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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. This is useful if we know the node is not needed but may need to
// keep the children around; this utility will automatically remove any children
// we do not actually need to keep, based on their effects.
//
// The caller must also pass in a last item to append to the output (which is
// typically what the original expression is replaced with).
inline Expression* getDroppedChildrenAndAppend(Expression* curr,
Module& wasm,
const PassOptions& options,
Expression* last) {
Builder builder(wasm);
std::vector<Expression*> contents;
for (auto* child : ChildIterator(curr)) {
if (!EffectAnalyzer(options, wasm, child).hasUnremovableSideEffects()) {
continue;
}
if (child->type.isConcrete()) {
contents.push_back(builder.makeDrop(child));
} else {
// The child is unreachable, or none (none is possible as a child of a
// block or loop, etc.); in both cases we do not need a drop.
contents.push_back(child);
}
}
contents.push_back(last);
if (contents.size() == 1) {
return contents[0];
}
return builder.makeBlock(contents);
}
// As the above, but 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. The above function
// simply returns all children in order, so it does this to if:
//
// (if
// (condition)
// (arm-A)
// (arm-B)
// )
// =>
// (drop
// (condition)
// )
// (drop
// (arm-A)
// )
// (drop
// (arm-B)
// )
// (appended last item)
//
// This is dangerous as it executes what were conditional children in an
// unconditional way. To avoid that issue, this function will only operate on
// unconditional children, and keep conditional ones as they were. That means
// it will not split up and drop the children of an if, for example. All we do
// in that case is drop the entire if and append the last item:
//
// (drop
// (if
// (condition)
// (arm-A)
// (arm-B)
// )
// )
// (appended last item)
//
// Also this function preserves other unremovable expressions like trys and
// pops.
inline Expression*
getDroppedUnconditionalChildrenAndAppend(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->type == Type::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.
if (effects.hasUnremovableSideEffects() || curr->is<If>() ||
curr->is<Try>() || curr->is<Pop>() ||
BranchUtils::getDefinedName(curr).is()) {
Builder builder(wasm);
return builder.makeSequence(builder.makeDrop(curr), last);
}
return getDroppedChildrenAndAppend(curr, wasm, options, last);
}
} // namespace wasm
#endif // wasm_ir_drop_h
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