Standard Portable Intermediate Representation

Standard Portable Intermediate Representation (SPIR) is an intermediate language for parallel compute and graphics by Khronos Group, originally developed for use with OpenCL. SPIR was rewritten into SPIR-V in March 2015.

SPIR
Developer(s)Khronos Group
Initial releaseJanuary 2014 (2014-01)
Stable release
SPIR-V 1.5 / September 13, 2019 (2019-09-13)
Operating systemCross-platform
PlatformCross-platform
TypeIntermediate language
Websitewww.khronos.org/spir

Purpose

OpenCL uses just-in-time compilation (JIT), necessitating one of two software distribution patterns: developers can distribute device-specific pre-compiled binaries, or they can distribute relevant source code, which is limited by the desire to protect intellectual property. SPIR enables the creation and distribution of device-independent binaries within the OpenCL stack.[1] The offline nature of compilation also allows for more flexible language improvements such as the use of newer C++ features.[2]

More importantly, SPIR-V allows the Vulkan API to use any shading language, including GLSL and HLSL.[3][4] SPIR-V can be compiled into several shading languages (GLSL, GLSL ES, MSL, HLSL) using SPIRV-Cross, so that these languages can be interconverted.[5] The textual shading language of WebGPU is designed as a thin layer over SPIR-V.[6]

Versions

SPIR was originally introduced in 2011 and SPIR-V was introduced in 2015.

SPIR and SPIR-V
SPIR 1.2 SPIR 2.0 SPIR-V 1.X
LLVM Interaction LLVM IR version 3.2 LLVM IR version 3.4 100% Khronos defined

Round-trip lossless conversion

Compute Constructs Metadata/Intrinsics Metadata/Intrinsics Native
Graphics Constructs No No Native
Supported Language & Feature Supported OpenCL C 1.2 OpenCL C 1.2

OpenCL C 2.0

 OpenCL C 1.2 / 2.X

OpenCL C++

GLSL

OpenCL Ingestion OpenCL 1.2 Extension OpenCL 2.0 Extension OpenCL 2.1/2.2 Core
Graphics API Ingestion NA NA Vulkan 1.X

OpenGL 4.6 Core

LLVM-based versions

SPIR prior to the 2015 SPIR-V release was based on the LLVM Intermediate Representation. A provisional specification for SPIR 1.0 was announced in 2012.[7] On July 22, 2013, a provisional specification SPIR 1.2 was announced at SIGGRAPH 2013.[8] The final SPIR 1.2 specification was released at HiPEAC 2014 on January 21, 2014.[9] On August 11th, 2014, a provisional specification for SPIR 2.0 was released at SIGGRAPH 2014.[10]

SPIR-V

SPIR-V 1.0 is a rewritten version of SPIR announced in March 2015,[11] and released on Nov. 16 2015.[12] The SPIR family now includes a true cross-API standard that is fully defined by Khronos with native support for shader and kernel features.

SPIR-V is a high-level intermediate language, exchanged in binary form. Functions are represented by a control flow graph of basic blocks, using static single assignment (SSA) form. Data structures retain high-level hierarchical representation. It is not lossy like previous byte-code or virtual machine-like intermediate representations used for graphical shaders. This allows closer to optimum performance on the target devices.[13] A separate program by the Khronos Group allows for interconversion with LLVM IR.[14]

Support for ingestion of SPIR-V is incorporated in the core specification of OpenCL 2.1, the Vulkan API, and OpenGL version 4.6.

SPIR-V 1.1

On April 18, 2016, SPIR-V 1.1 was released at IWOCL 2016 along with Provisional OpenCL 2.2. SPIR-V 1.1 added support for OpenCL C++, initializer/finalizer function execution modes, named barriers, subgroup execution, program scope pipes and pipe storage.[15]

SPIR-V 1.2

On May 16, 2017, SPIR-V 1.2 was released at IWOCL 2017 along with OpenCL 2.2. SPIR-V 1.2 added support for runtime specialization of key tuning parameters in OpenCL 2.2.[16]

SPIR-V 1.3

On March 7, 2018, SPIR-V 1.3 was released along with Vulkan 1.1. SPIR-V 1.3 Added support for subgroup operations and enables enhanced compiler optimizations.[17]

SPIR-V 1.4

On May 7, 2019, SPIR-V 1.4 was released.[18]

SPIR-V 1.5

On September 13, 2019, SPIR-V 1.5 was released.[19]

