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/*
* 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 "ir/eh-utils.h"
#include "ir/iteration.h"
#include "ir/properties.h"
#include "ir/type-updating.h"
#include "pass.h"
#include "vector"
#include "wasm-builder.h"
#include "wasm.h"
namespace wasm {
struct DeadCodeElimination
: public WalkerPass<
PostWalker<DeadCodeElimination,
UnifiedExpressionVisitor<DeadCodeElimination>>> {
bool isFunctionParallel() override { return true; }
// This pass removes dead code, which can only help validation (a dead
// local.get might have prevented validation).
bool requiresNonNullableLocalFixups() override { return false; }
std::unique_ptr<Pass> create() override {
return std::make_unique<DeadCodeElimination>();
}
// as we remove code, we must keep the types of other nodes valid
TypeUpdater typeUpdater;
// Information used to decide whether we need EH fixups at the end
bool hasPop = false; // Do we have a 'pop' in this function?
bool addedBlock = false; // Have we added blocks in this function?
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;
}
void doWalkFunction(Function* func) {
typeUpdater.walk(func->body);
walk(func->body);
}
void visitExpression(Expression* curr) {
if (curr->is<Pop>()) {
hasPop = true;
}
if (!Properties::isControlFlowStructure(curr)) {
// Control flow structures require special handling, but others are
// simple.
if (curr->type == Type::unreachable) {
// This may be dead code. Check if there is an unreachable child.
bool hasUnreachableChild = false;
for (auto* child : ChildIterator(curr)) {
if (child->type == Type::unreachable) {
hasUnreachableChild = true;
break;
}
}
if (hasUnreachableChild) {
// This is indeed unreachable code, made unreachable by that child.
Builder builder(*getModule());
std::vector<Expression*> remainingChildren;
bool afterUnreachable = false;
for (auto* child : ChildIterator(curr)) {
if (afterUnreachable) {
typeUpdater.noteRecursiveRemoval(child);
continue;
}
if (child->type == Type::unreachable) {
remainingChildren.push_back(child);
afterUnreachable = true;
} else {
remainingChildren.push_back(builder.makeDrop(child));
}
}
if (remainingChildren.size() == 1) {
replaceCurrent(remainingChildren[0]);
} else {
addedBlock = true;
replaceCurrent(builder.makeBlock(remainingChildren));
}
}
}
return;
}
// This is a control flow structure.
if (auto* block = curr->dynCast<Block>()) {
auto& list = block->list;
// The index from which to remove, which is one after the first
// unreachable instruction. Note that 0 is not a valid value, so we can
// use it as such.
Index removeFromHere = 0;
for (Index i = 0; i < list.size(); i++) {
if (list[i]->type == Type::unreachable) {
removeFromHere = i + 1;
break;
}
}
if (removeFromHere != 0) {
for (Index i = removeFromHere; i < list.size(); i++) {
typeUpdater.noteRecursiveRemoval(list[i]);
}
list.resize(removeFromHere);
if (list.size() == 1 && list[0]->is<Unreachable>()) {
replaceCurrent(list[0]);
return;
}
}
// Finally, if there is no need for a concrete type (which is when there
// is one marked, but nothing breaks to it, and also the block does not
// have a concrete value flowing out) then remove it, which may allow
// more reduction.
if (block->type.isConcrete() && list.back()->type == Type::unreachable &&
!typeUpdater.hasBreaks(block)) {
typeUpdater.changeType(block, Type::unreachable);
}
} else if (auto* iff = curr->dynCast<If>()) {
if (iff->condition->type == Type::unreachable) {
typeUpdater.noteRecursiveRemoval(iff->ifTrue);
if (iff->ifFalse) {
typeUpdater.noteRecursiveRemoval(iff->ifFalse);
}
replaceCurrent(iff->condition);
return;
}
// If both arms are unreachable, there is no need for a concrete type,
// which may allow more reduction.
if (iff->type != Type::unreachable && iff->ifFalse &&
iff->ifTrue->type == Type::unreachable &&
iff->ifFalse->type == Type::unreachable) {
typeUpdater.changeType(iff, Type::unreachable);
}
} else if (auto* loop = curr->dynCast<Loop>()) {
// The loop body may have unreachable type if it branches back to the
// loop top, for example. The only case we look for here is where we've
// already removed the entire body as dead code.
if (loop->body->is<Unreachable>()) {
replaceCurrent(loop->body);
}
} else if (auto* tryy = curr->dynCast<Try>()) {
// If both try body and catch body are unreachable, there is no need for a
// concrete type, which may allow more reduction.
bool allCatchesUnreachable = true;
for (auto* catchBody : tryy->catchBodies) {
allCatchesUnreachable &= catchBody->type == Type::unreachable;
}
if (tryy->type != Type::unreachable &&
tryy->body->type == Type::unreachable && allCatchesUnreachable) {
typeUpdater.changeType(tryy, Type::unreachable);
}
} else if (auto* tryTable = curr->dynCast<TryTable>()) {
// try_table can finish normally only if its body finishes normally.
if (tryTable->type != Type::unreachable &&
tryTable->body->type == Type::unreachable) {
typeUpdater.changeType(tryTable, Type::unreachable);
}
} else {
WASM_UNREACHABLE("unimplemented DCE control flow structure");
}
}
void visitFunction(Function* curr) {
// We conservatively run the EH pop fixup if this function has a 'pop' and
// if we have ever added blocks in the optimization, which may have moved
// pops into the blocks.
if (hasPop && addedBlock) {
EHUtils::handleBlockNestedPops(curr, *getModule());
}
}
};
Pass* createDeadCodeEliminationPass() { return new DeadCodeElimination(); }
} // namespace wasm
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