Two Shader Compilers of Direct3D 12

Mon
23
Dec 2019

If we write a game or other graphics application using Direct3D 12, we also need to write some shaders. We author these in high-level language called HLSL and compile them before passing to the DirectX API while creating pipeline state objects (ID3D12Device::CreateGraphicsPipelineState). There are currently two shader compilers available, both from Microsoft, each outputting different binary format:

  1. old compiler “FXC”
  2. new compiler “DXC”

Which one to choose? The new compiler, called DirectX Shader Compiler, is more modern, based on LLVM/Clang, and open source. We must use it if we want to use Shader Model 6 or above. On the other hand, shaders compiled with it require relatively recent version of Windows and graphics drivers installed, so they won’t work on systems not updated for years.

Shaders can be compiled offline using a command-line program (standalone executable compiler) and then bundled with your program in compiled binary form. That’s probably the best way to go for release version, but for development and debugging purposes it’s easier if we can change shader source just as we change the source of CPU code, easily rebuild or run, or even reload changed shader while the app is running. For this, it’s convenient to integrate shader compiler as part of your program, which is possible through a compiler API.

This gives us 4 different ways of compiling shaders. This article is a quick tutorial for all of them.

1. Old Compiler - Offline

The standalone executable of the old compiler is called “fxc.exe”. You can find it bundled with Windows SDK, which is installed together with Visual Studio. For example, in my system I located it in this path: “c:\Program Files (x86)\Windows Kits\10\bin\10.0.17763.0\x64\fxc.exe”.

To compile a shader from HLSL source to the old binary format, issue a command like this:

fxc.exe /T ps_5_0 /E main PS.hlsl /Fo PS.bin

/T is target profile
ps_5_0 means pixel shader with Shader Model 5.0
/E is the entry point - the name of the main shader function, “main” in my case
PS.hlsl is the text file with shader source
/Fo is binary output file to be written

There are many more command line parameters supported for this tool. You can display help about them by passing /? parameter. Using appropriate parameters you can change optimization level, other compilation settings, provide additional #include directories, #define macros, preview intermediate data (preprocessed source, compiled assembly), or even disassemble existing binary file.

2. Old compiler - API

To use the old compiler as a library in your C++ program:

Example:

CComPtr<ID3DBlob> code, errorMsgs;
HRESULT hr = D3DCompileFromFile(
    L"PS.hlsl", // pFileName
    nullptr, // pDefines
    nullptr, // pInclude
    "main", // pEntrypoint
    "PS_5_0", // pTarget
    0, // Flags1, can be e.g. D3DCOMPILE_DEBUG, D3DCOMPILE_SKIP_OPTIMIZATION
    0, // Flags2
    &code, // ppCode
    &errorMsgs); // ppErrorMsgs
if(FAILED(hr))
{
    if(errorMsgs)
    {
        wprintf(L"Compilation failed with errors:\n%hs\n",
            (const char*)errorMsgs->GetBufferPointer());
    }
    // Handle compilation error...
}

D3D12_GRAPHICS_PIPELINE_STATE_DESC psoDesc = {};
// (...)
psoDesc.PS.BytecodeLength = code->GetBufferSize();
psoDesc.PS.pShaderBytecode = code->GetBufferPointer();
CComPtr<ID3D12PipelineState> pso;
hr = device->CreateGraphicsPipelineState(&psoDesc, IID_PPV_ARGS(&pso));

First parameter is the path to the file that contains HLSL source. If you want to load the source in some other way, there is also a function that takes a buffer in memory: D3DCompile. Second parameter (optional) can specify preprocessor macros to be #define-d during compilation. Third parameter (optional) can point to your own implementation of ID3DInclude interface that would provide additional files requested via #include. Entry point and target platforms is a string just like in command-line compiler. Other options that have their command line parameters (e.g. /Zi, /Od) can be specified as bit flags.

Two objects returned from this function are just buffers of binary data. ID3DBlob is a simple interface that you can query for its size and pointer to its data. In case of a successful compilation, ppCode output parameter returns buffer with compiled shader binary. You should pass its data to ID3D12PipelineState creation. After successful creation, the blob can be Release-d. The second buffer ppErrorMsgs contains a null-terminated string with error messages generated during compilation. It can be useful even if the compilation succeeded, as it then contains warnings.

