src/passes/DeadCodeElimination.cpp - external/github.com/WebAssembly/binaryen - Git at Google

 /*
  * Copyright 2016 WebAssembly Community Group participants
  *
  * 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.
  */

 //
 // Removes dead, i.e. unreachable, code.
 //
 // We keep a record of when control flow is reachable. When it isn't, we
 // kill (turn into unreachable). We then fold away entire unreachable
 // expressions.
 //
 // When dead code causes an operation to not happen, like a store, a call
 // or an add, we replace with a block with a list of what does happen.
 // That isn't necessarily smaller, but blocks are friendlier to other
 // optimizations: blocks can be merged and eliminated, and they clearly
 // have no side effects.
 //

 #include <vector>
 #include <wasm.h>
 #include <pass.h>
 #include <wasm-builder.h>
 #include <ir/block-utils.h>
 #include <ir/branch-utils.h>
 #include <ir/type-updating.h>

 namespace wasm {

 struct DeadCodeElimination : public WalkerPass<PostWalker<DeadCodeElimination>> {
   bool isFunctionParallel() override { return true; }

   Pass* create() override { return new DeadCodeElimination; }

   // as we remove code, we must keep the types of other nodes valid
   TypeUpdater typeUpdater;

   Expression* replaceCurrent(Expression* expression) {
     auto* old = getCurrent();
     if (old == expression) return expression;
     super::replaceCurrent(expression);
     // also update the type updater
     typeUpdater.noteReplacement(old, expression);
     return expression;
   }

   // whether the current code is actually reachable
   bool reachable;

   void doWalkFunction(Function* func) {
     reachable = true;
     typeUpdater.walk(func->body);
     walk(func->body);
   }

   std::set<Name> reachableBreaks;

   void addBreak(Name name) {
     // we normally have already reduced unreachable code into (unreachable)
     // nodes, so we would not get to this function at all anyhow, the breaking
     // instruction itself would be removed. However, an exception are things
     // like  (block (result i32) (call $x) (unreachable)) , which has type i32
     // despite not being exited.
     // TODO: optimize such cases
     if (reachable) {
       reachableBreaks.insert(name);
     }
   }

   // if a child exists and is unreachable, we can replace ourselves with it
   bool isDead(Expression* child) {
     return child && child->type == unreachable;
   }

   // a similar check, assumes the child exists
   bool isUnreachable(Expression* child) {
     return child->type == unreachable;
   }

   // things that stop control flow

   void visitBreak(Break* curr) {
     if (isDead(curr->value)) {
       // the condition is evaluated last, so if the value was unreachable, the whole thing is
       replaceCurrent(curr->value);
       return;
     }
     if (isDead(curr->condition)) {
       if (curr->value) {
         auto* block = getModule()->allocator.alloc<Block>();
         block->list.resize(2);
         block->list[0] = drop(curr->value);
         block->list[1] = curr->condition;
         // if we previously returned a value, then this block
         // must have the same type, so it fits in the ast
         // properly. it ends in an unreachable
         // anyhow, so that is ok.
         block->finalize(curr->type);
         replaceCurrent(block);
       } else {
         replaceCurrent(curr->condition);
       }
       return;
     }
     addBreak(curr->name);
     if (!curr->condition) {
       reachable = false;
     }
   }

   void visitSwitch(Switch* curr) {
     if (isDead(curr->value)) {
       replaceCurrent(curr->value);
       return;
     }
     if (isUnreachable(curr->condition)) {
       if (curr->value) {
         auto* block = getModule()->allocator.alloc<Block>();
         block->list.resize(2);
         block->list[0] = drop(curr->value);
         block->list[1] = curr->condition;
         block->finalize(curr->type);
         replaceCurrent(block);
       } else {
         replaceCurrent(curr->condition);
       }
       return;
     }
     for (auto target : curr->targets) {
       addBreak(target);
     }
     addBreak(curr->default_);
     reachable = false;
   }

   void visitReturn(Return* curr) {
     if (isDead(curr->value)) {
       replaceCurrent(curr->value);
       return;
     }
     reachable = false;
   }

