src/wasm-module-building.h

/*
 * Copyright 2016 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 optimizing_incremental_module_builder_h
#define optimizing_incremental_module_builder_h

#include <wasm.h>
#include <support/threads.h>

namespace wasm {

#ifdef BINARYEN_THREAD_DEBUG
static std::mutex debug;
#define DEBUG_THREAD(x) { std::lock_guard<std::mutex> lock(debug); std::cerr << "[OptimizingIncrementalModuleBuilder Threading (thread: " << std::this_thread::get_id()  << ")] " << x; std::cerr << '\n'; }
#else
#define DEBUG_THREAD(x)
#endif

//
// OptimizingIncrementalModuleBuilder
//
// Helps build wasm modules efficiently. If you build a module by
// adding function by function, and you want to optimize them, this class
// starts optimizing using worker threads *while you are still adding*.
// It runs function optimization passes at that time, and then at the end
// it runs global module-level optimization passes. The result is a fully
// optimized module, optimized while being generated.
//
// This might also be faster than normal module optimization since it
// runs all passes on each function, then goes on to the next function
// which is better for data locality.
//
// Usage: Create an instance, passing it the module and the total
//        number of functions. Then call addFunction as you have
//        new functions to add (this also adds it to the module). Finally,
//        call finish() when all functions have been added.
//
// This avoids locking by using atomics. We allocate an array of nullptrs
// that represent all the functions, and as we add a function, we place it
// at the next index. Each worker will read from the start to the end,
// and when a non-nullptr is found, the worker optimizes that function and
// nulls it. There is also an end marker that is not nullptr nor the address of
// a valid function, which represents that beyond this point we have not
// yet filled in. In other words,
//    * the main thread fills everything with the end marker
//    * the main thread transforms a marker entry into a function
//    * workers pause when they see the marker
//    * workers skip over nullptrs
//    * workers transform functions into nullptrs, and optimize them
//    * we keep an atomic count of the number of active workers and
//      the number of optimized functions.
//    * after adding a function, the main thread wakes up workers if
//      it calculates there is work for them.
//    * a lock is used for going to sleep and waking up.
// Locking should be rare, as optimization is
// generally slower than generation; in the optimal case, we never
// lock beyond the first step, and all further work is lock-free.
//

class OptimizingIncrementalModuleBuilder {
  Module* wasm;
  uint32_t numFunctions;
  PassOptions passOptions;
  std::function<void (PassRunner&)> addPrePasses;
  Function* endMarker;
  std::atomic<Function*>* list;
  uint32_t nextFunction; // only used on main thread
  uint32_t numWorkers;
  std::vector<std::unique_ptr<std::thread>> threads;
  std::atomic<uint32_t> liveWorkers, activeWorkers, availableFuncs, finishedFuncs;
  std::mutex mutex;
  std::condition_variable condition;
  bool finishing;
  bool debug;
  bool validateGlobally;

public:
  // numFunctions must be equal to the number of functions allocated, or higher. Knowing
  // this bounds helps avoid locking.
  OptimizingIncrementalModuleBuilder(Module* wasm, Index numFunctions, PassOptions passOptions, std::function<void (PassRunner&)> addPrePasses, bool debug, bool validateGlobally)
      : wasm(wasm), numFunctions(numFunctions), passOptions(passOptions), addPrePasses(addPrePasses), endMarker(nullptr), list(nullptr), nextFunction(0),
        numWorkers(0), liveWorkers(0), activeWorkers(0), availableFuncs(0), finishedFuncs(0),
        finishing(false), debug(debug), validateGlobally(validateGlobally) {
    if (numFunctions == 0 || debug) {
      // if no functions to be optimized, or debug non-parallel mode, don't create any threads.
      return;
    }

    // Before parallelism, create all passes on the main thread here, to ensure
    // prepareToRun() is called for each pass before we start to optimize functions.
    {
      PassRunner passRunner(wasm, passOptions);
      addPrePasses(passRunner);
      passRunner.addDefaultFunctionOptimizationPasses();
    }

    // prepare work list
    endMarker = new Function();
    list = new std::atomic<Function*>[numFunctions];
    for (uint32_t i = 0; i < numFunctions; i++) {
      list[i].store(endMarker);
    }
    // create workers
    DEBUG_THREAD("creating workers");
    numWorkers = ThreadPool::getNumCores();
    assert(numWorkers >= 1);
    liveWorkers.store(0);
    activeWorkers.store(0);
    for (uint32_t i = 0; i < numWorkers; i++) { // TODO: one less, and add it at the very end, to not compete with main thread?
      createWorker();
    }
    waitUntilAllReady();
    DEBUG_THREAD("workers are ready");
    // prepare the rest of the initial state
    availableFuncs.store(0);
    finishedFuncs.store(0);
  }

  ~OptimizingIncrementalModuleBuilder() {
    delete[] list;
    delete endMarker;
  }

