_id(), preheader_phi_ops);

       // Add the OpPhi to the header bb.
       header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {new_phi->result_id()}});
       header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {bb->id()}});
     } else {
       // An OpPhi with a single entry is just a copy.  In this case use the same
       // instruction in the new header.
       header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {preheader_phi_ops[0]}});
       header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {bb->id()}});
     }

     phi->RemoveFromList();
     std::unique_ptr<Instruction> phi_owner(phi);
     phi->SetInOperands(std::move(header_phi_ops));
     new_header->begin()->InsertBefore(std::move(phi_owner));
     context->set_instr_block(phi, new_header);
     context->AnalyzeUses(phi);
   });

   // Add a branch to the new header.
   InstructionBuilder branch_builder(
       context, bb,
       IRContext::kAnalysisDefUse | IRContext::kAnalysisInstrToBlockMapping);
   bb->AddInstruction(
       MakeUnique<Instruction>(context, SpvOpBranch, 0, 0,
                               std::initializer_list<Operand>{
                                   {SPV_OPERAND_TYPE_ID, {new_header->id()}}}));
   context->AnalyzeUses(bb->terminator());
   context->set_instr_block(bb->terminator(), bb);
   label2preds_[new_header->id()].push_back(bb->id());

   // Update the latch to branch to the new header.
   latch_block->ForEachSuccessorLabel([bb, new_header_id](uint32_t* id) {
     if (*id == bb->id()) {
       *id = new_header_id;
     }
   });
   Instruction* latch_branch = latch_block->terminator();
   context->AnalyzeUses(latch_branch);
   label2preds_[new_header->id()].push_back(latch_block->id());

   auto& block_preds = label2preds_[bb->id()];
   auto latch_pos =
       std::find(block_preds.begin(), block_preds.end(), latch_block->id());
   assert(latch_pos != block_preds.end() && "The cfg was invalid.");
   block_preds.erase(latch_pos);

   // Update the loop descriptors
   if (context->AreAnalysesValid(IRContext::kAnalysisLoopAnalysis)) {
     LoopDescriptor* loop_desc = context->GetLoopDescriptor(bb->GetParent());
     Loop* loop = (*loop_desc)[bb->id()];

     loop->AddBasicBlock(new_header_id);
     loop->SetHeaderBlock(new_header);
     loop_desc->SetBasicBlockToLoop(new_header_id, loop);

     loop->RemoveBasicBlock(bb->id());
     loop->SetPreHeaderBlock(bb);

     Loop* parent_loop = loop->GetParent();
     if (parent_loop != nullptr) {
       parent_loop->AddBasicBlock(bb->id());
       loop_desc->SetBasicBlockToLoop(bb->id(), parent_loop);
     } else {
       loop_desc->SetBasicBlockToLoop(bb->id(), nullptr);
     }
   }
   return new_header;
 }

 }  // namespace opt
 }  // namespace spvtools
	// Copyright (c) 2017 Google Inc.
	//
	// 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 "source/opt/cfg.h"

	#include <memory>
	#include <utility>

	#include "source/cfa.h"
	#include "source/opt/ir_builder.h"
	#include "source/opt/ir_context.h"
	#include "source/opt/module.h"

	namespace spvtools {
	namespace opt {
	namespace {

	using cbb_ptr = const opt::BasicBlock*;

	// Universal Limit of ResultID + 1
	const int kMaxResultId = 0x400000;

	} // namespace

	CFG::CFG(Module* module)
	: module_(module),
	pseudo_entry_block_(std::unique_ptr<Instruction>(
	new Instruction(module->context(), SpvOpLabel, 0, 0, {}))),
	pseudo_exit_block_(std::unique_ptr<Instruction>(new Instruction(
	module->context(), SpvOpLabel, 0, kMaxResultId, {}))) {
	for (auto& fn : *module) {
	for (auto& blk : fn) {
	RegisterBlock(&blk);
	}
	}
	}

	void CFG::AddEdges(BasicBlock* blk) {
	uint32_t blk_id = blk->id();
	// Force the creation of an entry, not all basic block have predecessors
	// (such as the entry blocks and some unreachables).
	label2preds_[blk_id];
	const auto* const_blk = blk;
	const_blk->ForEachSuccessorLabel(
	[blk_id, this](const uint32_t succ_id) { AddEdge(blk_id, succ_id); });
	}

	void CFG::RemoveNonExistingEdges(uint32_t blk_id) {
	std::vector<uint32_t> updated_pred_list;
	for (uint32_t id : preds(blk_id)) {
	const BasicBlock* pred_blk = block(id);
	bool has_branch = false;
	pred_blk->ForEachSuccessorLabel([&has_branch, blk_id](uint32_t succ) {
	if (succ == blk_id) {
	has_branch = true;
	}
	});
	if (has_branch) updated_pred_list.push_back(id);
	}

	label2preds_.at(blk_id) = std::move(updated_pred_list);
	}

	void CFG::ComputeStructuredOrder(Function* func, BasicBlock* root,
	std::list<BasicBlock> order) {
	assert(module_->context()->get_feature_mgr()->HasCapability(
	SpvCapabilityShader) &&
	"This only works on structured control flow");