   void visitUnreachable(Unreachable* curr) {
     reachable = false;
   }

   void visitBlock(Block* curr) {
     auto& list = curr->list;
     // if we are currently unreachable (before we take into account
     // breaks to the block) then a child may be unreachable, and we
     // can shorten
     if (!reachable && list.size() > 1) {
       // to do here: nothing to remove after it)
       for (Index i = 0; i < list.size() - 1; i++) {
         if (list[i]->type == unreachable) {
           list.resize(i + 1);
           break;
         }
       }
     }
     if (curr->name.is()) {
       reachable = reachable || reachableBreaks.count(curr->name);
       reachableBreaks.erase(curr->name);
     }
     if (list.size() == 1 && isUnreachable(list[0])) {
       replaceCurrent(BlockUtils::simplifyToContentsWithPossibleTypeChange(curr, this));
     } else {
       // the block may have had a type, but can now be unreachable, which allows more reduction outside
       typeUpdater.maybeUpdateTypeToUnreachable(curr);
     }
   }

   void visitLoop(Loop* curr) {
     if (curr->name.is()) {
       reachableBreaks.erase(curr->name);
     }
     if (isUnreachable(curr->body) && !BranchUtils::BranchSeeker::hasNamed(curr->body, curr->name)) {
       replaceCurrent(curr->body);
       return;
     }
   }

   // ifs need special handling

   std::vector<bool> ifStack; // stack of reachable state, for forking and joining

   static void doAfterIfCondition(DeadCodeElimination* self, Expression** currp) {
     self->ifStack.push_back(self->reachable);
   }

   static void doAfterIfElseTrue(DeadCodeElimination* self, Expression** currp) {
     assert((*currp)->cast<If>()->ifFalse);
     bool reachableBefore = self->ifStack.back();
     self->ifStack.pop_back();
     self->ifStack.push_back(self->reachable);
     self->reachable = reachableBefore;
   }

   void visitIf(If* curr) {
     // the ifStack has the branch that joins us, either from before if just an if, or the ifTrue if an if-else
     reachable = reachable || ifStack.back();
     ifStack.pop_back();
     if (isUnreachable(curr->condition)) {
       replaceCurrent(curr->condition);
     }
     // the if may have had a type, but can now be unreachable, which allows more reduction outside
     typeUpdater.maybeUpdateTypeToUnreachable(curr);
   }

   static void scan(DeadCodeElimination* self, Expression** currp) {
     auto* curr = *currp;
     if (!self->reachable) {
       // convert to an unreachable safely
       #define DELEGATE(CLASS_TO_VISIT) { \
         auto* parent = self->typeUpdater.parents[curr]; \
         self->typeUpdater.noteRecursiveRemoval(curr); \
         ExpressionManipulator::convert<CLASS_TO_VISIT, Unreachable>(static_cast<CLASS_TO_VISIT*>(curr)); \
         self->typeUpdater.noteAddition(curr, parent); \
         break; \
       }
       switch (curr->_id) {
         case Expression::Id::BlockId: DELEGATE(Block);
         case Expression::Id::IfId: DELEGATE(If);
         case Expression::Id::LoopId: DELEGATE(Loop);
         case Expression::Id::BreakId: DELEGATE(Break);
         case Expression::Id::SwitchId: DELEGATE(Switch);
         case Expression::Id::CallId: DELEGATE(Call);
         case Expression::Id::CallImportId: DELEGATE(CallImport);
         case Expression::Id::CallIndirectId: DELEGATE(CallIndirect);
         case Expression::Id::GetLocalId: DELEGATE(GetLocal);
         case Expression::Id::SetLocalId: DELEGATE(SetLocal);
         case Expression::Id::GetGlobalId: DELEGATE(GetGlobal);
         case Expression::Id::SetGlobalId: DELEGATE(SetGlobal);
         case Expression::Id::LoadId: DELEGATE(Load);
         case Expression::Id::StoreId: DELEGATE(Store);
         case Expression::Id::ConstId: DELEGATE(Const);
         case Expression::Id::UnaryId: DELEGATE(Unary);
         case Expression::Id::BinaryId: DELEGATE(Binary);
         case Expression::Id::SelectId: DELEGATE(Select);
         case Expression::Id::DropId: DELEGATE(Drop);
         case Expression::Id::ReturnId: DELEGATE(Return);
         case Expression::Id::HostId: DELEGATE(Host);
         case Expression::Id::NopId: DELEGATE(Nop);
         case Expression::Id::UnreachableId: break;
         case Expression::Id::AtomicCmpxchgId: DELEGATE(AtomicCmpxchg);
         case Expression::Id::AtomicRMWId: DELEGATE(AtomicRMW);
         case Expression::Id::AtomicWaitId: DELEGATE(AtomicWait);
         case Expression::Id::AtomicWakeId: DELEGATE(AtomicWake);
         case Expression::Id::InvalidId:
         default: WASM_UNREACHABLE();
       }
       #undef DELEGATE
       return;
     }
     if (curr->is<If>()) {
       self->pushTask(DeadCodeElimination::doVisitIf, currp);
       if (curr->cast<If>()->ifFalse) {
         self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->ifFalse);
         self->pushTask(DeadCodeElimination::doAfterIfElseTrue, currp);
       }
       self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->ifTrue);
       self->pushTask(DeadCodeElimination::doAfterIfCondition, currp);
       self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->condition);
     } else {
       super::scan(self, currp);
     }
   }

