v8  10.1.124 (node 18.2.0)
V8 is Google's open source JavaScript engine
v8-platform.h
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1 // Copyright 2013 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #ifndef V8_V8_PLATFORM_H_
6 #define V8_V8_PLATFORM_H_
7 
8 #include <stddef.h>
9 #include <stdint.h>
10 #include <stdlib.h> // For abort.
11 #include <memory>
12 #include <string>
13 
14 #include "v8config.h" // NOLINT(build/include_directory)
15 
16 namespace v8 {
17 
18 class Isolate;
19 
20 // Valid priorities supported by the task scheduling infrastructure.
21 enum class TaskPriority : uint8_t {
22  /**
23  * Best effort tasks are not critical for performance of the application. The
24  * platform implementation should preempt such tasks if higher priority tasks
25  * arrive.
26  */
28  /**
29  * User visible tasks are long running background tasks that will
30  * improve performance and memory usage of the application upon completion.
31  * Example: background compilation and garbage collection.
32  */
34  /**
35  * User blocking tasks are highest priority tasks that block the execution
36  * thread (e.g. major garbage collection). They must be finished as soon as
37  * possible.
38  */
40 };
41 
42 /**
43  * A Task represents a unit of work.
44  */
45 class Task {
46  public:
47  virtual ~Task() = default;
48 
49  virtual void Run() = 0;
50 };
51 
52 /**
53  * An IdleTask represents a unit of work to be performed in idle time.
54  * The Run method is invoked with an argument that specifies the deadline in
55  * seconds returned by MonotonicallyIncreasingTime().
56  * The idle task is expected to complete by this deadline.
57  */
58 class IdleTask {
59  public:
60  virtual ~IdleTask() = default;
61  virtual void Run(double deadline_in_seconds) = 0;
62 };
63 
64 /**
65  * A TaskRunner allows scheduling of tasks. The TaskRunner may still be used to
66  * post tasks after the isolate gets destructed, but these tasks may not get
67  * executed anymore. All tasks posted to a given TaskRunner will be invoked in
68  * sequence. Tasks can be posted from any thread.
69  */
70 class TaskRunner {
71  public:
72  /**
73  * Schedules a task to be invoked by this TaskRunner. The TaskRunner
74  * implementation takes ownership of |task|.
75  */
76  virtual void PostTask(std::unique_ptr<Task> task) = 0;
77 
78  /**
79  * Schedules a task to be invoked by this TaskRunner. The TaskRunner
80  * implementation takes ownership of |task|. The |task| cannot be nested
81  * within other task executions.
82  *
83  * Tasks which shouldn't be interleaved with JS execution must be posted with
84  * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
85  * embedder may process tasks in a callback which is called during JS
86  * execution.
87  *
88  * In particular, tasks which execute JS must be non-nestable, since JS
89  * execution is not allowed to nest.
90  *
91  * Requires that |TaskRunner::NonNestableTasksEnabled()| is true.
92  */
93  virtual void PostNonNestableTask(std::unique_ptr<Task> task) {}
94 
95  /**
96  * Schedules a task to be invoked by this TaskRunner. The task is scheduled
97  * after the given number of seconds |delay_in_seconds|. The TaskRunner
98  * implementation takes ownership of |task|.
99  */
100  virtual void PostDelayedTask(std::unique_ptr<Task> task,
101  double delay_in_seconds) = 0;
102 
103  /**
104  * Schedules a task to be invoked by this TaskRunner. The task is scheduled
105  * after the given number of seconds |delay_in_seconds|. The TaskRunner
106  * implementation takes ownership of |task|. The |task| cannot be nested
107  * within other task executions.
108  *
109  * Tasks which shouldn't be interleaved with JS execution must be posted with
110  * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
111  * embedder may process tasks in a callback which is called during JS
112  * execution.
113  *
114  * In particular, tasks which execute JS must be non-nestable, since JS
115  * execution is not allowed to nest.
116  *
117  * Requires that |TaskRunner::NonNestableDelayedTasksEnabled()| is true.
118  */
119  virtual void PostNonNestableDelayedTask(std::unique_ptr<Task> task,
120  double delay_in_seconds) {}
121 
122  /**
123  * Schedules an idle task to be invoked by this TaskRunner. The task is
124  * scheduled when the embedder is idle. Requires that
125  * |TaskRunner::IdleTasksEnabled()| is true. Idle tasks may be reordered
126  * relative to other task types and may be starved for an arbitrarily long
127  * time if no idle time is available. The TaskRunner implementation takes
128  * ownership of |task|.
129  */
130  virtual void PostIdleTask(std::unique_ptr<IdleTask> task) = 0;
131 
132  /**
133  * Returns true if idle tasks are enabled for this TaskRunner.