	// Compute structured successors and do DFS.
	ComputeStructuredSuccessors(func);
	auto ignore_block = [](cbb_ptr) {};
	auto ignore_edge = [](cbb_ptr, cbb_ptr) {};
	auto get_structured_successors = [this](const BasicBlock* b) {
	return &(block2structured_succs_[b]);
	};

	// TODO(greg-lunarg): Get rid of const_cast by making moving const
	// out of the cfa.h prototypes and into the invoking code.
	auto post_order = [&](cbb_ptr b) {
	order->push_front(const_cast<BasicBlock*>(b));
	};
	CFA<BasicBlock>::DepthFirstTraversal(root, get_structured_successors,
	ignore_block, post_order, ignore_edge);
	}

	void CFG::ForEachBlockInPostOrder(BasicBlock* bb,
	const std::function<void(BasicBlock*)>& f) {
	std::vector<BasicBlock*> po;
	std::unordered_set<BasicBlock*> seen;
	ComputePostOrderTraversal(bb, &po, &seen);

	for (BasicBlock* current_bb : po) {
	if (!IsPseudoExitBlock(current_bb) && !IsPseudoEntryBlock(current_bb)) {
	f(current_bb);
	}
	}
	}

	void CFG::ForEachBlockInReversePostOrder(
	BasicBlock* bb, const std::function<void(BasicBlock*)>& f) {
	WhileEachBlockInReversePostOrder(bb, [f](BasicBlock* b) {
	f(b);
	return true;
	});
	}

	bool CFG::WhileEachBlockInReversePostOrder(
	BasicBlock* bb, const std::function<bool(BasicBlock*)>& f) {
	std::vector<BasicBlock*> po;
	std::unordered_set<BasicBlock*> seen;
	ComputePostOrderTraversal(bb, &po, &seen);

	for (auto current_bb = po.rbegin(); current_bb != po.rend(); ++current_bb) {
	if (!IsPseudoExitBlock(current_bb) && !IsPseudoEntryBlock(current_bb)) {
	if (!f(*current_bb)) {
	return false;
	}
	}
	}
	return true;
	}

	void CFG::ComputeStructuredSuccessors(Function* func) {
	block2structured_succs_.clear();
	for (auto& blk : *func) {
	// If no predecessors in function, make successor to pseudo entry.
	if (label2preds_[blk.id()].size() == 0)
	block2structured_succs_[&pseudo_entry_block_].push_back(&blk);

	// If header, make merge block first successor and continue block second
	// successor if there is one.
	uint32_t mbid = blk.MergeBlockIdIfAny();
	if (mbid != 0) {
	block2structured_succs_[&blk].push_back(block(mbid));
	uint32_t cbid = blk.ContinueBlockIdIfAny();
	if (cbid != 0) {
	block2structured_succs_[&blk].push_back(block(cbid));
	}
	}

	// Add true successors.
	const auto& const_blk = blk;
	const_blk.ForEachSuccessorLabel([&blk, this](const uint32_t sbid) {
	block2structured_succs_[&blk].push_back(block(sbid));
	});
	}
	}

	void CFG::ComputePostOrderTraversal(BasicBlock* bb,
	std::vector<BasicBlock> order,
	std::unordered_set<BasicBlock> seen) {
	std::vector<BasicBlock*> stack;
	stack.push_back(bb);
	while (!stack.empty()) {
	bb = stack.back();
	seen->insert(bb);
	static_cast<const BasicBlock*>(bb)->WhileEachSuccessorLabel(
	[&seen, &stack, this](const uint32_t sbid) {
	BasicBlock* succ_bb = id2block_[sbid];
	if (!seen->count(succ_bb)) {
	stack.push_back(succ_bb);
	return false;
	}
	return true;
	});
	if (stack.back() == bb) {
	order->push_back(bb);
	stack.pop_back();
	}
	}
	}

	BasicBlock* CFG::SplitLoopHeader(BasicBlock* bb) {
	assert(bb->GetLoopMergeInst() && "Expecting bb to be the header of a loop.");

	Function* fn = bb->GetParent();
	IRContext* context = module_->context();

	// Get the new header id up front. If we are out of ids, then we cannot split
	// the loop.
	uint32_t new_header_id = context->TakeNextId();
	if (new_header_id == 0) {
	return nullptr;
	}

	// Find the insertion point for the new bb.
	Function::iterator header_it = std::find_if(
	fn->begin(), fn->end(),
	[bb](BasicBlock& block_in_func) { return &block_in_func == bb; });
	assert(header_it != fn->end());

	const std::vector<uint32_t>& pred = preds(bb->id());
	// Find the back edge
	BasicBlock* latch_block = nullptr;
	Function::iterator latch_block_iter = header_it;
	while (++latch_block_iter != fn->end()) {
	// If blocks are in the proper order, then the only branch that appears
	// after the header is the latch.
	if (std::find(pred.begin(), pred.end(), latch_block_iter->id()) !=
	pred.end()) {
	break;
	}
	}
	assert(latch_block_iter != fn->end() && "Could not find the latch.");
	latch_block = &*latch_block_iter;