   // other things

   // we don't need to drop unreachable nodes
   Expression* drop(Expression* toDrop) {
     if (toDrop->type == unreachable) return toDrop;
     return Builder(*getModule()).makeDrop(toDrop);
   }

   template<typename T>
   Expression* handleCall(T* curr) {
     for (Index i = 0; i < curr->operands.size(); i++) {
       if (isUnreachable(curr->operands[i])) {
         if (i > 0) {
           auto* block = getModule()->allocator.alloc<Block>();
           Index newSize = i + 1;
           block->list.resize(newSize);
           Index j = 0;
           for (; j < newSize; j++) {
             block->list[j] = drop(curr->operands[j]);
           }
           block->finalize(curr->type);
           return replaceCurrent(block);
         } else {
           return replaceCurrent(curr->operands[i]);
         }
       }
     }
     return curr;
   }

   void visitCall(Call* curr) {
     handleCall(curr);
   }

   void visitCallImport(CallImport* curr) {
     handleCall(curr);
   }

   void visitCallIndirect(CallIndirect* curr) {
     if (handleCall(curr) != curr) return;
     if (isUnreachable(curr->target)) {
       auto* block = getModule()->allocator.alloc<Block>();
       for (auto* operand : curr->operands) {
         block->list.push_back(drop(operand));
       }
       block->list.push_back(curr->target);
       block->finalize(curr->type);
       replaceCurrent(block);
     }
   }

   // Append the reachable operands of the current node to a block, and replace
   // it with the block
   void blockifyReachableOperands(std::vector<Expression*>&& list, Type type) {
     for (size_t i = 0; i < list.size(); ++i) {
       auto* elem = list[i];
       if (isUnreachable(elem)) {
         auto* replacement = elem;
         if (i > 0) {
           auto* block = getModule()->allocator.alloc<Block>();
           for (size_t j = 0; j < i; ++j) {
             block->list.push_back(drop(list[j]));
           }
           block->list.push_back(list[i]);
           block->finalize(type);
           replacement = block;
         }
         replaceCurrent(replacement);
         return;
       }
     }
   }

   void visitSetLocal(SetLocal* curr) {
     blockifyReachableOperands({ curr->value }, curr->type);
   }

   void visitSetGlobal(SetGlobal* curr) {
     blockifyReachableOperands({ curr->value }, curr->type);
   }

   void visitLoad(Load* curr) {
     blockifyReachableOperands({ curr->ptr }, curr->type);
   }

   void visitStore(Store* curr) {
     blockifyReachableOperands({ curr->ptr, curr->value }, curr->type);
   }

   void visitAtomicRMW(AtomicRMW* curr) {
     blockifyReachableOperands({ curr->ptr, curr->value }, curr->type);
   }

   void visitAtomicCmpxchg(AtomicCmpxchg* curr) {
     blockifyReachableOperands({ curr->ptr, curr->expected, curr->replacement }, curr->type);
   }

   void visitUnary(Unary* curr) {
     blockifyReachableOperands({ curr->value }, curr->type);
   }

   void visitBinary(Binary* curr) {
     blockifyReachableOperands({ curr->left, curr->right }, curr->type);
   }

   void visitSelect(Select* curr) {
     blockifyReachableOperands({ curr->ifTrue, curr->ifFalse, curr->condition }, curr->type);
   }

   void visitDrop(Drop* curr) {
     blockifyReachableOperands({ curr->value }, curr->type);
   }

   void visitHost(Host* curr) {
     handleCall(curr);
   }

   void visitFunction(Function* curr) {
     assert(reachableBreaks.size() == 0);
   }
 };

