OpenCL2.2规范详解：GPU编程入门必备

OpenCL2.2

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"OpenCL2.2_API.pdf是一个关于OpenCL 2.2 API的详细文档，它是学习OpenCL编程的基础资料，包含最新的OpenCL标准，全英文内容。文档涵盖了OpenCL架构、平台模型、执行模型、内存模型以及OpenCL框架等核心概念。" OpenCL（Open Computing Language）是一种跨平台的并行编程框架，用于利用GPU（图形处理单元）、CPU和其他处理器进行计算。OpenCL 2.2 API是这个框架的一个版本，提供了丰富的接口和功能来开发高效的并行计算程序。 1. **OpenCL架构**：文档详细介绍了OpenCL的架构，包括平台模型、执行模型和内存模型。平台模型定义了如何在不同的硬件上组织OpenCL环境，包括主机设备（如CPU）和计算设备（如GPU）。执行模型描述了并行计算的工作方式，如工作项、工作组和计算单元的交互。内存模型则解释了数据在不同设备间如何共享和通信。 2. **平台层**：这部分讲解了如何查询平台信息，如可用的OpenCL平台和设备。开发者可以获取设备类型、特性、最大工作组大小等信息，以便选择合适的计算资源。 3. **运行时**：OpenCL运行时组件包括命令队列、缓冲区对象、图像对象、管道、内存管理、共享虚拟内存、采样器对象、程序对象、内核对象、执行内核、事件对象等功能。这些组件构成了OpenCL程序的核心部分，使得开发者能够创建、管理和调度计算任务。 4. **命令队列**：命令队列用于安排执行OpenCL命令，比如拷贝操作、内核执行等。它们可以同步，也可以异步执行，支持出序执行以优化性能。 5. **内存对象**：OpenCL支持缓冲区和图像对象，它们是数据传输和计算的基础。内存管理涉及缓冲区的映射、解锁、迁移、保留和释放，确保高效的数据交换。 6. **SPIR-V中间语言**：OpenCL 2.2引入了对SPIR-V的支持，这是一种通用的中间表示，允许编译器将源代码转换为多种目标架构，促进了跨平台的兼容性。 7. **其他高级功能**：文档还包含了对事件的等待、标记、屏障操作，以及内存对象和内核的性能分析工具。此外，flush和finish函数用于确保数据传输和计算的完成。 OpenCL 2.2 API为开发者提供了强大的工具，用于编写高性能的并行计算程序，充分利用现代多核硬件的计算能力。通过理解和应用这些API，开发者能够实现高度优化的计算解决方案，适用于科学计算、图像处理、机器学习等多个领域。

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Image Object

A memory object that stores a two- or three-dimensional structured array. Image data can only

be accessed with read and write functions. The read functions use a sampler.

The image object encapsulates the following information:

• Dimensions of the image.

• Description of each element in the image.

• Properties that describe usage information and which region to allocate from.

• Image data.

The elements of an image are selected from a list of predefined image formats.

Implementation Defined

Behavior that is explicitly allowed to vary between conforming implementations of OpenCL. An

OpenCL implementor is required to document the implementation-defined behavior.

Independent Forward Progress

If an entity supports independent forward progress, then if it is otherwise not dependent on any

actions due to be performed by any other entity (for example it does not wait on a lock held by,

and thus that must be released by, any other entity), then its execution cannot be blocked by the

execution of any other entity in the system (it will not be starved). Work items in a subgroup, for

example, typically do not support independent forward progress, so one work item in a

subgroup may be completely blocked (starved) if a different work item in the same subgroup

enters a spin loop.

In-order Execution

A model of execution in OpenCL where the commands in a command-queue are executed in

order of submission with each command running to completion before the next one begins. See

Out-of-order Execution.

Intermediate Language

A lower-level language that may be used to create programs. SPIR-V is a required IL for OpenCL

2.2 runtimes. Additional ILs may be accepted on an implementation-defined basis.

Kernel

A kernel is a function declared in a program and executed on an OpenCL device. A kernel is

identified by the kernel or kernel qualifier applied to any function defined in a program.