134  */
135  virtual bool IdleTasksEnabled() = 0;
136 
137  /**
138  * Returns true if non-nestable tasks are enabled for this TaskRunner.
139  */
140  virtual bool NonNestableTasksEnabled() const { return false; }
141 
142  /**
143  * Returns true if non-nestable delayed tasks are enabled for this TaskRunner.
144  */
145  virtual bool NonNestableDelayedTasksEnabled() const { return false; }
146 
147  TaskRunner() = default;
148  virtual ~TaskRunner() = default;
149 
150  TaskRunner(const TaskRunner&) = delete;
151  TaskRunner& operator=(const TaskRunner&) = delete;
152 };
153 
154 /**
155  * Delegate that's passed to Job's worker task, providing an entry point to
156  * communicate with the scheduler.
157  */
158 class JobDelegate {
159  public:
160  /**
161  * Returns true if this thread should return from the worker task on the
162  * current thread ASAP. Workers should periodically invoke ShouldYield (or
163  * YieldIfNeeded()) as often as is reasonable.
164  */
165  virtual bool ShouldYield() = 0;
166 
167  /**
168  * Notifies the scheduler that max concurrency was increased, and the number
169  * of worker should be adjusted accordingly. See Platform::PostJob() for more
170  * details.
171  */
172  virtual void NotifyConcurrencyIncrease() = 0;
173 
174  /**
175  * Returns a task_id unique among threads currently running this job, such
176  * that GetTaskId() < worker count. To achieve this, the same task_id may be
177  * reused by a different thread after a worker_task returns.
178  */
179  virtual uint8_t GetTaskId() = 0;
180 
181  /**
182  * Returns true if the current task is called from the thread currently
183  * running JobHandle::Join().
184  */
185  virtual bool IsJoiningThread() const = 0;
186 };
187 
188 /**
189  * Handle returned when posting a Job. Provides methods to control execution of
190  * the posted Job.
191  */
192 class JobHandle {
193  public:
194  virtual ~JobHandle() = default;
195 
196  /**
197  * Notifies the scheduler that max concurrency was increased, and the number
198  * of worker should be adjusted accordingly. See Platform::PostJob() for more
199  * details.
200  */
201  virtual void NotifyConcurrencyIncrease() = 0;
202 
203  /**
204  * Contributes to the job on this thread. Doesn't return until all tasks have
205  * completed and max concurrency becomes 0. When Join() is called and max
206  * concurrency reaches 0, it should not increase again. This also promotes
207  * this Job's priority to be at least as high as the calling thread's
208  * priority.
209  */
210  virtual void Join() = 0;
211 
212  /**
213  * Forces all existing workers to yield ASAP. Waits until they have all
214  * returned from the Job's callback before returning.
215  */
216  virtual void Cancel() = 0;
217 
218  /*
219  * Forces all existing workers to yield ASAP but doesn’t wait for them.
220  * Warning, this is dangerous if the Job's callback is bound to or has access
221  * to state which may be deleted after this call.
222  */
223  virtual void CancelAndDetach() = 0;
224 
225  /**
226  * Returns true if there's any work pending or any worker running.
227  */
228  virtual bool IsActive() = 0;
229 
230  /**
231  * Returns true if associated with a Job and other methods may be called.
232  * Returns false after Join() or Cancel() was called. This may return true
233  * even if no workers are running and IsCompleted() returns true
234  */
235  virtual bool IsValid() = 0;
236 
237  /**
238  * Returns true if job priority can be changed.
239  */
240  virtual bool UpdatePriorityEnabled() const { return false; }
241 
242  /**
243  * Update this Job's priority.
244  */
245  virtual void UpdatePriority(TaskPriority new_priority) {}
246 };
247 
248 /**
249  * A JobTask represents work to run in parallel from Platform::PostJob().
250  */
251 class JobTask {
252  public:
253  virtual ~JobTask() = default;
254 
255  virtual void Run(JobDelegate* delegate) = 0;
256 
257  /**
258  * Controls the maximum number of threads calling Run() concurrently, given
259  * the number of threads currently assigned to this job and executing Run().
260  * Run() is only invoked if the number of threads previously running Run() was
261  * less than the value returned. Since GetMaxConcurrency() is a leaf function,
262  * it must not call back any JobHandle methods.
263  */
264  virtual size_t GetMaxConcurrency(size_t worker_count) const = 0;
265 };
266 
267 /**
268  * The interface represents complex arguments to trace events.
269  */
271  public:
272  virtual ~ConvertableToTraceFormat() = default;
273 
274  /**
275  * Append the class info to the provided |out| string. The appended
276  * data must be a valid JSON object. Strings must be properly quoted, and
277  * escaped. There is no processing applied to the content after it is
278  * appended.
279  */
280  virtual void AppendAsTraceFormat(std::string* out) const = 0;
281 };
282 
283 /**
284  * V8 Tracing controller.
