src/ir/branch-utils.h

/*
 * Copyright 2017 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_branch_h
#define wasm_ir_branch_h

#include "wasm.h"
#include "wasm-traversal.h"

namespace wasm {

namespace BranchUtils {

// Some branches are obviously not actually reachable (e.g. (br $out (unreachable)))

inline bool isBranchReachable(Break* br) {
  return !(br->value     && br->value->type     == unreachable) &&
         !(br->condition && br->condition->type == unreachable);
}

inline bool isBranchReachable(Switch* sw) {
  return !(sw->value && sw->value->type     == unreachable) &&
                        sw->condition->type != unreachable;
}

inline bool isBranchReachable(Expression* expr) {
  if (auto* br = expr->dynCast<Break>()) {
    return isBranchReachable(br);
  } else if (auto* sw = expr->dynCast<Switch>()) {
    return isBranchReachable(sw);
  }
  WASM_UNREACHABLE();
}

// returns the set of targets to which we branch that are
// outside of a node
inline std::set<Name> getExitingBranches(Expression* ast) {
  struct Scanner : public PostWalker<Scanner> {
    std::set<Name> targets;

    void visitBreak(Break* curr) {
      targets.insert(curr->name);
    }
    void visitSwitch(Switch* curr) {
      for (auto target : targets) {
        targets.insert(target);
      }
      targets.insert(curr->default_);
    }
    void visitBlock(Block* curr) {
      if (curr->name.is()) {
        targets.erase(curr->name);
      }
    }
    void visitLoop(Loop* curr) {
      if (curr->name.is()) {
        targets.erase(curr->name);
      }
    }
  };
  Scanner scanner;
  scanner.walk(ast);
  // anything not erased is a branch out
  return scanner.targets;
}

// returns the list of all branch targets in a node

inline std::set<Name> getBranchTargets(Expression* ast) {
  struct Scanner : public PostWalker<Scanner> {
    std::set<Name> targets;

    void visitBlock(Block* curr) {
      if (curr->name.is()) {
        targets.insert(curr->name);
      }
    }
    void visitLoop(Loop* curr) {
      if (curr->name.is()) {
        targets.insert(curr->name);
      }
    }
  };
  Scanner scanner;
  scanner.walk(ast);
  return scanner.targets;
}

// Finds if there are branches targeting a name. Note that since names are
// unique in our IR, we just need to look for the name, and do not need
// to analyze scoping.
// By default we consider all branches, so any place there is a branch that
// names the target. You can unset 'named' to only note branches that appear
// reachable (i.e., are not obviously unreachable).
struct BranchSeeker : public PostWalker<BranchSeeker> {
  Name target;
  bool named = true;

  Index found;
  Type valueType;

  BranchSeeker(Name target) : target(target), found(0) {}

  void noteFound(Expression* value) {
    found++;
    if (found == 1) valueType = unreachable;
    if (!value) valueType = none;
    else if (value->type != unreachable) valueType = value->type;
  }

  void visitBreak(Break *curr) {
    if (!named) {
      // ignore an unreachable break
      if (curr->condition && curr->condition->type == unreachable) return;
      if (curr->value && curr->value->type == unreachable) return;
    }
    // check the break
    if (curr->name == target) noteFound(curr->value);
  }

  void visitSwitch(Switch *curr) {
    if (!named) {
      // ignore an unreachable switch
      if (curr->condition->type == unreachable) return;
      if (curr->value && curr->value->type == unreachable) return;
    }
    // check the switch
    for (auto name : curr->targets) {
      if (name == target) noteFound(curr->value);
    }
    if (curr->default_ == target) noteFound(curr->value);
  }

  static bool hasReachable(Expression* tree, Name target) {
    if (!target.is()) return false;
    BranchSeeker seeker(target);
    seeker.named = false;
    seeker.walk(tree);
    return seeker.found > 0;
  }

  static Index countReachable(Expression* tree, Name target) {
    if (!target.is()) return 0;
    BranchSeeker seeker(target);
    seeker.named = false;
    seeker.walk(tree);
    return seeker.found;
  }

  static bool hasNamed(Expression* tree, Name target) {
    if (!target.is()) return false;
    BranchSeeker seeker(target);
    seeker.walk(tree);
    return seeker.found > 0;
  }

  static Index countNamed(Expression* tree, Name target) {
    if (!target.is()) return 0;
    BranchSeeker seeker(target);
    seeker.walk(tree);
    return seeker.found;
  }
};

} // namespace BranchUtils

} // namespace wasm

#endif // wasm_ir_branch_h