What is OpenCL

Introduced by Khronos, OpenCL is an explicit, industrial standard framework for cross-platform development supporting CPU, GPU, mobile devices and embedded platforms. Being a standard framework, OpenCL delivers high levels of portability across a wide range of hardware. OpenCL is an explicit programming model: The OpenCL developer must explicitly define the platform, context and schedule the work across multiple devices. In OpenCL terminology, a user application is divided into two parts: a host program and device program (also called an OpenCL kernel). The host program provides the outer control logic for configuring, constructing and executing an accelerator-based program, while the device program represents the program executed on the selected accelerator.

In the following sections we will explore each part in detail.

OpenCL host program

The OpenCL host program is composed of the two following APIs

  1. OpenCL platform API : a set of functions for identifying available OpenCL platforms, OpenCL devices per platform and their capabilities, and creating OpenCL context for OpenCL applications.

  2. OpenCL runtime API: a set of functions to set the OpenCL context, create an OpenCL command queue,create OpenCL memory objects, build an OpenCL kernel, set arguments for an OpenCL kernel, transfer the data across host and device, and execute the OpenCL kernel.

OpenCL context

An OpenCL context represents a set of devices from the OpenCL platform that can share the same OpenCL memory objects. OpenCL context can contain more than one device. The OpenCL implementation will manage the communication across all the devices in the context. In particular, whenever pre-defined synchronization points occur, the different memory objects from the context are updated with the latest copy of the data.

An application can have more than one OpenCL context created from a different platform in order to distribute the tasks among multiple devices.

OpenCL command queue

An OpenCL command queue is used to dispatch operations to a certain device inside a given OpenCL context. Such operations can be transferring data between host and device, or launching and executing an OpenCL kernel for a particular device. Only one device can exist on a given queue. Operations across devices must be synchronized manually by the developer using OpenCL events and barriers.

OpenCL program object

A program object contains a collection of kernels, auxiliary functions and constant data that can be used by kernel functions.

OpenCL program objects are built either from OpenCL kernels in text source form (OpenCL C sources) or from pre-generated program binaries. In particular, OpenCL implementations allow the usage of intermediate representations (IR), such as SPIR or SPIR-V, enabling other compilers to parse high-level languages into binary files.

The OpenCL compiler can generate a program executable for an appropriate target device in an online or offline mode. A program object stores the compiled executable code for each kernel on each device attached to it

OpenCL kernel object

OpenCL kernel objects represent the compiled binary of a specific device function (kernel) from a program object. Developers can pass arguments to a kernel function that is contained within a program object using the OpenCL host API functions. Developers can also extract information from a specific kernel function, such as the amount of memory used or preferred work group size.

OpenCL Kernel programming language

The OpenCL kernel programming language (also called the OpenCL C programming Language) is a subset of ISO C99 standard. It is used for creating device programs. As it is a subset of ISO C99, the following features are not supported in OpenCL kernels:

  • Pointers to functions

  • Bit-fields

  • Recursive functions

  • Dynamic memory allocation

However, given its parallel nature, it features some additions on top of the ISO C99 standard: Native vector types: The OpenCL C compiler provides native handlers for different vector types that can be optimized by the backend later on. Memory address spaces: Given the nature of the memory layout of heterogeneous platforms, OpenCL C offers native support for discontinuous address spaces, enabling developers to use all memory spaces supported by the device.

OpenCL memory type

OpenCL memory objects are represented by buffer objects on the host. A buffer object is a one-dimensional array of bytes. Each buffer is allocated within a context and, therefore, it will be visible to all devices within that context. It is not possible to create a memory object that is shared across different contexts. In order to use an OpenCL buffer across multiple devices of different platforms, the buffer data must be manually transferred across contexts. OpenCL provides a relaxed memory model: Not all writes to a memory object will necessarily be visible to all reads following those writes. Memory is only guaranteed to be visible after certain synchronization points.

It is possible to create a sub-buffer from an OpenCL buffer. Each sub-buffer can be considered as a one-dimensional view of a region of a given buffer. Using a sub-buffer, two or more kernels using different sub-buffers, can be executed simultaneously, independent from each others.

OpenCL also provides provides another type of memory called image. An image can be considered as an specialized type of OpenCL memory objects that provides 2D and 3D access over the image data. OpenCL images are typically implemented using texture memory on GPU accelerators.

What is SYCL

SYCL (pronounced "sickle") is a royalty-free, cross-platform abstraction C++ programming model for OpenCL[@khronos2009khronos], introduced, by KhronosTM group. The SYCL abstraction layer provides a completely standard C++ single-source style programming model while maintaining the same performance portability as OpenCL, across other OpenCL-enabled platforms. This means that SYCL enables OpenCL kernels to be written inside C++ source files. Such abstraction has been designed to enable implementation across as wide a variety of platforms as possible, as well as ease of integration with other platform-specific technologies, thereby letting both users and implementors build on top of SYCL as an open platform for heterogeneous processing innovation.