285  *
286  * Can be implemented by an embedder to record trace events from V8.
287  */
289  public:
290  virtual ~TracingController() = default;
291 
292  // In Perfetto mode, trace events are written using Perfetto's Track Event
293  // API directly without going through the embedder. However, it is still
294  // possible to observe tracing being enabled and disabled.
295 #if !defined(V8_USE_PERFETTO)
296  /**
297  * Called by TRACE_EVENT* macros, don't call this directly.
298  * The name parameter is a category group for example:
299  * TRACE_EVENT0("v8,parse", "V8.Parse")
300  * The pointer returned points to a value with zero or more of the bits
301  * defined in CategoryGroupEnabledFlags.
302  **/
303  virtual const uint8_t* GetCategoryGroupEnabled(const char* name) {
304  static uint8_t no = 0;
305  return &no;
306  }
307 
308  /**
309  * Adds a trace event to the platform tracing system. These function calls are
310  * usually the result of a TRACE_* macro from trace_event_common.h when
311  * tracing and the category of the particular trace are enabled. It is not
312  * advisable to call these functions on their own; they are really only meant
313  * to be used by the trace macros. The returned handle can be used by
314  * UpdateTraceEventDuration to update the duration of COMPLETE events.
315  */
316  virtual uint64_t AddTraceEvent(
317  char phase, const uint8_t* category_enabled_flag, const char* name,
318  const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
319  const char** arg_names, const uint8_t* arg_types,
320  const uint64_t* arg_values,
321  std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
322  unsigned int flags) {
323  return 0;
324  }
325  virtual uint64_t AddTraceEventWithTimestamp(
326  char phase, const uint8_t* category_enabled_flag, const char* name,
327  const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
328  const char** arg_names, const uint8_t* arg_types,
329  const uint64_t* arg_values,
330  std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
331  unsigned int flags, int64_t timestamp) {
332  return 0;
333  }
334 
335  /**
336  * Sets the duration field of a COMPLETE trace event. It must be called with
337  * the handle returned from AddTraceEvent().
338  **/
339  virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
340  const char* name, uint64_t handle) {}
341 #endif // !defined(V8_USE_PERFETTO)
342 
344  public:
345  virtual ~TraceStateObserver() = default;
346  virtual void OnTraceEnabled() = 0;
347  virtual void OnTraceDisabled() = 0;
348  };
349 
350  /** Adds tracing state change observer. */
352 
353  /** Removes tracing state change observer. */
355 };
356 
357 /**
358  * A V8 memory page allocator.
359  *
360  * Can be implemented by an embedder to manage large host OS allocations.
361  */
363  public:
364  virtual ~PageAllocator() = default;
365 
366  /**
367  * Gets the page granularity for AllocatePages and FreePages. Addresses and
368  * lengths for those calls should be multiples of AllocatePageSize().
369  */
370  virtual size_t AllocatePageSize() = 0;
371 
372  /**
373  * Gets the page granularity for SetPermissions and ReleasePages. Addresses
374  * and lengths for those calls should be multiples of CommitPageSize().
375  */
376  virtual size_t CommitPageSize() = 0;
377 
378  /**
379  * Sets the random seed so that GetRandomMmapAddr() will generate repeatable
380  * sequences of random mmap addresses.
381  */
382  virtual void SetRandomMmapSeed(int64_t seed) = 0;
383 
384  /**
385  * Returns a randomized address, suitable for memory allocation under ASLR.
386  * The address will be aligned to AllocatePageSize.
387  */
388  virtual void* GetRandomMmapAddr() = 0;
389 
390  /**
391  * Memory permissions.
392  */
393  enum Permission {
399  // Set this when reserving memory that will later require kReadWriteExecute
400  // permissions. The resulting behavior is platform-specific, currently
401  // this is used to set the MAP_JIT flag on Apple Silicon.
402  // TODO(jkummerow): Remove this when Wasm has a platform-independent
403  // w^x implementation.
404  // TODO(saelo): Remove this once all JIT pages are allocated through the
405  // VirtualAddressSpace API.
407  };
408 
409  /**
410  * Allocates memory in range with the given alignment and permission.
411  */
412  virtual void* AllocatePages(void* address, size_t length, size_t alignment,
413  Permission permissions) = 0;
414 
415  /**
416  * Frees memory in a range that was allocated by a call to AllocatePages.
417  */
418  virtual bool FreePages(void* address, size_t length) = 0;
419 
420  /**
421  * Releases memory in a range that was allocated by a call to AllocatePages.
422  */
423  virtual bool ReleasePages(void* address, size_t length,
424  size_t new_length) = 0;
425 
426  /**
427  * Sets permissions on pages in an allocated range.
