src/passes/SpillPointers.cpp

/*
 * 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.
 */

//
// Spills values that might be pointers to the C stack. This allows
// Boehm-style GC to see them properly.
//
// To reduce the overhead of the extra operations added here, you
// should probably run optimizations after doing it.
// TODO: add a dead store elimination pass, which would help here
//
//  * There is currently no check that there is enough stack space.
//

#include "abi/stack.h"
#include "cfg/liveness-traversal.h"
#include "pass.h"
#include "wasm-builder.h"
#include "wasm.h"

namespace wasm {

struct SpillPointers
  : public WalkerPass<LivenessWalker<SpillPointers, Visitor<SpillPointers>>> {
  bool isFunctionParallel() override { return true; }

  // Adds writes to memory.
  bool addsEffects() override { return true; }

  std::unique_ptr<Pass> create() override {
    return std::make_unique<SpillPointers>();
  }

  // a mapping of the pointers to all the spillable things. We need to know
  // how to replace them, and as we spill we may modify them. This map
  // gives us, for an Expression** seen during the walk (and placed in the
  // basic block, which is what we iterate on for efficiency) => the
  // current actual pointer, which may have moded
  std::unordered_map<Expression**, Expression**> actualPointers;

  // note calls in basic blocks
  template<typename T> void visitSpillable(T* curr) {
    // if in unreachable code, ignore
    if (!currBasicBlock) {
      return;
    }
    auto* pointer = getCurrentPointer();
    currBasicBlock->contents.actions.emplace_back(pointer);
    // starts out as correct, may change later
    actualPointers[pointer] = pointer;
  }

  void visitCall(Call* curr) { visitSpillable(curr); }
  void visitCallIndirect(CallIndirect* curr) { visitSpillable(curr); }

  // main entry point

  void doWalkFunction(Function* func) {
    Super::doWalkFunction(func);
    spillPointers();
  }

  // map pointers to their offset in the spill area
  using PointerMap = std::unordered_map<Index, Index>;

  Type pointerType;

  void spillPointers() {
    pointerType = getModule()->memories[0]->addressType;

    // we only care about possible pointers
    auto* func = getFunction();
    PointerMap pointerMap;
    for (Index i = 0; i < func->getNumLocals(); i++) {
      if (func->getLocalType(i) == pointerType) {
        auto offset = pointerMap.size() * pointerType.getByteSize();
        pointerMap[i] = offset;
      }
    }
    // find calls and spill around them
    bool spilled = false;
    Index spillLocal = -1;
    for (auto& curr : basicBlocks) {
      if (!liveBlocks.contains(curr.get())) {
        continue; // ignore dead blocks
      }
      auto& liveness = curr->contents;
      auto& actions = liveness.actions;
      Index lastCall = -1;
      for (Index i = 0; i < actions.size(); i++) {
        auto& action = liveness.actions[i];
        if (action.isOther()) {
          lastCall = i;
        }
      }
      if (lastCall == Index(-1)) {
        continue; // nothing to see here
      }
      // scan through the block, spilling around the calls
      // TODO: we can filter on pointerMap everywhere
      SetOfLocals live = liveness.end;
      for (int i = int(actions.size()) - 1; i >= 0; i--) {
        auto& action = actions[i];
        if (action.isGet()) {
          live.insert(action.index);
        } else if (action.isSet()) {
          live.erase(action.index);
        } else if (action.isOther()) {
          std::vector<Index> toSpill;
          for (auto index : live) {
            if (pointerMap.contains(index)) {
              toSpill.push_back(index);
            }
          }
          if (!toSpill.empty()) {
            // we now have a call + the information about which locals
            // should be spilled
            if (!spilled) {
              // prepare stack support: get a pointer to stack space big enough
              // for all our data
              spillLocal = Builder::addVar(func, pointerType);
              spilled = true;
            }
            // the origin was seen at walk, but the thing may have moved
            auto* pointer = actualPointers[action.origin];
            spillPointersAroundCall(
              pointer, toSpill, spillLocal, pointerMap, func, getModule());
          }
        } else {
          WASM_UNREACHABLE("unexpected action");
        }
      }
    }
    if (spilled) {
      // get the stack space, and set the local to it
      ABI::getStackSpace(spillLocal,
                         func,
                         pointerType.getByteSize() * pointerMap.size(),
                         *getModule());
    }
  }

  void spillPointersAroundCall(Expression** origin,
                               std::vector<Index>& toSpill,
                               Index spillLocal,
                               PointerMap& pointerMap,
                               Function* func,
                               Module* module) {
    auto* call = *origin;
    if (call->type == Type::unreachable) {
      return; // the call is never reached anyhow, ignore
    }
    Builder builder(*module);
    auto* block = builder.makeBlock();
    // move the operands into locals, as we must spill after they are executed
    auto handleOperand = [&](Expression*& operand) {
      auto temp = builder.addVar(func, operand->type);
      auto* set = builder.makeLocalSet(temp, operand);
      block->list.push_back(set);
      block->finalize();
      if (actualPointers.contains(&operand)) {
        // this is something we track, and it's moving - update
        actualPointers[&operand] = &set->value;
      }
      operand = builder.makeLocalGet(temp, operand->type);
    };
    if (call->is<Call>()) {
      for (auto*& operand : call->cast<Call>()->operands) {
        handleOperand(operand);
      }
    } else if (call->is<CallIndirect>()) {
      for (auto*& operand : call->cast<CallIndirect>()->operands) {
        handleOperand(operand);
      }
      handleOperand(call->cast<CallIndirect>()->target);
    } else {
      WASM_UNREACHABLE("unexpected expr");
    }
    // add the spills
    for (auto index : toSpill) {
      block->list.push_back(
        builder.makeStore(pointerType.getByteSize(),
                          pointerMap[index],
                          pointerType.getByteSize(),
                          builder.makeLocalGet(spillLocal, pointerType),
                          builder.makeLocalGet(index, pointerType),
                          pointerType,
                          getModule()->memories[0]->name));
    }
    // add the (modified) call
    block->list.push_back(call);
    block->finalize();
    *origin = block;
  }
};

Pass* createSpillPointersPass() { return new SpillPointers(); }

} // namespace wasm