v8-platform.h 24.3 KB
Newer Older
1
// Copyright 2013 the V8 project authors. All rights reserved.
2 3
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
4 5 6 7

#ifndef V8_V8_PLATFORM_H_
#define V8_V8_PLATFORM_H_

8
#include <stddef.h>
9
#include <stdint.h>
10
#include <stdlib.h>  // For abort.
11 12
#include <memory>
#include <string>
13

14
#include "v8config.h"  // NOLINT(build/include_directory)
15

16 17
namespace v8 {

18 19
class Isolate;

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
// Valid priorities supported by the task scheduling infrastructure.
enum class TaskPriority : uint8_t {
  /**
   * Best effort tasks are not critical for performance of the application. The
   * platform implementation should preempt such tasks if higher priority tasks
   * arrive.
   */
  kBestEffort,
  /**
   * User visible tasks are long running background tasks that will
   * improve performance and memory usage of the application upon completion.
   * Example: background compilation and garbage collection.
   */
  kUserVisible,
  /**
   * User blocking tasks are highest priority tasks that block the execution
   * thread (e.g. major garbage collection). They must be finished as soon as
   * possible.
   */
  kUserBlocking,
};

42 43 44 45 46
/**
 * A Task represents a unit of work.
 */
class Task {
 public:
47
  virtual ~Task() = default;
48 49 50 51

  virtual void Run() = 0;
};

52
/**
53 54 55 56 57
 * An IdleTask represents a unit of work to be performed in idle time.
 * The Run method is invoked with an argument that specifies the deadline in
 * seconds returned by MonotonicallyIncreasingTime().
 * The idle task is expected to complete by this deadline.
 */
58 59
class IdleTask {
 public:
60
  virtual ~IdleTask() = default;
61 62 63
  virtual void Run(double deadline_in_seconds) = 0;
};

64 65 66 67 68 69 70 71 72 73 74 75 76 77
/**
 * A TaskRunner allows scheduling of tasks. The TaskRunner may still be used to
 * post tasks after the isolate gets destructed, but these tasks may not get
 * executed anymore. All tasks posted to a given TaskRunner will be invoked in
 * sequence. Tasks can be posted from any thread.
 */
class TaskRunner {
 public:
  /**
   * Schedules a task to be invoked by this TaskRunner. The TaskRunner
   * implementation takes ownership of |task|.
   */
  virtual void PostTask(std::unique_ptr<Task> task) = 0;

78 79 80 81 82
  /**
   * Schedules a task to be invoked by this TaskRunner. The TaskRunner
   * implementation takes ownership of |task|. The |task| cannot be nested
   * within other task executions.
   *
83 84 85 86 87 88 89 90
   * Tasks which shouldn't be interleaved with JS execution must be posted with
   * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
   * embedder may process tasks in a callback which is called during JS
   * execution.
   *
   * In particular, tasks which execute JS must be non-nestable, since JS
   * execution is not allowed to nest.
   *
91 92 93 94
   * Requires that |TaskRunner::NonNestableTasksEnabled()| is true.
   */
  virtual void PostNonNestableTask(std::unique_ptr<Task> task) {}

95 96 97 98 99 100 101 102
  /**
   * Schedules a task to be invoked by this TaskRunner. The task is scheduled
   * after the given number of seconds |delay_in_seconds|. The TaskRunner
   * implementation takes ownership of |task|.
   */
  virtual void PostDelayedTask(std::unique_ptr<Task> task,
                               double delay_in_seconds) = 0;

103 104 105 106 107 108
  /**
   * Schedules a task to be invoked by this TaskRunner. The task is scheduled
   * after the given number of seconds |delay_in_seconds|. The TaskRunner
   * implementation takes ownership of |task|. The |task| cannot be nested
   * within other task executions.
   *
109 110 111 112 113 114 115 116
   * Tasks which shouldn't be interleaved with JS execution must be posted with
   * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
   * embedder may process tasks in a callback which is called during JS
   * execution.
   *
   * In particular, tasks which execute JS must be non-nestable, since JS
   * execution is not allowed to nest.
   *
117 118 119 120 121
   * Requires that |TaskRunner::NonNestableDelayedTasksEnabled()| is true.
   */
  virtual void PostNonNestableDelayedTask(std::unique_ptr<Task> task,
                                          double delay_in_seconds) {}