428  */
429  virtual bool SetPermissions(void* address, size_t length,
430  Permission permissions) = 0;
431 
432  /**
433  * Frees memory in the given [address, address + size) range. address and size
434  * should be operating system page-aligned. The next write to this
435  * memory area brings the memory transparently back. This should be treated as
436  * a hint to the OS that the pages are no longer needed. It does not guarantee
437  * that the pages will be discarded immediately or at all.
438  */
439  virtual bool DiscardSystemPages(void* address, size_t size) { return true; }
440 
441  /**
442  * Decommits any wired memory pages in the given range, allowing the OS to
443  * reclaim them, and marks the region as inacessible (kNoAccess). The address
444  * range stays reserved and can be accessed again later by changing its
445  * permissions. However, in that case the memory content is guaranteed to be
446  * zero-initialized again. The memory must have been previously allocated by a
447  * call to AllocatePages. Returns true on success, false otherwise.
448  */
449  virtual bool DecommitPages(void* address, size_t size) = 0;
450 
451  /**
452  * INTERNAL ONLY: This interface has not been stabilised and may change
453  * without notice from one release to another without being deprecated first.
454  */
456  public:
457  // Implementations are expected to free the shared memory mapping in the
458  // destructor.
459  virtual ~SharedMemoryMapping() = default;
460  virtual void* GetMemory() const = 0;
461  };
462 
463  /**
464  * INTERNAL ONLY: This interface has not been stabilised and may change
465  * without notice from one release to another without being deprecated first.
466  */
467  class SharedMemory {
468  public:
469  // Implementations are expected to free the shared memory in the destructor.
470  virtual ~SharedMemory() = default;
471  virtual std::unique_ptr<SharedMemoryMapping> RemapTo(
472  void* new_address) const = 0;
473  virtual void* GetMemory() const = 0;
474  virtual size_t GetSize() const = 0;
475  };
476 
477  /**
478  * INTERNAL ONLY: This interface has not been stabilised and may change
479  * without notice from one release to another without being deprecated first.
480  *
481  * Reserve pages at a fixed address returning whether the reservation is
482  * possible. The reserved memory is detached from the PageAllocator and so
483  * should not be freed by it. It's intended for use with
484  * SharedMemory::RemapTo, where ~SharedMemoryMapping would free the memory.
485  */
486  virtual bool ReserveForSharedMemoryMapping(void* address, size_t size) {
487  return false;
488  }
489 
490  /**
491  * INTERNAL ONLY: This interface has not been stabilised and may change
492  * without notice from one release to another without being deprecated first.
493  *
494  * Allocates shared memory pages. Not all PageAllocators need support this and
495  * so this method need not be overridden.
496  * Allocates a new read-only shared memory region of size |length| and copies
497  * the memory at |original_address| into it.
498  */
499  virtual std::unique_ptr<SharedMemory> AllocateSharedPages(
500  size_t length, const void* original_address) {
501  return {};
502  }
503 
504  /**
505  * INTERNAL ONLY: This interface has not been stabilised and may change
506  * without notice from one release to another without being deprecated first.
507  *
508  * If not overridden and changed to return true, V8 will not attempt to call
509  * AllocateSharedPages or RemapSharedPages. If overridden, AllocateSharedPages
510  * and RemapSharedPages must also be overridden.
511  */
512  virtual bool CanAllocateSharedPages() { return false; }
513 };
514 
515 // Opaque type representing a handle to a shared memory region.
516 using PlatformSharedMemoryHandle = intptr_t;
517 static constexpr PlatformSharedMemoryHandle kInvalidSharedMemoryHandle = -1;
518 
519 // Conversion routines from the platform-dependent shared memory identifiers
520 // into the opaque PlatformSharedMemoryHandle type. These use the underlying
521 // types (e.g. unsigned int) instead of the typedef'd ones (e.g. mach_port_t)
522 // to avoid pulling in large OS header files into this header file. Instead,
523 // the users of these routines are expected to include the respecitve OS
524 // headers in addition to this one.
525 #if V8_OS_MACOS
526 // Convert between a shared memory handle and a mach_port_t referencing a memory
527 // entry object.
529  unsigned int port) {
530  return static_cast<PlatformSharedMemoryHandle>(port);
531 }
532 inline unsigned int MachMemoryEntryFromSharedMemoryHandle(
534  return static_cast<unsigned int>(handle);
535 }
536 #elif V8_OS_FUCHSIA
537 // Convert between a shared memory handle and a zx_handle_t to a VMO.
539  return static_cast<PlatformSharedMemoryHandle>(handle);
540 }
542  return static_cast<uint32_t>(handle);
543 }
544 #elif V8_OS_WIN
545 // Convert between a shared memory handle and a Windows HANDLE to a file mapping
546 // object.
548  void* handle) {
549  return reinterpret_cast<PlatformSharedMemoryHandle>(handle);
550 }
553  return reinterpret_cast<void*>(handle);
554 }
555 #else
556 // Convert between a shared memory handle and a file descriptor.
