| # 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 Vulkan-compatible |
| GLSL. Vulkan-compatible GLSL matches 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 OpenGL |
| [line segment rasterization](OpenGLLineSegmentRasterization.md). For more info see |
| [TranslatorVulkan.cpp][TranslatorVulkan.cpp]. After initial compilation the shaders 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 markers are left in the shader for link-time substitution. |
| |
| The translator outputs some feature code conditional to Vulkan specialization constants, which are |
| resolved at draw-time. For example, |
| [Bresenham line rasterization](OpenGLLineSegmentRasterization.md) emulation. |
| |
| 1. **Link-Time Compilation and Transformation**: During a call to `glLinkProgram` the Vulkan |
| back-end can know the necessary locations and properties to write to connect the shader stage |
| interfaces. We get the completed shader source using ANGLE's |
| [GlslangWrapperVk][GlslangWrapperVk.cpp] helper class. At this time, we use Khronos' |
| [glslang][glslang] to convert the Vulkan-compatible GLSL into SPIR-V. 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 and inactive |
| varyings are removed from the shader interface. We currently don't generate `VkShaderModules` at |
| this time, but that could be a future optimization. |
| |
| 1. **Draw-time Pipeline Creation**: Once the application records a draw call, the SPIR-V is compiled |
| into `VkShaderModule`s. The appropriate specialization constants are then resolved and the |
| `VkPipeline` object is created. 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 "GlslangWrapperVk" |
| participant "Glslang" |
| |
| 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 Vulkan-compatible GLSL |
| |
| Note right of "ANGLE Front-end": Source is using bogus\nVulkan qualifiers 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"->GlslangWrapperVk: GlslangWrapperVk::GetShaderSpirvCode |
| GlslangWrapperVk->Glslang: GlslangToSpv |
| Glslang- ->GlslangWrapperVk: Return SPIR-V |
| |
| Note right of GlslangWrapperVk: Transform SPIR-V |
| |
| GlslangWrapperVk- ->"Vulkan Back-end": return transformed SPIR-V |
| "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 |
| |
| 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/master/extensions/khr/GL_KHR_vulkan_glsl.txt |
| [glslang]: https://github.com/KhronosGroup/glslang |
| [GlslangWrapperVk.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/libANGLE/renderer/vulkan/GlslangWrapperVk.cpp |
| [SPIRV-Tools]: https://github.com/KhronosGroup/SPIRV-Tools |
| [translator]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/ |
| [TranslatorVulkan.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/TranslatorVulkan.cpp |
| [VkShaderModule]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkShaderModule.html |