Kernel-instance

The work carried out by an OpenCL program occurs through the execution of kernel-instances

on devices. The kernel instance is the kernel object, the values associated with the arguments to

the kernel, and the parameters that define the NDRange index space.

Kernel Object

A kernel object encapsulates a specific

__kernel function declared in a program and the

argument values to be used when executing this __kernel function.

Kernel Language

A language that is used to create source code for kernel. Supported kernel languages include

OpenCL C, OpenCL C++, and OpenCL dialect of SPIR-V.

Launch

The transition of a command from the submitted state to the ready state. See Ready.

Local ID

A local ID specifies a unique work-item ID within a given work-group that is executing a kernel.

The local ID is a N-dimensional value that starts at (0, 0, … 0). See also Global ID.

Local Memory

A memory region associated with a work-group and accessible only by work-items in that work-

group. Local memory is included within the generic address space that includes the private and

global address spaces.

Marker

A command queued in a command-queue that can be used to tag all commands queued before the

marker in the command-queue. The marker command returns an event which can be used by the

application to queue a wait on the marker event i.e. wait for all commands queued before the

marker command to complete.

Memory Consistency Model

Rules that define which values are observed when multiple units of execution load data from

any shared memory plus the synchronization operations that constrain the order of memory

operations and define synchronization relationships. The memory consistency model in OpenCL

is based on the memory model from the ISO C11 programming language.

Memory Objects

A memory object is a handle to a reference counted region of Global Memory. Also see Buffer

Object and Image Object.

Memory Regions (or Pools)

A distinct address space in OpenCL. Memory regions may overlap in physical memory though

OpenCL will treat them as logically distinct. The memory regions are denoted as private, local,

constant, and global.

Memory Scopes

These memory scopes define a hierarchy of visibilities when analyzing the ordering constraints

of memory operations. They are defined by the values of the memory_scope enumeration

constant. Current values are memory_scope_work_item (memory constraints only apply to a

single work-item and in practice apply only to image operations), memory_scope_sub_group

(memory-ordering constraints only apply to work-items executing in a sub-group),

memory_scope_work_group (memory-ordering constraints only apply to work-items executing

in a work-group), memory_scope_device (memory-ordering constraints only apply to work-

items executing on a single device) and memory_scope_all_svm_devices (memory-ordering

constraints only apply to work-items executing across multiple devices and when using shared

virtual memory).

Modification Order

All modifications to a particular atomic object M occur in some particular total order, called the

modification order of M. If A and B are modifications of an atomic object M, and A happens-

before B, then A shall precede B in the modification order of M. Note that the modification order

of an atomic object M is independent of whether M is in local or global memory.

Nested Parallelism

See device-side enqueue.

Object

Objects are abstract representation of the resources that can be manipulated by the OpenCL API.

Examples include program objects, kernel objects, and memory objects.

Out-of-Order Execution

A model of execution in which commands placed in the work queue may begin and complete

execution in any order consistent with constraints imposed by event wait lists_and_command-

queue barrier. See In-order Execution.

Parent device

The OpenCL device which is partitioned to create sub-devices. Not all parent devices are root

devices. A root device might be partitioned and the sub-devices partitioned again. In this case, the

first set of sub-devices would be parent devices of the second set, but not the root devices. Also see

Device, parent device and root device.

Parent kernel

see Device-side enqueue.

Pipe

The pipe memory object conceptually is an ordered sequence of data items. A pipe has two

endpoints: a write endpoint into which data items are inserted, and a read endpoint from which

data items are removed. At any one time, only one kernel instance may write into a pipe, and

only one kernel instance may read from a pipe. To support the producer consumer design

pattern, one kernel instance connects to the write endpoint (the producer) while another kernel

instance connects to the reading endpoint (the consumer).

Platform

The host plus a collection of devices managed by the OpenCL framework that allow an

application to share resources and execute kernels on devices in the platform.

Private Memory

A region of memory private to a work-item. Variables defined in one work-items private memory

are not visible to another work-item.

Processing Element

A virtual scalar processor. A work-item may execute on one or more processing elements.