557 inline PlatformSharedMemoryHandle SharedMemoryHandleFromFileDescriptor(int fd) {
558  return static_cast<PlatformSharedMemoryHandle>(fd);
559 }
561  PlatformSharedMemoryHandle handle) {
562  return static_cast<int>(handle);
563 }
564 #endif
565 
566 /**
567  * Possible permissions for memory pages.
568  */
569 enum class PagePermissions {
570  kNoAccess,
571  kRead,
572  kReadWrite,
574  kReadExecute,
575 };
576 
577 /**
578  * Class to manage a virtual memory address space.
579  *
580  * This class represents a contiguous region of virtual address space in which
581  * sub-spaces and (private or shared) memory pages can be allocated, freed, and
582  * modified. This interface is meant to eventually replace the PageAllocator
583  * interface, and can be used as an alternative in the meantime.
584  *
585  * This API is not yet stable and may change without notice!
586  */
588  public:
589  using Address = uintptr_t;
590 
591  VirtualAddressSpace(size_t page_size, size_t allocation_granularity,
592  Address base, size_t size,
593  PagePermissions max_page_permissions)
594  : page_size_(page_size),
595  allocation_granularity_(allocation_granularity),
596  base_(base),
597  size_(size),
598  max_page_permissions_(max_page_permissions) {}
599 
600  virtual ~VirtualAddressSpace() = default;
601 
602  /**
603  * The page size used inside this space. Guaranteed to be a power of two.
604  * Used as granularity for all page-related operations except for allocation,
605  * which use the allocation_granularity(), see below.
606  *
607  * \returns the page size in bytes.
608  */
609  size_t page_size() const { return page_size_; }
610 
611  /**
612  * The granularity of page allocations and, by extension, of subspace
613  * allocations. This is guaranteed to be a power of two and a multiple of the
614  * page_size(). In practice, this is equal to the page size on most OSes, but
615  * on Windows it is usually 64KB, while the page size is 4KB.
616  *
617  * \returns the allocation granularity in bytes.
618  */
619  size_t allocation_granularity() const { return allocation_granularity_; }
620 
621  /**
622  * The base address of the address space managed by this instance.
623  *
624  * \returns the base address of this address space.
625  */
626  Address base() const { return base_; }
627 
628  /**
629  * The size of the address space managed by this instance.
630  *
631  * \returns the size of this address space in bytes.
632  */
633  size_t size() const { return size_; }
634 
635  /**
636  * The maximum page permissions that pages allocated inside this space can
637  * obtain.
638  *
639  * \returns the maximum page permissions.
640  */
641  PagePermissions max_page_permissions() const { return max_page_permissions_; }
642 
643  /**
644  * Sets the random seed so that GetRandomPageAddress() will generate
645  * repeatable sequences of random addresses.
646  *
647  * \param The seed for the PRNG.
648  */
649  virtual void SetRandomSeed(int64_t seed) = 0;
650 
651  /**
652  * Returns a random address inside this address space, suitable for page
653  * allocations hints.
654  *
655  * \returns a random address aligned to allocation_granularity().
656  */
657  virtual Address RandomPageAddress() = 0;
658 
659  /**
660  * Allocates private memory pages with the given alignment and permissions.
661  *
662  * \param hint If nonzero, the allocation is attempted to be placed at the
663  * given address first. If that fails, the allocation is attempted to be
664  * placed elsewhere, possibly nearby, but that is not guaranteed. Specifying
665  * zero for the hint always causes this function to choose a random address.
666  * The hint, if specified, must be aligned to the specified alignment.
667  *
668  * \param size The size of the allocation in bytes. Must be a multiple of the
669  * allocation_granularity().
670  *
671  * \param alignment The alignment of the allocation in bytes. Must be a
672  * multiple of the allocation_granularity() and should be a power of two.
673  *
674  * \param permissions The page permissions of the newly allocated pages.
675  *
676  * \returns the start address of the allocated pages on success, zero on
677  * failure.
678  */
679  static constexpr Address kNoHint = 0;
680  virtual V8_WARN_UNUSED_RESULT Address
681  AllocatePages(Address hint, size_t size, size_t alignment,
682  PagePermissions permissions) = 0;
683 
684  /**
685  * Frees previously allocated pages.
686  *
687  * This function will terminate the process on failure as this implies a bug
688  * in the client. As such, there is no return value.
689  *
690  * \param address The start address of the pages to free. This address must
691  * have been obtained through a call to AllocatePages.
692  *
693  * \param size The size in bytes of the region to free. This must match the
694  * size passed to AllocatePages when the pages were allocated.
695  */
696  virtual void FreePages(Address address, size_t size) = 0;
697 
698  /**
699  * Sets permissions of all allocated pages in the given range.
