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/*
* Copyright 2022 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.
*/
#ifndef wasm_ir_drop_h
#define wasm_ir_drop_h
#include "ir/branch-utils.h"
#include "ir/effects.h"
#include "ir/iteration.h"
#include "wasm-builder.h"
#include "wasm.h"
namespace wasm {
// Given an expression, returns a new expression that drops the given
// expression's children that cannot be removed outright due to their side
// effects. Note that this only operates on children that execute
// unconditionally. That is the case in almost all expressions, except for those
// with conditional execution, like if, which unconditionally executes the
// condition but then conditionally executes one of the two arms.
Expression* getDroppedChildrenAndAppend(Expression* curr,
Module& wasm,
const PassOptions& options,
Expression* last) {
// We check for shallow effects here, since we may be able to remove |curr|
// itself but keep its children around - we don't want effects in the children
// to stop us from improving the code. Note that there are cases where the
// combined curr+children has fewer effects than curr itself, such as if curr
// is a block and the child branches to it, but in such cases we cannot remove
// curr anyhow (those cases are ruled out below), so looking at non-shallow
// effects would never help us (and would be slower to run).
ShallowEffectAnalyzer effects(options, wasm, curr);
// Ignore a trap, as the unreachable replacement would trap too.
if (last->is<Unreachable>()) {
effects.trap = false;
}
// We cannot remove
// 1. Expressions with unremovable side effects
// 2. if: 'if's contains conditional expressions
// 3. try: Removing a try could leave a pop without a proper parent
// 4. pop: Pops are struturally necessary in catch bodies
// 5. Branch targets: We will need the target for the branches to it to
// validate.
Builder builder(wasm);
if (effects.hasUnremovableSideEffects() || curr->is<If>() ||
curr->is<Try>() || curr->is<Pop>() ||
BranchUtils::getDefinedName(curr).is()) {
// If curr is concrete we must drop it. Or, if it is unreachable or none,
// then we can leave it as it is.
if (curr->type.isConcrete()) {
curr = builder.makeDrop(curr);
}
return builder.makeSequence(curr, last);
}
std::vector<Expression*> contents;
for (auto* child : ChildIterator(curr)) {
if (!EffectAnalyzer(options, wasm, child).hasUnremovableSideEffects()) {
continue;
}
// See above.
if (child->type.isConcrete()) {
contents.push_back(builder.makeDrop(child));
} else {
contents.push_back(child);
}
}
if (contents.empty()) {
return last;
}
contents.push_back(last);
return builder.makeBlock(contents);
}
} // namespace wasm
#endif // wasm_ir_drop_h
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