 Pass *createDeadCodeEliminationPass() {
   return new DeadCodeElimination();
 }

 } // namespace wasm
	/*
	* Copyright 2016 WebAssembly Community Group participants
	*
	* 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.
	*/

	//
	// Removes dead, i.e. unreachable, code.
	//
	// We keep a record of when control flow is reachable. When it isn't, we
	// kill (turn into unreachable). We then fold away entire unreachable
	// expressions.
	//
	// When dead code causes an operation to not happen, like a store, a call
	// or an add, we replace with a block with a list of what does happen.
	// That isn't necessarily smaller, but blocks are friendlier to other
	// optimizations: blocks can be merged and eliminated, and they clearly
	// have no side effects.
	//

	#include <vector>
	#include <wasm.h>
	#include <pass.h>
	#include <wasm-builder.h>
	#include <ir/block-utils.h>
	#include <ir/branch-utils.h>
	#include <ir/type-updating.h>

	namespace wasm {

	struct DeadCodeElimination : public WalkerPass<PostWalker<DeadCodeElimination>> {
	bool isFunctionParallel() override { return true; }

	Pass* create() override { return new DeadCodeElimination; }

	// as we remove code, we must keep the types of other nodes valid
	TypeUpdater typeUpdater;

	Expression* replaceCurrent(Expression* expression) {
	auto* old = getCurrent();
	if (old == expression) return expression;
	super::replaceCurrent(expression);
	// also update the type updater
	typeUpdater.noteReplacement(old, expression);
	return expression;
	}

	// whether the current code is actually reachable
	bool reachable;

	void doWalkFunction(Function* func) {
	reachable = true;
	typeUpdater.walk(func->body);
	walk(func->body);
	}

	std::set<Name> reachableBreaks;

	void addBreak(Name name) {
	// we normally have already reduced unreachable code into (unreachable)
	// nodes, so we would not get to this function at all anyhow, the breaking
	// instruction itself would be removed. However, an exception are things
	// like (block (result i32) (call $x) (unreachable)) , which has type i32
	// despite not being exited.
	// TODO: optimize such cases
	if (reachable) {
	reachableBreaks.insert(name);
	}
	}

	// if a child exists and is unreachable, we can replace ourselves with it
	bool isDead(Expression* child) {
	return child && child->type == unreachable;
	}

	// a similar check, assumes the child exists
	bool isUnreachable(Expression* child) {
	return child->type == unreachable;
	}

	// things that stop control flow

	void visitBreak(Break* curr) {
	if (isDead(curr->value)) {
	// the condition is evaluated last, so if the value was unreachable, the whole thing is
	replaceCurrent(curr->value);
	return;
	}
	if (isDead(curr->condition)) {
	if (curr->value) {
	auto* block = getModule()->allocator.alloc<Block>();
	block->list.resize(2);
	block->list[0] = drop(curr->value);
	block->list[1] = curr->condition;
	// if we previously returned a value, then this block
	// must have the same type, so it fits in the ast
	// properly. it ends in an unreachable
	// anyhow, so that is ok.
	block->finalize(curr->type);
	replaceCurrent(block);
	} else {
	replaceCurrent(curr->condition);
	}
	return;
	}
	addBreak(curr->name);
	if (!curr->condition) {
	reachable = false;
	}
	}

	void visitSwitch(Switch* curr) {
	if (isDead(curr->value)) {
	replaceCurrent(curr->value);
	return;
	}
	if (isUnreachable(curr->condition)) {
	if (curr->value) {
	auto* block = getModule()->allocator.alloc<Block>();
	block->list.resize(2);
	block->list[0] = drop(curr->value);
	block->list[1] = curr->condition;
	block->finalize(curr->type);
	replaceCurrent(block);
	} else {
	replaceCurrent(curr->condition);
	}
	return;
	}
	for (auto target : curr->targets) {
	addBreak(target);
	}
	addBreak(curr->default_);
	reachable = false;
	}

	void visitReturn(Return* curr) {
	if (isDead(curr->value)) {
	replaceCurrent(curr->value);
	return;
	}
	reachable = false;
	}

