Architecture and Design of DXIL Support in LLVM — LLVM 21.1.1 documentation

# Architecture and Design of DXIL Support in LLVM

## Introduction

LLVM supports reading and writing the DirectX Intermediate Language., or DXIL. DXIL is essentially LLVM 3.7 era bitcode with some restrictions and various semantically important operations and metadata.

LLVM’s implementation philosophy for DXIL support is to treat DXIL as merely a representation format as much as possible. When reading DXIL, we should translate everything to generic LLVM constructs when possible. Similarly, we should introduce DXIL-specific constructs as late as possible in the process of lowering to the format.

There are three places to look for DXIL related code in LLVM: The DirectX backend, for writing DXIL; The DXILUpgrade pass, for reading; and in library code that is shared between writing and reading. We’ll describe these in reverse order.

## Common Code for Reading and Writing

There’s quite a bit of logic that needs to be shared between reading and writing DXIL in order to avoid code duplication. While we don’t have a hard and fast rule about where such code should live, there are generally three sensible places. Simple definitions of enums and values that must stay fixed to match DXIL’s ABI can be found in Support/DXILABI.h, utilities to translate bidirectionally between DXIL and modern LLVM constructs live in lib/Transforms/Utils, and more analyses that are needed to derive or preserve information are implemented as typical lib/Analysis passes.

## The DXILUpgrade Pass

Translating DXIL to LLVM IR takes advantage of the fact that DXIL is compatible with LLVM 3.7 bitcode, and that modern LLVM is capable of “upgrading” older bitcode into modern IR. Simply relying on the bitcode upgrade process isn’t sufficient though, since that leaves a number of DXIL specific constructs around. Thus, we have the DXILUpgrade pass to transform DXIL operations to LLVM operations and smooth over differences in metadata representation. We call this pass “upgrade” to reflect that it follows LLVM’s standard bitcode upgrade process and simply finishes the job for DXIL constructs - while “reader” or “lifting” might also be reasonable names, they could be a bit misleading.

The DXILUpgrade pass itself is fairly lightweight. It mostly relies on the utilities described in “Common Code” above in order to share logic with both the DirectX backend and with Clang’s codegen of HLSL support as much as possible.

## The DirectX Intrinsic Expansion Pass

There are intrinsics that don’t map directly to DXIL Ops. In some cases an intrinsic needs to be expanded to a set of LLVM IR instructions. In other cases an intrinsic needs modifications to the arguments or return values of a DXIL Op. The DXILIntrinsicExpansion pass handles all the cases where our intrinsics don’t have a one to one mapping. This pass may also be used when the expansion is specific to DXIL to keep implementation details out of CodeGen. Finally, there is an expectation that we maintain vector types through this pass. Therefore, best practice would be to avoid scalarization in this pass.

## The DirectX Backend

The DirectX backend lowers LLVM IR into DXIL. As we’re transforming to an intermediate format rather than a specific ISA, this backend does not follow the instruction selection patterns you might be familiar with from other backends. There are two parts to lowering DXIL - a set of passes that mutate various constructs into a form that matches how DXIL represents those constructs, followed by a limited bitcode “downgrader pass”.

Before emitting DXIL, the DirectX backend needs to modify the LLVM IR such that external operations, types, and metadata is represented in the way that DXIL expects. For example, DXILOpLowering translates intrinsics into dx.op calls. These passes are essentially the inverse of the DXILUpgrade pass. It’s best to do this downgrading process as IR to IR passes when possible, as that means that they can be easily tested with opt and FileCheck without the need for external tooling.

The second part of DXIL emission is more or less an LLVM bitcode downgrader. We need to emit bitcode that matches the LLVM 3.7 representation. For this, we have DXILWriter, which is an alternate version of LLVM’s BitcodeWriter. At present, this is able to leverage LLVM’s current bitcode libraries to do a lot of the work, but it’s possible that at some point in the future it will need to be completely separate as modern LLVM bitcode evolves.

### DirectX Backend Flow

The code generation flow for DXIL is broken into a series of passes. The passes are grouped into two flows:

1. Generating DXIL IR.

2. Generating DXIL Binary.

The passes to generate DXIL IR follow the flow:

> DXILOpLowering -> DXILPrepare -> DXILTranslateMetadata

Each of these passes has a defined responsibility:

1. DXILOpLowering translates LLVM intrinsic calls to dx.op calls.

2. DXILPrepare transforms the DXIL IR to be compatible with LLVM 3.7, and inserts bitcasts to allow typed pointers to be inserted.

3. DXILTranslateMetadata emits the DXIL Metadata structures.

The passes to encode DXIL to binary in the DX Container follow the flow:

> DXILEmbedder -> DXContainerGlobals -> AsmPrinter

Each of these passes have the following defined responsibilities:

1. DXILEmbedder runs the DXIL bitcode writer to generate a bitcode stream and embeds the binary data inside a global in the original module.

2. DXContainerGlobals generates binary data globals for the other DX Container parts based on computed analysis passes.

3. AsmPrinter is the standard LLVM infrastructure for emitting object files.

When emitting DXIL into a DX Container file the MC layer is used in a similar way to how the Clang `-fembed-bitcode` option operates. The DX Container object writer knows how to construct the headers and structural fields of the container, and reads global variables from the module to fill in the remaining part data.

### DirectX Container

The DirectX container format is treated in LLVM as an object file format. Reading is implemented between the BinaryFormat and Object libraries, and writing is implemented in the MC layer. Additional testing and inspection support are implemented in the ObjectYAML library and tools.

## Testing

A lot of DXIL testing can be done with typical IR to IR tests using opt and FileCheck, since a lot of the support is implemented in terms of IR level passes as described in the previous sections. You can see examples of this in llvm/test/CodeGen/DirectX as well as llvm/test/Transforms/DXILUpgrade, and this type of testing should be leveraged as much as possible.

However, when it comes to testing the DXIL format itself, IR passes are insufficient for testing. For now, the best option we have available is using the DXC project’s tools in order to round trip. These tests are currently found in test/tools/dxil-dis and are only available if the LLVM_INCLUDE_DXIL_TESTS cmake option is set. Note that we do not currently have the equivalent testing set up for the DXIL reading path.

As soon as we are able, we will also want to round trip using the DXIL writing and reading paths in order to ensure self consistency and to get test coverage when dxil-dis isn’t available.