	RemoveSuccessorEdges(bb);

	// Create the new header bb basic bb.
	// Leave the phi instructions behind.
	auto iter = bb->begin();
	while (iter->opcode() == SpvOpPhi) {
	++iter;
	}

	BasicBlock* new_header = bb->SplitBasicBlock(context, new_header_id, iter);
	context->AnalyzeDefUse(new_header->GetLabelInst());

	// Update cfg
	RegisterBlock(new_header);

	// Update bb mappings.
	context->set_instr_block(new_header->GetLabelInst(), new_header);
	new_header->ForEachInst([new_header, context](Instruction* inst) {
	context->set_instr_block(inst, new_header);
	});

	// Adjust the OpPhi instructions as needed.
	bb->ForEachPhiInst([latch_block, bb, new_header, context](Instruction* phi) {
	std::vector<uint32_t> preheader_phi_ops;
	std::vector<Operand> header_phi_ops;

	// Identify where the original inputs to original OpPhi belong: header or
	// preheader.
	for (uint32_t i = 0; i < phi->NumInOperands(); i += 2) {
	uint32_t def_id = phi->GetSingleWordInOperand(i);
	uint32_t branch_id = phi->GetSingleWordInOperand(i + 1);
	if (branch_id == latch_block->id()) {
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {def_id}});
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {branch_id}});
	} else {
	preheader_phi_ops.push_back(def_id);
	preheader_phi_ops.push_back(branch_id);
	}
	}

	// Create a phi instruction if and only if the preheader_phi_ops has more
	// than one pair.
	if (preheader_phi_ops.size() > 2) {
	InstructionBuilder builder(
	context, &*bb->begin(),
	IRContext::kAnalysisDefUse \| IRContext::kAnalysisInstrToBlockMapping);

	Instruction* new_phi = builder.AddPhi(phi->type_id(), preheader_phi_ops);

	// Add the OpPhi to the header bb.
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {new_phi->result_id()}});
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {bb->id()}});
	} else {
	// An OpPhi with a single entry is just a copy. In this case use the same
	// instruction in the new header.
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {preheader_phi_ops[0]}});
	header_phi_ops.push_back({SPV_OPERAND_TYPE_ID, {bb->id()}});
	}

	phi->RemoveFromList();
	std::unique_ptr<Instruction> phi_owner(phi);
	phi->SetInOperands(std::move(header_phi_ops));
	new_header->begin()->InsertBefore(std::move(phi_owner));
	context->set_instr_block(phi, new_header);
	context->AnalyzeUses(phi);
	});

	// Add a branch to the new header.
	InstructionBuilder branch_builder(
	context, bb,
	IRContext::kAnalysisDefUse \| IRContext::kAnalysisInstrToBlockMapping);
	bb->AddInstruction(
	MakeUnique<Instruction>(context, SpvOpBranch, 0, 0,
	std::initializer_list<Operand>{
	{SPV_OPERAND_TYPE_ID, {new_header->id()}}}));
	context->AnalyzeUses(bb->terminator());
	context->set_instr_block(bb->terminator(), bb);
	label2preds_[new_header->id()].push_back(bb->id());

	// Update the latch to branch to the new header.
	latch_block->ForEachSuccessorLabel([bb, new_header_id](uint32_t* id) {
	if (*id == bb->id()) {
	*id = new_header_id;
	}
	});
	Instruction* latch_branch = latch_block->terminator();
	context->AnalyzeUses(latch_branch);
	label2preds_[new_header->id()].push_back(latch_block->id());

	auto& block_preds = label2preds_[bb->id()];
	auto latch_pos =
	std::find(block_preds.begin(), block_preds.end(), latch_block->id());
	assert(latch_pos != block_preds.end() && "The cfg was invalid.");
	block_preds.erase(latch_pos);

	// Update the loop descriptors
	if (context->AreAnalysesValid(IRContext::kAnalysisLoopAnalysis)) {
	LoopDescriptor* loop_desc = context->GetLoopDescriptor(bb->GetParent());
	Loop* loop = (*loop_desc)[bb->id()];

	loop->AddBasicBlock(new_header_id);
	loop->SetHeaderBlock(new_header);
	loop_desc->SetBasicBlockToLoop(new_header_id, loop);

	loop->RemoveBasicBlock(bb->id());
	loop->SetPreHeaderBlock(bb);

	Loop* parent_loop = loop->GetParent();
	if (parent_loop != nullptr) {
	parent_loop->AddBasicBlock(bb->id());
	loop_desc->SetBasicBlockToLoop(bb->id(), parent_loop);
	} else {
	loop_desc->SetBasicBlockToLoop(bb->id(), nullptr);
	}
	}
	return new_header;
	}

	} // namespace opt
	} // namespace spvtools