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v8/include/cppgc/platform.h

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+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef INCLUDE_CPPGC_PLATFORM_H_
+#define INCLUDE_CPPGC_PLATFORM_H_
+
+#include <memory>
+
+#include "cppgc/source-location.h"
+#include "v8-platform.h"  // NOLINT(build/include_directory)
+#include "v8config.h"     // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+// TODO(v8:10346): Create separate includes for concepts that are not
+// V8-specific.
+using IdleTask = v8::IdleTask;
+using JobHandle = v8::JobHandle;
+using JobDelegate = v8::JobDelegate;
+using JobTask = v8::JobTask;
+using PageAllocator = v8::PageAllocator;
+using Task = v8::Task;
+using TaskPriority = v8::TaskPriority;
+using TaskRunner = v8::TaskRunner;
+using TracingController = v8::TracingController;
+
+/**
+ * Platform interface used by Heap. Contains allocators and executors.
+ */
+class V8_EXPORT Platform {
+ public:
+  virtual ~Platform() = default;
+
+  /**
+   * Returns the allocator used by cppgc to allocate its heap and various
+   * support structures.
+   */
+  virtual PageAllocator* GetPageAllocator() = 0;
+
+  /**
+   * Monotonically increasing time in seconds from an arbitrary fixed point in
+   * the past. This function is expected to return at least
+   * millisecond-precision values. For this reason,
+   * it is recommended that the fixed point be no further in the past than
+   * the epoch.
+   **/
+  virtual double MonotonicallyIncreasingTime() = 0;
+
+  /**
+   * Foreground task runner that should be used by a Heap.
+   */
+  virtual std::shared_ptr<TaskRunner> GetForegroundTaskRunner() {
+    return nullptr;
+  }
+
+  /**
+   * Posts `job_task` to run in parallel. Returns a `JobHandle` associated with
+   * the `Job`, which can be joined or canceled.
+   * This avoids degenerate cases:
+   * - Calling `CallOnWorkerThread()` for each work item, causing significant
+   *   overhead.
+   * - Fixed number of `CallOnWorkerThread()` calls that split the work and
+   *   might run for a long time. This is problematic when many components post
+   *   "num cores" tasks and all expect to use all the cores. In these cases,
+   *   the scheduler lacks context to be fair to multiple same-priority requests
+   *   and/or ability to request lower priority work to yield when high priority
+   *   work comes in.
+   * A canonical implementation of `job_task` looks like:
+   * \code
+   * class MyJobTask : public JobTask {
+   *  public:
+   *   MyJobTask(...) : worker_queue_(...) {}
+   *   // JobTask implementation.
+   *   void Run(JobDelegate* delegate) override {
+   *     while (!delegate->ShouldYield()) {
+   *       // Smallest unit of work.
+   *       auto work_item = worker_queue_.TakeWorkItem(); // Thread safe.
+   *       if (!work_item) return;
+   *       ProcessWork(work_item);
+   *     }
+   *   }
+   *
+   *   size_t GetMaxConcurrency() const override {
+   *     return worker_queue_.GetSize(); // Thread safe.
+   *   }
+   * };
+   *
+   * // ...
+   * auto handle = PostJob(TaskPriority::kUserVisible,
+   *                       std::make_unique<MyJobTask>(...));
+   * handle->Join();
+   * \endcode
+   *
+   * `PostJob()` and methods of the returned JobHandle/JobDelegate, must never
+   * be called while holding a lock that could be acquired by `JobTask::Run()`
+   * or `JobTask::GetMaxConcurrency()` -- that could result in a deadlock. This
+   * is because (1) `JobTask::GetMaxConcurrency()` may be invoked while holding
+   * internal lock (A), hence `JobTask::GetMaxConcurrency()` can only use a lock
+   * (B) if that lock is *never* held while calling back into `JobHandle` from
+   * any thread (A=>B/B=>A deadlock) and (2) `JobTask::Run()` or
+   * `JobTask::GetMaxConcurrency()` may be invoked synchronously from
+   * `JobHandle` (B=>JobHandle::foo=>B deadlock).
+   *
+   * A sufficient `PostJob()` implementation that uses the default Job provided
+   * in libplatform looks like:
+   * \code
+   * std::unique_ptr<JobHandle> PostJob(
+   *     TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
+   *   return std::make_unique<DefaultJobHandle>(
+   *       std::make_shared<DefaultJobState>(
+   *           this, std::move(job_task), kNumThreads));
+   * }
+   * \endcode
+   */
+  virtual std::unique_ptr<JobHandle> PostJob(
+      TaskPriority priority, std::unique_ptr<JobTask> job_task) {
+    return nullptr;
+  }
+
+  /**
+   * Returns an instance of a `TracingController`. This must be non-nullptr. The
+   * default implementation returns an empty `TracingController` that consumes
+   * trace data without effect.
+   */
+  virtual TracingController* GetTracingController();
+};
+
+/**
+ * Process-global initialization of the garbage collector. Must be called before
+ * creating a Heap.
+ *
+ * Can be called multiple times when paired with `ShutdownProcess()`.
+ *
+ * \param page_allocator The allocator used for maintaining meta data. Must stay
+ *   always alive and not change between multiple calls to InitializeProcess.
+ */
+V8_EXPORT void InitializeProcess(PageAllocator* page_allocator);
+
+/**
+ * Must be called after destroying the last used heap. Some process-global
+ * metadata may not be returned and reused upon a subsequent
+ * `InitializeProcess()` call.
+ */
+V8_EXPORT void ShutdownProcess();
+
+namespace internal {
+
+V8_EXPORT void Fatal(const std::string& reason = std::string(),
+                     const SourceLocation& = SourceLocation::Current());
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_PLATFORM_H_