700  *
701  * \param address The start address of the range. Must be aligned to
702  * page_size().
703  *
704  * \param size The size in bytes of the range. Must be a multiple
705  * of page_size().
706  *
707  * \param permissions The new permissions for the range.
708  *
709  * \returns true on success, false otherwise.
710  */
712  Address address, size_t size, PagePermissions permissions) = 0;
713 
714  /**
715  * Creates a guard region at the specified address.
716  *
717  * Guard regions are guaranteed to cause a fault when accessed and generally
718  * do not count towards any memory consumption limits. Further, allocating
719  * guard regions can usually not fail in subspaces if the region does not
720  * overlap with another region, subspace, or page allocation.
721  *
722  * \param address The start address of the guard region. Must be aligned to
723  * the allocation_granularity().
724  *
725  * \param size The size of the guard region in bytes. Must be a multiple of
726  * the allocation_granularity().
727  *
728  * \returns true on success, false otherwise.
729  */
730  virtual V8_WARN_UNUSED_RESULT bool AllocateGuardRegion(Address address,
731  size_t size) = 0;
732 
733  /**
734  * Frees an existing guard region.
735  *
736  * This function will terminate the process on failure as this implies a bug
737  * in the client. As such, there is no return value.
738  *
739  * \param address The start address of the guard region to free. This address
740  * must have previously been used as address parameter in a successful
741  * invocation of AllocateGuardRegion.
742  *
743  * \param size The size in bytes of the guard region to free. This must match
744  * the size passed to AllocateGuardRegion when the region was created.
745  */
746  virtual void FreeGuardRegion(Address address, size_t size) = 0;
747 
748  /**
749  * Allocates shared memory pages with the given permissions.
750  *
751  * \param hint Placement hint. See AllocatePages.
752  *
753  * \param size The size of the allocation in bytes. Must be a multiple of the
754  * allocation_granularity().
755  *
756  * \param permissions The page permissions of the newly allocated pages.
757  *
758  * \param handle A platform-specific handle to a shared memory object. See
759  * the SharedMemoryHandleFromX routines above for ways to obtain these.
760  *
761  * \param offset The offset in the shared memory object at which the mapping
762  * should start. Must be a multiple of the allocation_granularity().
763  *
764  * \returns the start address of the allocated pages on success, zero on
765  * failure.
766  */
767  virtual V8_WARN_UNUSED_RESULT Address
768  AllocateSharedPages(Address hint, size_t size, PagePermissions permissions,
769  PlatformSharedMemoryHandle handle, uint64_t offset) = 0;
770 
771  /**
772  * Frees previously allocated shared pages.
773  *
774  * This function will terminate the process on failure as this implies a bug
775  * in the client. As such, there is no return value.
776  *
777  * \param address The start address of the pages to free. This address must
778  * have been obtained through a call to AllocateSharedPages.
779  *
780  * \param size The size in bytes of the region to free. This must match the
781  * size passed to AllocateSharedPages when the pages were allocated.
782  */
783  virtual void FreeSharedPages(Address address, size_t size) = 0;
784 
785  /**
786  * Whether this instance can allocate subspaces or not.
787  *
788  * \returns true if subspaces can be allocated, false if not.
789  */
790  virtual bool CanAllocateSubspaces() = 0;
791 
792  /*
793  * Allocate a subspace.
794  *
795  * The address space of a subspace stays reserved in the parent space for the
796  * lifetime of the subspace. As such, it is guaranteed that page allocations
797  * on the parent space cannot end up inside a subspace.
798  *
799  * \param hint Hints where the subspace should be allocated. See
800  * AllocatePages() for more details.
801  *
802  * \param size The size in bytes of the subspace. Must be a multiple of the
803  * allocation_granularity().
804  *
805  * \param alignment The alignment of the subspace in bytes. Must be a multiple
806  * of the allocation_granularity() and should be a power of two.
807  *
808  * \param max_page_permissions The maximum permissions that pages allocated in
809  * the subspace can obtain.
810  *
811  * \returns a new subspace or nullptr on failure.
812  */
813  virtual std::unique_ptr<VirtualAddressSpace> AllocateSubspace(
814  Address hint, size_t size, size_t alignment,
815  PagePermissions max_page_permissions) = 0;
816 
817  //
818  // TODO(v8) maybe refactor the methods below before stabilizing the API. For
819  // example by combining them into some form of page operation method that
820  // takes a command enum as parameter.
821  //
822 
823  /**
824  * Frees memory in the given [address, address + size) range. address and
825  * size should be aligned to the page_size(). The next write to this memory
826  * area brings the memory transparently back. This should be treated as a
827  * hint to the OS that the pages are no longer needed. It does not guarantee
828  * that the pages will be discarded immediately or at all.