122 123 124
  /**
   * Schedules an idle task to be invoked by this TaskRunner. The task is
   * scheduled when the embedder is idle. Requires that
125
   * |TaskRunner::IdleTasksEnabled()| is true. Idle tasks may be reordered
126 127 128 129 130 131 132 133 134 135 136
   * relative to other task types and may be starved for an arbitrarily long
   * time if no idle time is available. The TaskRunner implementation takes
   * ownership of |task|.
   */
  virtual void PostIdleTask(std::unique_ptr<IdleTask> task) = 0;

  /**
   * Returns true if idle tasks are enabled for this TaskRunner.
   */
  virtual bool IdleTasksEnabled() = 0;

137 138 139 140 141
  /**
   * Returns true if non-nestable tasks are enabled for this TaskRunner.
   */
  virtual bool NonNestableTasksEnabled() const { return false; }

142 143 144 145 146
  /**
   * Returns true if non-nestable delayed tasks are enabled for this TaskRunner.
   */
  virtual bool NonNestableDelayedTasksEnabled() const { return false; }

147 148 149 150 151 152 153
  TaskRunner() = default;
  virtual ~TaskRunner() = default;

  TaskRunner(const TaskRunner&) = delete;
  TaskRunner& operator=(const TaskRunner&) = delete;
};

154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
/**
 * Delegate that's passed to Job's worker task, providing an entry point to
 * communicate with the scheduler.
 */
class JobDelegate {
 public:
  /**
   * Returns true if this thread should return from the worker task on the
   * current thread ASAP. Workers should periodically invoke ShouldYield (or
   * YieldIfNeeded()) as often as is reasonable.
   */
  virtual bool ShouldYield() = 0;

  /**
   * Notifies the scheduler that max concurrency was increased, and the number
   * of worker should be adjusted accordingly. See Platform::PostJob() for more
   * details.
   */
  virtual void NotifyConcurrencyIncrease() = 0;
173 174 175 176 177 178

  /**
   * Returns a task_id unique among threads currently running this job, such
   * that GetTaskId() < worker count. To achieve this, the same task_id may be
   * reused by a different thread after a worker_task returns.
   */
179
  virtual uint8_t GetTaskId() = 0;
180 181 182 183 184 185 186

  /**
   * Returns true if the current task is called from the thread currently
   * running JobHandle::Join().
   * TODO(etiennep): Make pure virtual once custom embedders implement it.
   */
  virtual bool IsJoiningThread() const { return false; }
187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218
};

/**
 * Handle returned when posting a Job. Provides methods to control execution of
 * the posted Job.
 */
class JobHandle {
 public:
  virtual ~JobHandle() = default;

  /**
   * Notifies the scheduler that max concurrency was increased, and the number
   * of worker should be adjusted accordingly. See Platform::PostJob() for more
   * details.
   */
  virtual void NotifyConcurrencyIncrease() = 0;

  /**
   * Contributes to the job on this thread. Doesn't return until all tasks have
   * completed and max concurrency becomes 0. When Join() is called and max
   * concurrency reaches 0, it should not increase again. This also promotes
   * this Job's priority to be at least as high as the calling thread's
   * priority.
   */
  virtual void Join() = 0;

  /**
   * Forces all existing workers to yield ASAP. Waits until they have all
   * returned from the Job's callback before returning.
   */
  virtual void Cancel() = 0;

219 220 221 222 223 224 225 226
  /*
   * Forces all existing workers to yield ASAP but doesn’t wait for them.
   * Warning, this is dangerous if the Job's callback is bound to or has access
   * to state which may be deleted after this call.
   * TODO(etiennep): Cleanup once implemented by all embedders.
   */
  virtual void CancelAndDetach() { Cancel(); }

