| # 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. They are templated with markers that are filled in later at link time. |
| |
| 1. **Link-Time Translation**: 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. We still |
| cannot generate `VkShaderModules` since some ANGLE features like [OpenGL line |
| rasterization](OpenGLLineSegmentRasterization.md) emulation depend on draw-time information. |
| |
| 1. **Draw-time SPIR-V Generation**: Once the application records a draw call we use Khronos' |
| [glslang][glslang] to convert the Vulkan-compatible GLSL into SPIR-V with the correct draw-time |
| defines. The SPIR-V is then compiled into `VkShaderModules`. For details please see |
| [GlslangWrapperVk.cpp][GlslangWrapperVk.cpp]. The `VkShaderModules` are then used by `VkPipelines`. 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/ |
| 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 templated\nwith markers to be\nfilled 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::GetShaderSource |
| GlslangWrapperVk- ->"Vulkan Back-end": return filled-in sources |
| |
| Note right of "Vulkan Back-end": Source is templated with\ndefines to be resolved at\ndraw time. |
| |
| "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"->GlslangWrapperVk: GlslangWrapperVk::GetShaderCode (with defines) |
| GlslangWrapperVk->Glslang: GlslangToSpv |
| Glslang- ->"Vulkan Back-end": Return 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/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 |