SPIR-V for GLSL Cross-Compilation

Subsequent versions of the OpenGL Shading Language (GLSL) introduced a number of backwards-incompatible language changes. As a result, GLSL shaders that target multiple OpenGL versions are rewritten once for each version, which is exacerbated by practical compatibility concerns due implementations that are pegged to a specific older version of GLSL, such as for WebGL 1.0 and Apple's OpenGL implementation. One of the notable use cases of SPIR-V is its ability to be used as an interchange format between GLSL versions, using tools maintained by the Khronos Group for compiling GLSL to SPIR-V glslangValidator,[20] optimizing SPIR-V spirv-opt,[21] and cross-compiling to SPIR-V to different GLSL targets spirv-cross.[22]

As a format, however, SPIR-V has some notable limitations for cross-compilation. Perhaps most critically is the requirement that every SPIR-V module have at least one entry-point symbol. This prevents the format from being easily used for separate compilation, where complex shaders could be assembled by a series of partial compile steps followed by a linking step. The analogy for a CPU-compiled language, is that SPIR-V can be used only for the creation of executables, but not for object files .o, shared object files .so or static libraries .a. This runs counter to the stated goals of some SPIR-V tools such as spirv-link,[23] which aims to provide full linking functionality for SPIR-V binary code.

See also

References
  1. Farber, Rob (11 August 2014). "Commercial OpenCL! SPIR 2.0 Protects IP Yet Allows Powerful, Portable, Source Code Free Kernels". TechEnablement. Retrieved 5 April 2015.
  2. Trevett, Neil. "OpenCL Overview and Update" (PDF).
  3. "Shader modules". Vulkan Tutorial.
  4. "HLSL as a First Class Vulkan Shading Language". The Khronos Group. 15 January 2020.
  5. KhronosGroup/SPIRV-Cross, The Khronos Group, 2019-09-06, retrieved 2019-09-08
  6. WebGPU Shading Language specification
  7. Larabel, Michael (12 September 2012). "Khronos SPIR For OpenCL Brings Binary Compatibility". Phoronix. Retrieved 25 July 2015.
  8. Smith, Ryan (22 July 2013). "Khronos @ SIGGRAPH 2013: OpenGL 4.4, OpenCL 2.0, & OpenCL 1.2 SPIR Announced". Anandtech. Retrieved 5 April 2015.
  9. "Khronos Releases SPIR 1.2 Specification for Portable Encoding of OpenCL Device Programs". The Khronos Group. 2014-01-20. Retrieved 2019-08-05.
  10. Smith, Ryan (11 August 2014). "Khronos Announces OpenCL SPIR 2.0". Anandtech. Retrieved 5 April 2015.
  11. Parkerson, Stuart (4 March 2015). "Khronos Group Introduces New Vulkan Hardware Driver API and SPIR-V Intermediate Language Shared by Vulkan and OpenCL 2.1". App Developer Magazine. Retrieved 5 April 2015.
  12. "Khronos Releases OpenCL 2.1 and SPIR-V 1.0 Specifications for Heterogeneous Parallel Programming". www.Khronos.org. 16 November 2015. Retrieved 16 November 2015.
  13. Kessenich, John. "An Introduction to SPIR-V" (PDF). Khronos. Retrieved 25 July 2015.
  14. "SPIRV-LLVM-Translator: A tool and a library for bi-directional translation between SPIR-V and LLVM IR". GitHub. The Khronos Group.
  15. Trevett, Neil (18 April 2016). "OpenCL A State of the Union (IWOCL 2016)" (PDF). Khronos Group.
  16. Trevett, Neil (16 May 2017). "OpenCL State of the Nation (IWOCL 2017)" (PDF). IWOCL.
  17. "SPIR - The Industry Open Standard Intermediate Language for Parallel Compute and Graphics". The Khronos Group. 2014-01-20. Retrieved 2019-08-05.
  18. "Khronos SPIR-V Registry - The Khronos Group Inc". www.khronos.org. Retrieved 2019-08-05.
  19. "Khronos SPIR-V Registry - The Khronos Group Inc". www.khronos.org. Retrieved 2020-01-30.
  20. "OpenGL / OpenGL ES Reference Compiler". www.khronos.org. Retrieved 2020-10-01.
  21. "SPIR-V Tools". www.github.com. Retrieved 2020-10-01.
  22. "SPIR-V Cross". www.github.com. Retrieved 2020-10-01.
  23. "SPIR-V Tools". www.github.com. Retrieved 2020-10-01.
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