227
  /**
228
   * Returns true if there's any work pending or any worker running.
229
   */
230 231 232 233 234
  virtual bool IsActive() = 0;

  // TODO(etiennep): Clean up once all overrides are removed.
  V8_DEPRECATED("Use !IsActive() instead.")
  virtual bool IsCompleted() { return !IsActive(); }
235

236 237
  /**
   * Returns true if associated with a Job and other methods may be called.
238 239
   * Returns false after Join() or Cancel() was called. This may return true
   * even if no workers are running and IsCompleted() returns true
240
   */
241 242 243 244 245
  virtual bool IsValid() = 0;

  // TODO(etiennep): Clean up once all overrides are removed.
  V8_DEPRECATED("Use IsValid() instead.")
  virtual bool IsRunning() { return IsValid(); }
246 247 248 249 250 251 252 253 254 255

  /**
   * Returns true if job priority can be changed.
   */
  virtual bool UpdatePriorityEnabled() const { return false; }

  /**
   *  Update this Job's priority.
   */
  virtual void UpdatePriority(TaskPriority new_priority) {}
256 257 258 259 260 261 262 263 264 265 266 267
};

/**
 * A JobTask represents work to run in parallel from Platform::PostJob().
 */
class JobTask {
 public:
  virtual ~JobTask() = default;

  virtual void Run(JobDelegate* delegate) = 0;

  /**
268 269 270 271 272
   * Controls the maximum number of threads calling Run() concurrently, given
   * the number of threads currently assigned to this job and executing Run().
   * Run() is only invoked if the number of threads previously running Run() was
   * less than the value returned. Since GetMaxConcurrency() is a leaf function,
   * it must not call back any JobHandle methods.
273
   */
274 275 276 277 278
  virtual size_t GetMaxConcurrency(size_t worker_count) const = 0;

  // TODO(1114823): Clean up once all overrides are removed.
  V8_DEPRECATED("Use the version that takes |worker_count|.")
  virtual size_t GetMaxConcurrency() const { return 0; }
279 280
};

281 282 283 284 285 286 287 288 289 290 291 292 293 294 295
/**
 * The interface represents complex arguments to trace events.
 */
class ConvertableToTraceFormat {
 public:
  virtual ~ConvertableToTraceFormat() = default;

  /**
   * Append the class info to the provided |out| string. The appended
   * data must be a valid JSON object. Strings must be properly quoted, and
   * escaped. There is no processing applied to the content after it is
   * appended.
   */
  virtual void AppendAsTraceFormat(std::string* out) const = 0;
};
296

297 298 299 300 301 302 303 304 305
/**
 * V8 Tracing controller.
 *
 * Can be implemented by an embedder to record trace events from V8.
 */
class TracingController {
 public:
  virtual ~TracingController() = default;

306 307 308 309
  // In Perfetto mode, trace events are written using Perfetto's Track Event
  // API directly without going through the embedder. However, it is still
  // possible to observe tracing being enabled and disabled.
#if !defined(V8_USE_PERFETTO)
310 311 312 313 314 315 316 317 318 319 320 321 322
  /**
   * Called by TRACE_EVENT* macros, don't call this directly.
   * The name parameter is a category group for example:
   * TRACE_EVENT0("v8,parse", "V8.Parse")
   * The pointer returned points to a value with zero or more of the bits
   * defined in CategoryGroupEnabledFlags.
   **/
  virtual const uint8_t* GetCategoryGroupEnabled(const char* name) {
    static uint8_t no = 0;
    return &no;
  }

