src/ir/drop.cpp

/*
 * 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()) {
    return builder.makeSequence(builder.makeDrop(curr), last);
  }

  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);
    }
  }
  if (contents.empty()) {
    return last;
  }
  contents.push_back(last);
  return builder.makeBlock(contents);
}

} // namespace wasm

#endif // wasm_ir_drop_h