| # Shader Module Compilation |
| |
| ANGLE converts application shaders into Vulkan [VkShaderModules][VkShaderModule] through a series |
| of steps: |
| |
| 1. **ANGLE Internal Translation**: The initial calls to `glCompileShader` are passed to the [ANGLE |
| shader translator][translator]. The translator compiles application shaders into SPIR-V, after |
| transforming GLSL to conform to the [GL_KHR_vulkan_glsl][GL_KHR_vulkan_glsl] extension spec with |
| some additional workarounds and emulation. We emulate OpenGL's different depth range, viewport y |
| flipping, default uniforms, and emulate a number of extensions among others. For more info see |
| [TranslatorVulkan.cpp][TranslatorVulkan.cpp]. After initial compilation, the SPIR-V blobs are not |
| complete. The translator initially assigns resources and in/out variables arbitrary descriptor set, |
| binding and location indices. The correct values are determined at link time. For the sake of |
| transform feedback, some additional code is generated to be removed or modified during SPIR-V |
| transformation. |
| |
| The translator outputs some feature code conditional to Vulkan specialization constants, which are |
| resolved at draw-time. For example, for emulating Dithering and Android surface rotation. |
| |
| 1. **Link Time**: During a call to `glLinkProgram` the Vulkan back-end can know the necessary |
| locations and properties to write to connect the shader stage interfaces. However, some draw-time |
| information is still necessary to finalize the SPIR-V; for example whether "early fragment tests" |
| can be enabled. As an optimization, the SPIR-V is transformed with arbitrary settings and a Vulkan |
| pipeline object is created in an attempt to warm the Vulkan pipeline cache. |
| |
| 1. **Draw-Time SPIR-V Transformation and Pipeline Creation**: Once the application records a draw |
| call, a transformation pass is done on the generated SPIR-V to update the arbitrary descriptor set, |
| binding and location indices set in step 1. Additionally, component and various transform feedback |
| decorations are added, inactive varyings are removed, early fragment tests are enabled or disabled, |
| debug info is removed and pre-rotation is applied. At this time, `VkShaderModule`s are created (and |
| cached). The appropriate specialization constants are then resolved and the `VkPipeline` object is |
| created (see details [here](PipelineCreation)). Note that we currently don't use |
| [SPIRV-Tools][SPIRV-Tools] to perform any SPIR-V optimization. This could be something to improve on |
| in the future. |
| |
| See the below diagram for a high-level view of the shader translation flow: |
| |
| <!-- Generated from https://bramp.github.io/js-sequence-diagrams/ |
| Note: remove whitespace in - -> arrows. |
| participant App |
| participant "ANGLE Front-end" |
| participant "Vulkan Back-end" |
| participant "ANGLE Translator" |
| participant "Link-Time SPIR-V Transformer" |
| |
| App->"ANGLE Front-end": glCompileShader (VS) |
| "ANGLE Front-end"->"Vulkan Back-end": ShaderVk::compile |
| "Vulkan Back-end"->"ANGLE Translator": sh::Compile |
| "ANGLE Translator"- ->"ANGLE Front-end": return SPIR-V |
| |
| Note right of "ANGLE Front-end": SPIR-V is using bogus\ndecorations to be\ncorrected at link time. |
| |
| Note right of App: Same for FS, GS, etc... |
| |
| App->"ANGLE Front-end": glCreateProgram (...) |
| App->"ANGLE Front-end": glAttachShader (...) |
| App->"ANGLE Front-end": glLinkProgram |
| "ANGLE Front-end"->"Vulkan Back-end": ProgramVk::link |
| |
| Note right of "Vulkan Back-end": ProgramVk inits uniforms,\nlayouts, and descriptors. |
| |
| "Vulkan Back-end"->"Link-Time SPIR-V Transformer": SpvGetShaderSpirvCode |
| "Link-Time SPIR-V Transformer"- ->"Vulkan Back-end": retrieve SPIR-V and determine decorations |
| "Vulkan Back-end"- ->"ANGLE Front-end": return success |
| |
| Note right of App: App execution continues... |
| |
| App->"ANGLE Front-end": glDrawArrays (any draw) |
| "ANGLE Front-end"->"Vulkan Back-end": ContextVk::drawArrays |
| |
| "Vulkan Back-end"->"Link-Time SPIR-V Transformer": SpvTransformSpirvCode |
| "Link-Time SPIR-V Transformer"- ->"Vulkan Back-end": return transformed SPIR-V |
| |
| Note right of "Vulkan Back-end": We init VkShaderModules\nand VkPipeline then\nrecord the draw. |
| |
| "Vulkan Back-end"- ->"ANGLE Front-end": return success |
| --> |
| |
|  |
| |
| [GL_KHR_vulkan_glsl]: https://github.com/KhronosGroup/GLSL/blob/main/extensions/khr/GL_KHR_vulkan_glsl.txt |
| [SPIRV-Tools]: https://github.com/KhronosGroup/SPIRV-Tools |
| [translator]: https://chromium.googlesource.com/angle/angle/+/refs/heads/main/src/compiler/translator/ |
| [TranslatorVulkan.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/main/src/compiler/translator/TranslatorVulkan.cpp |
| [VkShaderModule]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkShaderModule.html |
| [PipelineCreation]: PipelineCreation.md |