	void visitUnreachable(Unreachable* curr) {
	reachable = false;
	}

	void visitBlock(Block* curr) {
	auto& list = curr->list;
	// if we are currently unreachable (before we take into account
	// breaks to the block) then a child may be unreachable, and we
	// can shorten
	if (!reachable && list.size() > 1) {
	// to do here: nothing to remove after it)
	for (Index i = 0; i < list.size() - 1; i++) {
	if (list[i]->type == unreachable) {
	list.resize(i + 1);
	break;
	}
	}
	}
	if (curr->name.is()) {
	reachable = reachable \|\| reachableBreaks.count(curr->name);
	reachableBreaks.erase(curr->name);
	}
	if (list.size() == 1 && isUnreachable(list[0])) {
	replaceCurrent(BlockUtils::simplifyToContentsWithPossibleTypeChange(curr, this));
	} else {
	// the block may have had a type, but can now be unreachable, which allows more reduction outside
	typeUpdater.maybeUpdateTypeToUnreachable(curr);
	}
	}

	void visitLoop(Loop* curr) {
	if (curr->name.is()) {
	reachableBreaks.erase(curr->name);
	}
	if (isUnreachable(curr->body) && !BranchUtils::BranchSeeker::hasNamed(curr->body, curr->name)) {
	replaceCurrent(curr->body);
	return;
	}
	}

	// ifs need special handling

	std::vector<bool> ifStack; // stack of reachable state, for forking and joining

	static void doAfterIfCondition(DeadCodeElimination* self, Expression** currp) {
	self->ifStack.push_back(self->reachable);
	}

	static void doAfterIfElseTrue(DeadCodeElimination* self, Expression** currp) {
	assert((*currp)->cast<If>()->ifFalse);
	bool reachableBefore = self->ifStack.back();
	self->ifStack.pop_back();
	self->ifStack.push_back(self->reachable);
	self->reachable = reachableBefore;
	}

	void visitIf(If* curr) {
	// the ifStack has the branch that joins us, either from before if just an if, or the ifTrue if an if-else
	reachable = reachable \|\| ifStack.back();
	ifStack.pop_back();
	if (isUnreachable(curr->condition)) {
	replaceCurrent(curr->condition);
	}
	// the if may have had a type, but can now be unreachable, which allows more reduction outside
	typeUpdater.maybeUpdateTypeToUnreachable(curr);
	}

	static void scan(DeadCodeElimination* self, Expression** currp) {
	auto* curr = *currp;
	if (!self->reachable) {
	// convert to an unreachable safely
	#define DELEGATE(CLASS_TO_VISIT) { \
	auto* parent = self->typeUpdater.parents[curr]; \
	self->typeUpdater.noteRecursiveRemoval(curr); \
	ExpressionManipulator::convert<CLASS_TO_VISIT, Unreachable>(static_cast<CLASS_TO_VISIT*>(curr)); \
	self->typeUpdater.noteAddition(curr, parent); \
	break; \
	}
	switch (curr->_id) {
	case Expression::Id::BlockId: DELEGATE(Block);
	case Expression::Id::IfId: DELEGATE(If);
	case Expression::Id::LoopId: DELEGATE(Loop);
	case Expression::Id::BreakId: DELEGATE(Break);
	case Expression::Id::SwitchId: DELEGATE(Switch);
	case Expression::Id::CallId: DELEGATE(Call);
	case Expression::Id::CallImportId: DELEGATE(CallImport);
	case Expression::Id::CallIndirectId: DELEGATE(CallIndirect);
	case Expression::Id::GetLocalId: DELEGATE(GetLocal);
	case Expression::Id::SetLocalId: DELEGATE(SetLocal);
	case Expression::Id::GetGlobalId: DELEGATE(GetGlobal);
	case Expression::Id::SetGlobalId: DELEGATE(SetGlobal);
	case Expression::Id::LoadId: DELEGATE(Load);
	case Expression::Id::StoreId: DELEGATE(Store);
	case Expression::Id::ConstId: DELEGATE(Const);
	case Expression::Id::UnaryId: DELEGATE(Unary);
	case Expression::Id::BinaryId: DELEGATE(Binary);
	case Expression::Id::SelectId: DELEGATE(Select);
	case Expression::Id::DropId: DELEGATE(Drop);
	case Expression::Id::ReturnId: DELEGATE(Return);
	case Expression::Id::HostId: DELEGATE(Host);
	case Expression::Id::NopId: DELEGATE(Nop);
	case Expression::Id::UnreachableId: break;
	case Expression::Id::AtomicCmpxchgId: DELEGATE(AtomicCmpxchg);
	case Expression::Id::AtomicRMWId: DELEGATE(AtomicRMW);
	case Expression::Id::AtomicWaitId: DELEGATE(AtomicWait);
	case Expression::Id::AtomicWakeId: DELEGATE(AtomicWake);
	case Expression::Id::InvalidId:
	default: WASM_UNREACHABLE();
	}
	#undef DELEGATE
	return;
	}
	if (curr->is<If>()) {
	self->pushTask(DeadCodeElimination::doVisitIf, currp);
	if (curr->cast<If>()->ifFalse) {
	self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->ifFalse);
	self->pushTask(DeadCodeElimination::doAfterIfElseTrue, currp);
	}
	self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->ifTrue);
	self->pushTask(DeadCodeElimination::doAfterIfCondition, currp);
	self->pushTask(DeadCodeElimination::scan, &curr->cast<If>()->condition);
	} else {
	super::scan(self, currp);
	}
	}