Program

An OpenCL program consists of a set of kernels. Programs may also contain auxiliary functions

called by the __kernel functions and constant data.

Program Object

A program object encapsulates the following information:

• A reference to an associated context.

• A program source or binary.

• The latest successfully built program executable, the list of devices for which the program

executable is built, the build options used and a build log.

• The number of kernel objects currently attached.

Queued

The first state in the six state model for the execution of a command. The transition into this

state occurs when the command is enqueued into a command-queue.

Ready

The third state in the six state model for the execution of a command. The transition into this

state occurs when pre-requisites constraining execution of a command have been met; i.e. the

command has been launched. When a kernel-enqueue command is launched, work-groups

associated with the command are placed in a devices work-pool from which they are scheduled

for execution.

Re-converged Control Flow

see Control flow.

Reference Count

The life span of an OpenCL object is determined by its reference count, an internal count of the

number of references to the object. When you create an object in OpenCL, its reference count is

set to one. Subsequent calls to the appropriate retain API (such as clRetainContext,

clRetainCommandQueue) increment the reference count. Calls to the appropriate release API

(such as clReleaseContext, clReleaseCommandQueue) decrement the reference count.

Implementations may also modify the reference count, e.g. to track attached objects or to ensure

correct operation of in-progress or scheduled activities. The object becomes inaccessible to host

code when the number of release operations performed matches the number of retain

operations plus the allocation of the object. At this point the reference count may be zero but

this is not guaranteed.

Relaxed Consistency

A memory consistency model in which the contents of memory visible to different work-items or

commands may be different except at a barrier or other explicit synchronization points.

Relaxed Semantics

A memory order semantics for atomic operations that implies no order constraints. The

operation is atomic but it has no impact on the order of memory operations.

Release Semantics

One of the memory order semantics defined for synchronization operations. Release semantics

apply to atomic operations that store to memory. Given two units of execution, A and B, acting

on a shared atomic object M, if A uses an atomic store of M with release semantics to

synchronize-with an atomic load to M by B that used acquire semantics, then A's atomic store

will occur after any prior operations by A. Note that the memory orders acquire, sequentially

consistent, and acquire_release all include acquire semantics and effectively pair with a store

using release semantics.

Remainder work-groups

When the work-groups associated with a kernel-instance are defined, the sizes of a work-group

in each dimension may not evenly divide the size of the NDRange in the corresponding

dimensions. The result is a collection of work-groups on the boundaries of the NDRange that are

smaller than the base work-group size. These are known as remainder work-groups.

Running

The fourth state in the six state model for the execution of a command. The transition into this

state occurs when the execution of the command starts. When a Kernel-enqueue command

starts, one or more work-groups associated with the command start to execute.

Root device

A root device is an OpenCL device that has not been partitioned. Also see Device, Parent device

and Root device.

Resource

A class of objects defined by OpenCL. An instance of a resource is an object. The most common

resources are the context, command-queue, program objects, kernel objects, and memory objects.

Computational resources are hardware elements that participate in the action of advancing a

program counter. Examples include the host, devices, compute units and processing elements.

Retain, Release

The action of incrementing (retain) and decrementing (release) the reference count using an

OpenCL object. This is a book keeping functionality to make sure the system doesn’t remove an

object before all instances that use this object have finished. Refer to Reference Count.

Sampler

An object that describes how to sample an image when the image is read in the kernel. The image

read functions take a sampler as an argument. The sampler specifies the image addressing-mode

i.e. how out-of-range image coordinates are handled, the filter mode, and whether the input

image coordinate is a normalized or unnormalized value.

Scope inclusion

Two actions A and B are defined to have an inclusive scope if they have the same scope P such

that: (1) if P is memory_scope_sub_group, and A and B are executed by work-items within the

same sub-group, or (2) if P is memory_scope_work_group, and A and B are executed by work-

items within the same work-group, or (3) if P is memory_scope_device, and A and B are

executed by work-items on the same device, or (4) if P is memory_scope_all_svm_devices, if A

and B are executed by host threads or by work-items on one or more devices that can share SVM

memory with each other and the host process.

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