829  *
830  * \returns true on success, false otherwise. Since this method is only a
831  * hint, a successful invocation does not imply that pages have been removed.
832  */
833  virtual V8_WARN_UNUSED_RESULT bool DiscardSystemPages(Address address,
834  size_t size) {
835  return true;
836  }
837  /**
838  * Decommits any wired memory pages in the given range, allowing the OS to
839  * reclaim them, and marks the region as inacessible (kNoAccess). The address
840  * range stays reserved and can be accessed again later by changing its
841  * permissions. However, in that case the memory content is guaranteed to be
842  * zero-initialized again. The memory must have been previously allocated by a
843  * call to AllocatePages.
844  *
845  * \returns true on success, false otherwise.
846  */
847  virtual V8_WARN_UNUSED_RESULT bool DecommitPages(Address address,
848  size_t size) = 0;
849 
850  private:
851  const size_t page_size_;
852  const size_t allocation_granularity_;
853  const Address base_;
854  const size_t size_;
855  const PagePermissions max_page_permissions_;
856 };
857 
858 /**
859  * V8 Allocator used for allocating zone backings.
860  */
862  public:
863  using MallocFn = void* (*)(size_t);
864  using FreeFn = void (*)(void*);
865 
866  virtual MallocFn GetMallocFn() const { return ::malloc; }
867  virtual FreeFn GetFreeFn() const { return ::free; }
868 };
869 
870 /**
871  * Observer used by V8 to notify the embedder about entering/leaving sections
872  * with high throughput of malloc/free operations.
873  */
875  public:
876  virtual void EnterSection() {}
877  virtual void LeaveSection() {}
878 };
879 
880 /**
881  * V8 Platform abstraction layer.
882  *
883  * The embedder has to provide an implementation of this interface before
884  * initializing the rest of V8.
885  */
886 class Platform {
887  public:
888  virtual ~Platform() = default;
889 
890  /**
891  * Allows the embedder to manage memory page allocations.
892  */
894  // TODO(bbudge) Make this abstract after all embedders implement this.
895  return nullptr;
896  }
897 
898  /**
899  * Allows the embedder to specify a custom allocator used for zones.
900  */
902  static ZoneBackingAllocator default_allocator;
903  return &default_allocator;
904  }
905 
906  /**
907  * Enables the embedder to respond in cases where V8 can't allocate large
908  * blocks of memory. V8 retries the failed allocation once after calling this
909  * method. On success, execution continues; otherwise V8 exits with a fatal
910  * error.
911  * Embedder overrides of this function must NOT call back into V8.
912  */
913  virtual void OnCriticalMemoryPressure() {
914  // TODO(bbudge) Remove this when embedders override the following method.
915  // See crbug.com/634547.
916  }
917 
918  /**
919  * Enables the embedder to respond in cases where V8 can't allocate large
920  * memory regions. The |length| parameter is the amount of memory needed.
921  * Returns true if memory is now available. Returns false if no memory could
922  * be made available. V8 will retry allocations until this method returns
923  * false.
924  *
925  * Embedder overrides of this function must NOT call back into V8.
926  */
927  virtual bool OnCriticalMemoryPressure(size_t length) { return false; }
928 
929  /**
930  * Gets the number of worker threads used by
931  * Call(BlockingTask)OnWorkerThread(). This can be used to estimate the number
932  * of tasks a work package should be split into. A return value of 0 means
933  * that there are no worker threads available. Note that a value of 0 won't
934  * prohibit V8 from posting tasks using |CallOnWorkerThread|.
935  */
936  virtual int NumberOfWorkerThreads() = 0;
937 
938  /**
939  * Returns a TaskRunner which can be used to post a task on the foreground.
940  * The TaskRunner's NonNestableTasksEnabled() must be true. This function
941  * should only be called from a foreground thread.
942  */
943  virtual std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(
944  Isolate* isolate) = 0;
945 
946  /**
947  * Schedules a task to be invoked on a worker thread.
948  */
949  virtual void CallOnWorkerThread(std::unique_ptr<Task> task) = 0;
950 
951  /**
952  * Schedules a task that blocks the main thread to be invoked with
953  * high-priority on a worker thread.
954  */
955  virtual void CallBlockingTaskOnWorkerThread(std::unique_ptr<Task> task) {
956  // Embedders may optionally override this to process these tasks in a high
957  // priority pool.
958  CallOnWorkerThread(std::move(task));
959  }
960 
961  /**
962  * Schedules a task to be invoked with low-priority on a worker thread.
963  */
964  virtual void CallLowPriorityTaskOnWorkerThread(std::unique_ptr<Task> task) {
965  // Embedders may optionally override this to process these tasks in a low
966  // priority pool.
967  CallOnWorkerThread(std::move(task));
968  }
969 
970  /**
971  * Schedules a task to be invoked on a worker thread after |delay_in_seconds|
972  * expires.
