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.

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. The offline nature of compilation also allows for more flexible language improvements such as the use of newer C++ features.

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

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

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. On July 22, 2013, a provisional specification SPIR 1.2 was announced at SIGGRAPH 2013. The final SPIR 1.2 specification was released at HiPEAC 2014 on January 21, 2014. On August 11th, 2014, a provisional specification for SPIR 2.0 was released at SIGGRAPH 2014.

SPIR-V
SPIR-V 1.0 is a rewritten version of SPIR announced in March 2015, and released on Nov. 16 2015. 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. A separate program by the Khronos Group allows for interconversion with LLVM IR.

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.

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.

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.

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

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

SPIR-V 1.6
On December 16, 2021, SPIR-V 1.6 was released.

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, optimizing SPIR-V spirv-opt, and cross-compiling to SPIR-V to different GLSL targets spirv-cross.

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, which aims to provide full linking functionality for SPIR-V binary code.

See also
 High-Level Shading Language (HLSL)
 Cg (programming language)
 OpenGL Shading Language (GLSL)
 Tungsten Graphics Shader Infrastructure
 Compute kernel
 Parallel Thread Execution

References

External links
 Khronos SPIR webpage
 Provisional SPIR-V Specification

Compiler construction
Shading languages