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The C and C++ Include Header Files

/usr/include/nodejs/deps/v8/include/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	   */
    27	  kBestEffort,
    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	   */
    33	  kUserVisible,
    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	   */
    39	  kUserBlocking,
    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	 */
   270	class ConvertableToTraceFormat {
   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	 */
   288	class TracingController {
   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	
   343	  class TraceStateObserver {
   344	   public:
   345	    virtual ~TraceStateObserver() = default;
   346	    virtual void OnTraceEnabled() = 0;
   347	    virtual void OnTraceDisabled() = 0;
   348	  };
   349	
   350	  /** Adds tracing state change observer. */
   351	  virtual void AddTraceStateObserver(TraceStateObserver*) {}
   352	
   353	  /** Removes tracing state change observer. */
   354	  virtual void RemoveTraceStateObserver(TraceStateObserver*) {}
   355	};
   356	
   357	/**
   358	 * A V8 memory page allocator.
   359	 *
   360	 * Can be implemented by an embedder to manage large host OS allocations.
   361	 */
   362	class PageAllocator {
   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 {
   394	    kNoAccess,
   395	    kRead,
   396	    kReadWrite,
   397	    kReadWriteExecute,
   398	    kReadExecute,
   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.
   406	    kNoAccessWillJitLater
   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	   */
   455	  class SharedMemoryMapping {
   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.
   528	inline PlatformSharedMemoryHandle SharedMemoryHandleFromMachMemoryEntry(
   529	    unsigned int port) {
   530	  return static_cast<PlatformSharedMemoryHandle>(port);
   531	}
   532	inline unsigned int MachMemoryEntryFromSharedMemoryHandle(
   533	    PlatformSharedMemoryHandle handle) {
   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.
   538	inline PlatformSharedMemoryHandle SharedMemoryHandleFromVMO(uint32_t handle) {
   539	  return static_cast<PlatformSharedMemoryHandle>(handle);
   540	}
   541	inline uint32_t VMOFromSharedMemoryHandle(PlatformSharedMemoryHandle handle) {
   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.
   547	inline PlatformSharedMemoryHandle SharedMemoryHandleFromFileMapping(
   548	    void* handle) {
   549	  return reinterpret_cast<PlatformSharedMemoryHandle>(handle);
   550	}
   551	inline void* FileMappingFromSharedMemoryHandle(
   552	    PlatformSharedMemoryHandle handle) {
   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	}
   560	inline int FileDescriptorFromSharedMemoryHandle(
   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,
   573	  kReadWriteExecute,
   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	 */
   587	class VirtualAddressSpace {
   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	   */
   711	  virtual V8_WARN_UNUSED_RESULT bool SetPagePermissions(
   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	 */
   861	class ZoneBackingAllocator {
   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	 */
   874	class HighAllocationThroughputObserver {
   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	   */
   893	  virtual PageAllocator* GetPageAllocator() {
   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	   */
   901	  virtual ZoneBackingAllocator* GetZoneBackingAllocator() {
   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	   */
  1063	  virtual TracingController* GetTracingController() = 0;
  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	   */
  1075	  virtual HighAllocationThroughputObserver*
  1076	  GetHighAllocationThroughputObserver() {
  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	   */
  1087	  V8_EXPORT static double SystemClockTimeMillis();
  1088	};
  1089	
  1090	}  // namespace v8
  1091	
  1092	#endif  // V8_V8_PLATFORM_H_