Update: "d3dcompiler_47.dll" file is needed. Typically some version of it is available on the machine, but generally you still want to redistribute the exact version you're using from the Win10 SDK. Otherwise you could end up compiling with an older or newer version on an end-user's machine.

3. New Compiler - Offline

Using the new compiler in its standalone form is very similar to the old one. The executable is called “dxc.exe” and it’s also bundled with Windows SDK, in the same directory. Documentation of command line syntax mentions parameters starting with "-", but old "/" also seems to work. To compile the same shader using Shader Model 6.0 issue following command, which looks almost the same as for "fxc.exe":

dxc.exe -T ps_6_0 -E main PS.hlsl -Fo PS.bin

Despite using a new binary format (called “DXIL”, based on LLVM IR), you can load it and pass it to D3D12 PSO creation the same way as before. There is a tricky issue though. You need to attach file “dxil.dll” to your program. Otherwise, the PSO creation will fail! You can find this file in Windows SDK path like: “c:\Program Files (x86)\Windows Kits\10\Redist\D3D\x64\dxil.dll”. Just copy it to the directory with target EXE of your project or the one that you use as working directory.

4. New Compiler - API

The new compiler can also be used programatically as a library, but its usage is a bit more difficult. Just as with any C++ library, start with:

This time though you need to bundle additional DLL to your program (next to “dxil.dll” mentioned above): “dxcompiler.dll”, to be found in the same “Redist\D3D\x64” directory. There is more code needed to perform the compilation. First create IDxcLibrary and IDxcCompiler objects. They can stay alive for the whole lifetime of your application or as long as you need to compile more shaders. Then for each shader, load it from a file (or any source of your choice) to a blob, call Compile method, and inspect its result, whether it’s an error + a blob with error messages, or a success + a blob with compiled shader binary.

CComPtr<IDxcLibrary> library;
HRESULT hr = DxcCreateInstance(CLSID_DxcLibrary, IID_PPV_ARGS(&library));
//if(FAILED(hr)) Handle error...

CComPtr<IDxcCompiler> compiler;
hr = DxcCreateInstance(CLSID_DxcCompiler, IID_PPV_ARGS(&compiler));
//if(FAILED(hr)) Handle error...

uint32_t codePage = CP_UTF8;
CComPtr<IDxcBlobEncoding> sourceBlob;
hr = library->CreateBlobFromFile(L"PS.hlsl", &codePage, &sourceBlob);
//if(FAILED(hr)) Handle file loading error...

CComPtr<IDxcOperationResult> result;
hr = compiler->Compile(
    sourceBlob, // pSource
    L"PS.hlsl", // pSourceName
    L"main", // pEntryPoint
    L"PS_6_0", // pTargetProfile
    NULL, 0, // pArguments, argCount
    NULL, 0, // pDefines, defineCount
    NULL, // pIncludeHandler
    &result); // ppResult
if(SUCCEEDED(hr))
    result->GetStatus(&hr);
if(FAILED(hr))
{
    if(result)
    {
        CComPtr<IDxcBlobEncoding> errorsBlob;
        hr = result->GetErrorBuffer(&errorsBlob);
        if(SUCCEEDED(hr) && errorsBlob)
        {
            wprintf(L"Compilation failed with errors:\n%hs\n",
                (const char*)errorsBlob->GetBufferPointer());
        }
    }
    // Handle compilation error...
}
CComPtr<IDxcBlob> code;
result->GetResult(&code);

D3D12_GRAPHICS_PIPELINE_STATE_DESC psoDesc = {};
// (...)
psoDesc.PS.BytecodeLength = code->GetBufferSize();
psoDesc.PS.pShaderBytecode = code->GetBufferPointer();
CComPtr<ID3D12PipelineState> pso;
hr = device->CreateGraphicsPipelineState(&psoDesc, IID_PPV_ARGS(&pso));

Compilation function also takes strings with entry point and target profile, but in Unicode format this time. The way to pass additional flags also changed. Instead of using bit flags, parameter pArguments and argCount take an array of strings that can specify additional parameters same as you would pass to the command-line compiler, e.g. L"-Zi" to attach debug information or L"-Od" to disable optimizations.

Update 2020-01-05: Thanks @MyNameIsMJP for your feedback!

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