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).

Table 111 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.. The format is documented in the LLVM project docs as well (see DirectX Container).

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.