  /**
323
   * Adds a trace event to the platform tracing system. These function calls are
324 325
   * usually the result of a TRACE_* macro from trace_event_common.h when
   * tracing and the category of the particular trace are enabled. It is not
326 327
   * advisable to call these functions on their own; they are really only meant
   * to be used by the trace macros. The returned handle can be used by
328 329 330 331 332 333 334 335 336 337 338
   * UpdateTraceEventDuration to update the duration of COMPLETE events.
   */
  virtual uint64_t AddTraceEvent(
      char phase, const uint8_t* category_enabled_flag, const char* name,
      const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
      const char** arg_names, const uint8_t* arg_types,
      const uint64_t* arg_values,
      std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
      unsigned int flags) {
    return 0;
  }
339 340 341 342 343 344 345 346 347
  virtual uint64_t AddTraceEventWithTimestamp(
      char phase, const uint8_t* category_enabled_flag, const char* name,
      const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
      const char** arg_names, const uint8_t* arg_types,
      const uint64_t* arg_values,
      std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
      unsigned int flags, int64_t timestamp) {
    return 0;
  }
348 349 350 351 352 353 354

  /**
   * Sets the duration field of a COMPLETE trace event. It must be called with
   * the handle returned from AddTraceEvent().
   **/
  virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                        const char* name, uint64_t handle) {}
355
#endif  // !defined(V8_USE_PERFETTO)
356 357 358 359 360 361 362 363 364 365 366 367 368 369 370

  class TraceStateObserver {
   public:
    virtual ~TraceStateObserver() = default;
    virtual void OnTraceEnabled() = 0;
    virtual void OnTraceDisabled() = 0;
  };

  /** Adds tracing state change observer. */
  virtual void AddTraceStateObserver(TraceStateObserver*) {}

  /** Removes tracing state change observer. */
  virtual void RemoveTraceStateObserver(TraceStateObserver*) {}
};

371
/**
372
 * A V8 memory page allocator.
373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404
 *
 * Can be implemented by an embedder to manage large host OS allocations.
 */
class PageAllocator {
 public:
  virtual ~PageAllocator() = default;

  /**
   * Gets the page granularity for AllocatePages and FreePages. Addresses and
   * lengths for those calls should be multiples of AllocatePageSize().
   */
  virtual size_t AllocatePageSize() = 0;

  /**
   * Gets the page granularity for SetPermissions and ReleasePages. Addresses
   * and lengths for those calls should be multiples of CommitPageSize().
   */
  virtual size_t CommitPageSize() = 0;

  /**
   * Sets the random seed so that GetRandomMmapAddr() will generate repeatable
   * sequences of random mmap addresses.
   */
  virtual void SetRandomMmapSeed(int64_t seed) = 0;

  /**
   * Returns a randomized address, suitable for memory allocation under ASLR.
   * The address will be aligned to AllocatePageSize.
   */
  virtual void* GetRandomMmapAddr() = 0;

  /**
405
   * Memory permissions.
406 407 408
   */
  enum Permission {
    kNoAccess,
409
    kRead,
410
    kReadWrite,
411 412
    // TODO(hpayer): Remove this flag. Memory should never be rwx.
    kReadWriteExecute,
413 414 415 416 417 418 419
    kReadExecute,
    // Set this when reserving memory that will later require kReadWriteExecute
    // permissions. The resulting behavior is platform-specific, currently
    // this is used to set the MAP_JIT flag on Apple Silicon.
    // TODO(jkummerow): Remove this when Wasm has a platform-independent
    // w^x implementation.
    kNoAccessWillJitLater
420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443
  };

  /**
   * Allocates memory in range with the given alignment and permission.
   */
  virtual void* AllocatePages(void* address, size_t length, size_t alignment,
                              Permission permissions) = 0;

  /**
   * Frees memory in a range that was allocated by a call to AllocatePages.
   */
  virtual bool FreePages(void* address, size_t length) = 0;

  /**
   * Releases memory in a range that was allocated by a call to AllocatePages.
   */
  virtual bool ReleasePages(void* address, size_t length,
                            size_t new_length) = 0;

  /**
   * Sets permissions on pages in an allocated range.
   */
  virtual bool SetPermissions(void* address, size_t length,
                              Permission permissions) = 0;
444 445 446 447 448 449 450

  /**
   * Frees memory in the given [address, address + size) range. address and size
   * should be operating system page-aligned. The next write to this
   * memory area brings the memory transparently back.
   */
  virtual bool DiscardSystemPages(void* address, size_t size) { return true; }
451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513