973  */
974  virtual void CallDelayedOnWorkerThread(std::unique_ptr<Task> task,
975  double delay_in_seconds) = 0;
976 
977  /**
978  * Returns true if idle tasks are enabled for the given |isolate|.
979  */
980  virtual bool IdleTasksEnabled(Isolate* isolate) { return false; }
981 
982  /**
983  * Posts |job_task| to run in parallel. Returns a JobHandle associated with
984  * the Job, which can be joined or canceled.
985  * This avoids degenerate cases:
986  * - Calling CallOnWorkerThread() for each work item, causing significant
987  * overhead.
988  * - Fixed number of CallOnWorkerThread() calls that split the work and might
989  * run for a long time. This is problematic when many components post
990  * "num cores" tasks and all expect to use all the cores. In these cases,
991  * the scheduler lacks context to be fair to multiple same-priority requests
992  * and/or ability to request lower priority work to yield when high priority
993  * work comes in.
994  * A canonical implementation of |job_task| looks like:
995  * class MyJobTask : public JobTask {
996  * public:
997  * MyJobTask(...) : worker_queue_(...) {}
998  * // JobTask:
999  * void Run(JobDelegate* delegate) override {
1000  * while (!delegate->ShouldYield()) {
1001  * // Smallest unit of work.
1002  * auto work_item = worker_queue_.TakeWorkItem(); // Thread safe.
1003  * if (!work_item) return;
1004  * ProcessWork(work_item);
1005  * }
1006  * }
1007  *
1008  * size_t GetMaxConcurrency() const override {
1009  * return worker_queue_.GetSize(); // Thread safe.
1010  * }
1011  * };
1012  * auto handle = PostJob(TaskPriority::kUserVisible,
1013  * std::make_unique<MyJobTask>(...));
1014  * handle->Join();
1015  *
1016  * PostJob() and methods of the returned JobHandle/JobDelegate, must never be
1017  * called while holding a lock that could be acquired by JobTask::Run or
1018  * JobTask::GetMaxConcurrency -- that could result in a deadlock. This is
1019  * because [1] JobTask::GetMaxConcurrency may be invoked while holding
1020  * internal lock (A), hence JobTask::GetMaxConcurrency can only use a lock (B)
1021  * if that lock is *never* held while calling back into JobHandle from any
1022  * thread (A=>B/B=>A deadlock) and [2] JobTask::Run or
1023  * JobTask::GetMaxConcurrency may be invoked synchronously from JobHandle
1024  * (B=>JobHandle::foo=>B deadlock).
1025  *
1026  * A sufficient PostJob() implementation that uses the default Job provided in
1027  * libplatform looks like:
1028  * std::unique_ptr<JobHandle> PostJob(
1029  * TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
1030  * return v8::platform::NewDefaultJobHandle(
1031  * this, priority, std::move(job_task), NumberOfWorkerThreads());
1032  * }
1033  */
1034  virtual std::unique_ptr<JobHandle> PostJob(
1035  TaskPriority priority, std::unique_ptr<JobTask> job_task) = 0;
1036 
1037  /**
1038  * Monotonically increasing time in seconds from an arbitrary fixed point in
1039  * the past. This function is expected to return at least
1040  * millisecond-precision values. For this reason,
1041  * it is recommended that the fixed point be no further in the past than
1042  * the epoch.
1043  **/
1044  virtual double MonotonicallyIncreasingTime() = 0;
1045 
1046  /**
1047  * Current wall-clock time in milliseconds since epoch.
1048  * This function is expected to return at least millisecond-precision values.
1049  */
1050  virtual double CurrentClockTimeMillis() = 0;
1051 
1052  typedef void (*StackTracePrinter)();
1053 
1054  /**
1055  * Returns a function pointer that print a stack trace of the current stack
1056  * on invocation. Disables printing of the stack trace if nullptr.
1057  */
1058  virtual StackTracePrinter GetStackTracePrinter() { return nullptr; }
1059 
1060  /**
1061  * Returns an instance of a v8::TracingController. This must be non-nullptr.
1062  */
1064 
1065  /**
1066  * Tells the embedder to generate and upload a crashdump during an unexpected
1067  * but non-critical scenario.
1068  */
1069  virtual void DumpWithoutCrashing() {}
1070 
1071  /**
1072  * Allows the embedder to observe sections with high throughput allocation
1073  * operations.
1074  */
1077  static HighAllocationThroughputObserver default_observer;
1078  return &default_observer;
1079  }
1080 
1081  protected:
1082  /**
1083  * Default implementation of current wall-clock time in milliseconds
1084  * since epoch. Useful for implementing |CurrentClockTimeMillis| if
1085  * nothing special needed.
1086  */
1088 };
1089 
1090 } // namespace v8
1091 
1092 #endif // V8_V8_PLATFORM_H_