	// other things

	// we don't need to drop unreachable nodes
	Expression* drop(Expression* toDrop) {
	if (toDrop->type == unreachable) return toDrop;
	return Builder(*getModule()).makeDrop(toDrop);
	}

	template<typename T>
	Expression* handleCall(T* curr) {
	for (Index i = 0; i < curr->operands.size(); i++) {
	if (isUnreachable(curr->operands[i])) {
	if (i > 0) {
	auto* block = getModule()->allocator.alloc<Block>();
	Index newSize = i + 1;
	block->list.resize(newSize);
	Index j = 0;
	for (; j < newSize; j++) {
	block->list[j] = drop(curr->operands[j]);
	}
	block->finalize(curr->type);
	return replaceCurrent(block);
	} else {
	return replaceCurrent(curr->operands[i]);
	}
	}
	}
	return curr;
	}

	void visitCall(Call* curr) {
	handleCall(curr);
	}

	void visitCallImport(CallImport* curr) {
	handleCall(curr);
	}

	void visitCallIndirect(CallIndirect* curr) {
	if (handleCall(curr) != curr) return;
	if (isUnreachable(curr->target)) {
	auto* block = getModule()->allocator.alloc<Block>();
	for (auto* operand : curr->operands) {
	block->list.push_back(drop(operand));
	}
	block->list.push_back(curr->target);
	block->finalize(curr->type);
	replaceCurrent(block);
	}
	}

	// Append the reachable operands of the current node to a block, and replace
	// it with the block
	void blockifyReachableOperands(std::vector<Expression*>&& list, Type type) {
	for (size_t i = 0; i < list.size(); ++i) {
	auto* elem = list[i];
	if (isUnreachable(elem)) {
	auto* replacement = elem;
	if (i > 0) {
	auto* block = getModule()->allocator.alloc<Block>();
	for (size_t j = 0; j < i; ++j) {
	block->list.push_back(drop(list[j]));
	}
	block->list.push_back(list[i]);
	block->finalize(type);
	replacement = block;
	}
	replaceCurrent(replacement);
	return;
	}
	}
	}

	void visitSetLocal(SetLocal* curr) {
	blockifyReachableOperands({ curr->value }, curr->type);
	}

	void visitSetGlobal(SetGlobal* curr) {
	blockifyReachableOperands({ curr->value }, curr->type);
	}

	void visitLoad(Load* curr) {
	blockifyReachableOperands({ curr->ptr }, curr->type);
	}

	void visitStore(Store* curr) {
	blockifyReachableOperands({ curr->ptr, curr->value }, curr->type);
	}

	void visitAtomicRMW(AtomicRMW* curr) {
	blockifyReachableOperands({ curr->ptr, curr->value }, curr->type);
	}

	void visitAtomicCmpxchg(AtomicCmpxchg* curr) {
	blockifyReachableOperands({ curr->ptr, curr->expected, curr->replacement }, curr->type);
	}

	void visitUnary(Unary* curr) {
	blockifyReachableOperands({ curr->value }, curr->type);
	}

	void visitBinary(Binary* curr) {
	blockifyReachableOperands({ curr->left, curr->right }, curr->type);
	}

	void visitSelect(Select* curr) {
	blockifyReachableOperands({ curr->ifTrue, curr->ifFalse, curr->condition }, curr->type);
	}

	void visitDrop(Drop* curr) {
	blockifyReachableOperands({ curr->value }, curr->type);
	}

	void visitHost(Host* curr) {
	handleCall(curr);
	}

	void visitFunction(Function* curr) {
	assert(reachableBreaks.size() == 0);
	}
	};

	Pass *createDeadCodeEliminationPass() {
	return new DeadCodeElimination();
	}

	} // namespace wasm