  /**
   * INTERNAL ONLY: This interface has not been stabilised and may change
   * without notice from one release to another without being deprecated first.
   */
  class SharedMemoryMapping {
   public:
    // Implementations are expected to free the shared memory mapping in the
    // destructor.
    virtual ~SharedMemoryMapping() = default;
    virtual void* GetMemory() const = 0;
  };

  /**
   * INTERNAL ONLY: This interface has not been stabilised and may change
   * without notice from one release to another without being deprecated first.
   */
  class SharedMemory {
   public:
    // Implementations are expected to free the shared memory in the destructor.
    virtual ~SharedMemory() = default;
    virtual std::unique_ptr<SharedMemoryMapping> RemapTo(
        void* new_address) const = 0;
    virtual void* GetMemory() const = 0;
    virtual size_t GetSize() const = 0;
  };

  /**
   * INTERNAL ONLY: This interface has not been stabilised and may change
   * without notice from one release to another without being deprecated first.
   *
   * Reserve pages at a fixed address returning whether the reservation is
   * possible. The reserved memory is detached from the PageAllocator and so
   * should not be freed by it. It's intended for use with
   * SharedMemory::RemapTo, where ~SharedMemoryMapping would free the memory.
   */
  virtual bool ReserveForSharedMemoryMapping(void* address, size_t size) {
    return false;
  }

  /**
   * INTERNAL ONLY: This interface has not been stabilised and may change
   * without notice from one release to another without being deprecated first.
   *
   * Allocates shared memory pages. Not all PageAllocators need support this and
   * so this method need not be overridden.
   * Allocates a new read-only shared memory region of size |length| and copies
   * the memory at |original_address| into it.
   */
  virtual std::unique_ptr<SharedMemory> AllocateSharedPages(
      size_t length, const void* original_address) {
    return {};
  }

  /**
   * INTERNAL ONLY: This interface has not been stabilised and may change
   * without notice from one release to another without being deprecated first.
   *
   * If not overridden and changed to return true, V8 will not attempt to call
   * AllocateSharedPages or RemapSharedPages. If overridden, AllocateSharedPages
   * and RemapSharedPages must also be overridden.
   */
  virtual bool CanAllocateSharedPages() { return false; }
514 515
};

516 517 518 519 520 521 522 523
/**
 * V8 Platform abstraction layer.
 *
 * The embedder has to provide an implementation of this interface before
 * initializing the rest of V8.
 */
class Platform {
 public:
524
  virtual ~Platform() = default;
525

526
  /**
527
   * Allows the embedder to manage memory page allocations.
528 529 530 531 532 533
   */
  virtual PageAllocator* GetPageAllocator() {
    // TODO(bbudge) Make this abstract after all embedders implement this.
    return nullptr;
  }

534 535 536 537 538 539 540
  /**
   * Enables the embedder to respond in cases where V8 can't allocate large
   * blocks of memory. V8 retries the failed allocation once after calling this
   * method. On success, execution continues; otherwise V8 exits with a fatal
   * error.
   * Embedder overrides of this function must NOT call back into V8.
   */
541
  virtual void OnCriticalMemoryPressure() {
542
    // TODO(bbudge) Remove this when embedders override the following method.
543 544
    // See crbug.com/634547.
  }
545

546 547 548 549 550 551 552 553 554 555 556
  /**
   * Enables the embedder to respond in cases where V8 can't allocate large
   * memory regions. The |length| parameter is the amount of memory needed.
   * Returns true if memory is now available. Returns false if no memory could
   * be made available. V8 will retry allocations until this method returns
   * false.
   *
   * Embedder overrides of this function must NOT call back into V8.
   */
  virtual bool OnCriticalMemoryPressure(size_t length) { return false; }