Using C++ features such as inheritance and templated libraries enables more developers to use OpenCL capabilities and devices to build simple, yet efficient, interfaces that were not possible to use in OpenCL 1.2. Software developers can develop and use generic algorithms and data structures using standard C++ template techniques, while still supporting the multi-platform, multi-device heterogeneous execution of OpenCL.

SYCL is designed to be as close to standard C++ as possible. In practice, this means that as long as no dependency is created on SYCL's integration with OpenCL, a standard C++ compiler can compile the SYCL programs and it will run correctly on the host CPU.

SYCL derives its device execution and memory models from the OpenCL kernel programming language. In particular, SYCL 1.2.1 is based on the OpenCL 1.2 specification. However, SYCL enables C++ programming on the device, instead of ISO C99. Due to OpenCL device model limitations, some features are not available:

  • Function pointers

  • Recursion

  • Virtual function calls (due to above)

  • Exceptions

  • RTTI (Runtime Type Information)

  • Dynamic memory allocation

Note that OpenCL defines some hardware features that may not be available in all OpenCL platforms. This limits the availability of certain C++ features on all platforms, like double floating point precision.

Single-source programming model

Unlike OpenCL-1.2, the single-source programming model in SYCL allows the kernel code to be embedded in the host code. This ability can be useful especially when re-using functionality in kernels for various operations. Moreover, embedding kernel code inside the host code makes SYCL programming easier for new programmers and allows them to focus on parallelization concepts rather than the API details.

Single-source multiple-compiler passes

SYCL supports the single-source multiple compiler-passes (SMCP) technique which allows a single source file to be parsed by multiple compilers for generating device binaries alongside program binaries.

For example, a compilation flow within SMCP uses a SYCL device compiler to parse a SYCL application and generate SPIR binaries. A standard host compiler can then parse the same SYCL application to produce the native host code. The SYCL runtime library will load the SPIR binaries at runtime. The source file is then parsed twice by two different compilers. Although SYCL standard supports the SMCP model, it does not preclude the use of a single compiler flow.

SMCP offers better integration with existing tool chains than a single compiler. An application that has been implemented using a certain compiler can continue to do so when using SYCL, as it is only the code that will be executed on the device that requires a different compilation step. This design simplifies porting of pre-existing applications to SYCL. In addition, the SMCP allows developers to use different device compilers from different vendors according to the target platform preferred toolchain.

This also helps when a CPU C++ compiler has been validated for safety as that compiler can be used unmodified, along with a SYCL device compiler for just the kernels.

OpenCL interoperability

SYCL provides the complete OpenCL feature set using the SYCL API. However, when interacting with existing OpenCL code or libraries, it is possible to directly call OpenCL APIs in SYCL through interoperability functions.

The main interoperability features used by developers are constructing SYCL buffers from OpenCL buffers, and obtaining an OpenCL queue from the SYCL queue.

SYCL object scope

SYCL relies heavily on C++ object scope rules and RAII (Resource Acquisition Is Initialization) to initialize and destroy device objects, including device memory. When a SYCL object is created, underlying OpenCL objects are created automatically by the runtime. Whenever a SYCL object is copied or moved, the same underlying objects are used: No extra overhead of creation/destruction low-level structures is introduced. Additional rules apply to memory objects. See SYCL Memory Types in the section below for details.

SYCL queue

A SYCL queue object schedules SYCL command groups (see section below) on a given SYCL device. A SYCL queue can map to one or multiple OpenCL command queues, or to a host queue. For the former, the SYCL APIs will be mapped to the underlying OpenCL APIs to execute a SYCL application. For the latter, the host queue, the SYCL API will be mapped to the host standard library. The host C++ APIs only require a C++ compiler and there is no need for OpenCL support.

SYCL command group

In SYCL, users can define a kernel and its dependencies inside command group_s. A _command group is a functor (or lambda) that takes a SYCL handler object as parameter. The handler provides the functionality that is available in the command group. The underlying OpenCL objects are generated or re-used during the submission. Submitting a SYCL command group object to SYCL queues will generate the required implicit memory update operations. Users can also dispatch explicit copy operations.

SYCL Memory

SYCL memory objects can be created either in the form of buffers, sub-buffers or images. However, SYCL memory objects offer various additional functionalities over OpenCL. The SYCL memory objects can be an array of one, two or three dimensions. Unlike the OpenCL ones, SYCL memory objects are not bound to any context, except if it has been explicitly bounded to an OpenCL context or it has been directly constructed from OpenCL memory objects. A SYCL buffer can migrate data across multiple OpenCL contexts, using any optimized mechanism available for the platform (e.g. fast DMA transfers).

Data access in SYCL is separated from data storage. By relying on the C++-style resource acquisition is initialization (RAII) idiom to capture data dependencies between device code blocks, the runtime library can track data movement and provide correct behavior without the complexity of manually managing event dependencies between kernel instances and without the programming having to explicitly move data. This approach enables the SYCL scheduler to automatically construct the data flow graph. By constructing a data flow graph, SYCL is able to automatically handle the coarse-grain task-based parallelism to identify the kernels' dependency and dispatch independent kernels concurrently whenever it is possible. This is an extension to the data parallelism, supported by the OpenCL execution model, and allows developers to build up SYCL programs easily and safely.


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