User Guide for the DirectX Target

Warning

Disclaimer: The DirectX backend is experimental and under active development. It is not yet feature complete or ready to be used outside of experimental or demonstration contexts.

Introduction

The DirectX target implements the DirectX programmability interfaces. These interfaces are documented in the DirectX Specifications.

Initially the backend is aimed at supporting DirectX 12, and support for DirectX 11 is planned at a later date.

The DirectX backend is currently experimental and is not shipped with any release builds of LLVM tools. To enable building the DirectX backend locally add DirectX to the LLVM_EXPERIMENTAL_TARGETS_TO_BUILD CMake option. For more information on building LLVM see the Building LLVM with CMake documentation.

Target Triples

At present the DirectX target only supports the dxil architecture, which generates code for the DirectX Intermediate Language.

In addition to target architecture, the DirectX backend also needs to know the target runtime version and pipeline stage. These are expressed using the OS and Environment triple component.

Presently the DirectX backend requires targeting the shadermodel OS, and supports versions 6.0+ (at time of writing the latest announced version is 6.7).

DirectX Environments

Environment	Description
pixel	Pixel shader
vertex	Vertex shader
geometry	Geometry shader
hull	Hull shader (tesselation)
domain	Domain shader (tesselation)
compute	Compute kernel
library	Linkable dxil library
raygeneration	Ray generation (ray tracing)
intersection	Ray intersection (ray tracing)
anyhit	Ray any collision (ray tracing)
closesthit	Ray closest collision (ray tracing)
miss	Ray miss (ray tracing)
callable	Callable shader (ray tracing)
mesh	Mesh shader
amplification	Amplification shader

Output Binaries

The DirectX runtime APIs read a file format based on the DirectX Specification.. In different codebases the file format is referred to by different names (specifically DXBC and DXILContainer). Since the format is used to store both DXBC and DXIL outputs, and the ultimate goal is to support both as code generation targets in LLVM, the LLVM codebase uses a more neutral name, DXContainer.

The DXContainer format is sparsely documented in the functional specification, but a reference implementation exists in the DirectXShaderCompiler..

Support for generating DXContainer files in LLVM, is being added to the LLVM MC layer for object streamers and writers, and to the Object and ObjectYAML libraries for testing and object file tooling.

For dxil targeting, bitcode emission into DXContainer files follows a similar model to the -fembed-bitcode flag supported by clang for other targets.