557
  /**
558 559 560 561 562
   * Gets the number of worker threads used by
   * Call(BlockingTask)OnWorkerThread(). This can be used to estimate the number
   * of tasks a work package should be split into. A return value of 0 means
   * that there are no worker threads available. Note that a value of 0 won't
   * prohibit V8 from posting tasks using |CallOnWorkerThread|.
563
   */
564
  virtual int NumberOfWorkerThreads() = 0;
565

566 567
  /**
   * Returns a TaskRunner which can be used to post a task on the foreground.
568 569
   * The TaskRunner's NonNestableTasksEnabled() must be true. This function
   * should only be called from a foreground thread.
570
   */
571
  virtual std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(
572
      Isolate* isolate) = 0;
573 574 575 576

  /**
   * Schedules a task to be invoked on a worker thread.
   */
577
  virtual void CallOnWorkerThread(std::unique_ptr<Task> task) = 0;
578

579 580 581 582
  /**
   * Schedules a task that blocks the main thread to be invoked with
   * high-priority on a worker thread.
   */
583 584 585 586
  virtual void CallBlockingTaskOnWorkerThread(std::unique_ptr<Task> task) {
    // Embedders may optionally override this to process these tasks in a high
    // priority pool.
    CallOnWorkerThread(std::move(task));
587 588
  }

589 590 591 592 593 594 595 596 597
  /**
   * Schedules a task to be invoked with low-priority on a worker thread.
   */
  virtual void CallLowPriorityTaskOnWorkerThread(std::unique_ptr<Task> task) {
    // Embedders may optionally override this to process these tasks in a low
    // priority pool.
    CallOnWorkerThread(std::move(task));
  }

598 599 600 601 602
  /**
   * Schedules a task to be invoked on a worker thread after |delay_in_seconds|
   * expires.
   */
  virtual void CallDelayedOnWorkerThread(std::unique_ptr<Task> task,
603
                                         double delay_in_seconds) = 0;
604

605 606 607
  /**
   * Returns true if idle tasks are enabled for the given |isolate|.
   */
608
  virtual bool IdleTasksEnabled(Isolate* isolate) { return false; }
609

610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657
  /**
   * 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:
   * class MyJobTask : public JobTask {
   *  public:
   *   MyJobTask(...) : worker_queue_(...) {}
   *   // JobTask:
   *   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();
   *
   * 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:
   *  std::unique_ptr<JobHandle> PostJob(
   *      TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
658 659
   *    return v8::platform::NewDefaultJobHandle(
   *        this, priority, std::move(job_task), NumberOfWorkerThreads());
660 661 662
   * }
   */
  virtual std::unique_ptr<JobHandle> PostJob(
663
      TaskPriority priority, std::unique_ptr<JobTask> job_task) = 0;
664

665 666 667 668 669 670 671
  /**
   * 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.
   **/
672
  virtual double MonotonicallyIncreasingTime() = 0;
673 674 675 676 677

  /**
   * Current wall-clock time in milliseconds since epoch.
   * This function is expected to return at least millisecond-precision values.
   */
678
  virtual double CurrentClockTimeMillis() = 0;
679

680 681 682 683 684 685 686 687 688 689 690
  typedef void (*StackTracePrinter)();

  /**
   * Returns a function pointer that print a stack trace of the current stack
   * on invocation. Disables printing of the stack trace if nullptr.
   */
  virtual StackTracePrinter GetStackTracePrinter() { return nullptr; }

  /**
   * Returns an instance of a v8::TracingController. This must be non-nullptr.
   */
691
  virtual TracingController* GetTracingController() = 0;
692

693 694 695 696 697 698
  /**
   * Tells the embedder to generate and upload a crashdump during an unexpected
   * but non-critical scenario.
   */
  virtual void DumpWithoutCrashing() {}

699 700 701 702 703 704
 protected:
  /**
   * Default implementation of current wall-clock time in milliseconds
   * since epoch. Useful for implementing |CurrentClockTimeMillis| if
   * nothing special needed.
   */
705
  V8_EXPORT static double SystemClockTimeMillis();
706 707 708 709 710
};

}  // namespace v8

#endif  // V8_V8_PLATFORM_H_