  // Add a function to the module, and to be optimized
  void addFunction(Function* func) {
    wasm->addFunction(func);
    if (debug) return; // we optimize at the end if debugging
    queueFunction(func);
    // wake workers if needed
    auto wake = availableFuncs.load();
    for (uint32_t i = 0; i < wake; i++) {
      wakeWorker();
    }
  }

  // All functions have been added, block until all are optimized, and then do
  // global optimizations. When this returns, the module is ready and optimized.
  void finish() {
    if (debug) {
      // in debug mode, optimize each function now that we are done adding functions,
      // then optimize globally
      PassRunner passRunner(wasm, passOptions);
      passRunner.setDebug(true);
      passRunner.setValidateGlobally(validateGlobally);
      addPrePasses(passRunner);
      passRunner.addDefaultFunctionOptimizationPasses();
      passRunner.addDefaultGlobalOptimizationPasses();
      passRunner.run();
      return;
    }
    DEBUG_THREAD("finish()ing");
    assert(nextFunction == numFunctions);
    wakeAllWorkers();
    waitUntilAllFinished();
    optimizeGlobally();
    // TODO: clear side thread allocators from module allocator, as these threads were transient
  }

private:
  void createWorker() {
    DEBUG_THREAD("create a worker");
    threads.emplace_back(make_unique<std::thread>(workerMain, this));
  }

  void wakeWorker() {
    DEBUG_THREAD("wake a worker");
    std::lock_guard<std::mutex> lock(mutex);
    condition.notify_one();
  }

  void wakeAllWorkers() {
    DEBUG_THREAD("wake all workers");
    std::lock_guard<std::mutex> lock(mutex);
    condition.notify_all();
  }

  void waitUntilAllReady() {
    DEBUG_THREAD("wait until all workers are ready");
    std::unique_lock<std::mutex> lock(mutex);
    if (liveWorkers.load() < numWorkers) {
      condition.wait(lock, [this]() { return liveWorkers.load() == numWorkers; });
    }
  }

  void waitUntilAllFinished() {
    DEBUG_THREAD("wait until all workers are finished");
    {
      std::unique_lock<std::mutex> lock(mutex);
      finishing = true;
      if (liveWorkers.load() > 0) {
        condition.wait(lock, [this]() { return liveWorkers.load() == 0; });
      }
    }
    DEBUG_THREAD("joining");
    for (auto& thread : threads) thread->join();
    DEBUG_THREAD("joined");
  }

  void queueFunction(Function* func) {
    DEBUG_THREAD("queue function");
    assert(nextFunction < numFunctions); // TODO: if we are given more than we expected, use a slower work queue?
    list[nextFunction++].store(func);
    availableFuncs++;
  }

  void optimizeGlobally() {
    PassRunner passRunner(wasm, passOptions);
    passRunner.addDefaultGlobalOptimizationPasses();
    passRunner.run();
  }

  // worker code

  void optimizeFunction(Function* func) {
    PassRunner passRunner(wasm, passOptions);
    addPrePasses(passRunner);
    passRunner.addDefaultFunctionOptimizationPasses();
    passRunner.runFunction(func);
  }

  static void workerMain(OptimizingIncrementalModuleBuilder* self) {
    DEBUG_THREAD("workerMain");
    {
      std::lock_guard<std::mutex> lock(self->mutex);
      self->liveWorkers++;
      self->activeWorkers++;
      self->condition.notify_all();
    }
    for (uint32_t i = 0; i < self->numFunctions; i++) {
      DEBUG_THREAD("workerMain iteration " << i);
      if (self->list[i].load() == self->endMarker) {
        // sleep, this entry isn't ready yet
        DEBUG_THREAD("workerMain sleep");
        self->activeWorkers--;
        {
          std::unique_lock<std::mutex> lock(self->mutex);
          if (!self->finishing) { // while waiting for the lock, things may have ended
            self->condition.wait(lock);
          }
        }
        // continue
        DEBUG_THREAD("workerMain continue");
        self->activeWorkers++;
        i--;
        continue;
      }
      DEBUG_THREAD("workerMain exchange item");
      auto* func = self->list[i].exchange(nullptr);
      if (func == nullptr) {
        DEBUG_THREAD("workerMain sees was already taken");
        continue; // someone else has taken this one
      }
      // we have work to do!
      DEBUG_THREAD("workerMain work on " << size_t(func));
      self->availableFuncs--;
      self->optimizeFunction(func);
      self->finishedFuncs++;
    }
    DEBUG_THREAD("workerMain ready to exit");
    {
      std::lock_guard<std::mutex> lock(self->mutex);
      self->liveWorkers--;
      self->condition.notify_all();
    }
    DEBUG_THREAD("workerMain exiting");
  }
};

} // namespace wasm

#endif // optimizing_incremental_module_builder_h